WO2015005006A1 - 中性子線減速材用フッ化物焼結体及びその製造方法 - Google Patents
中性子線減速材用フッ化物焼結体及びその製造方法 Download PDFInfo
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Definitions
- the present invention relates to a fluoride sintered body and a method for producing the same, and more specifically, a fluoride for a neutron beam moderator having a dense structure suitable for a moderator for suppressing the radiation speed of various types of radiation such as neutron beams.
- the present invention relates to a sintered body and a method for manufacturing the same.
- CaF 2 single crystal As the fluoride, calcium fluoride (CaF 2 ) single crystal, magnesium fluoride (MgF 2 ) single crystal and the like are relatively widely used in the optical field. There are very few cases where fluoride is used outside the optical field, and CaF 2 single crystals are rarely used in semiconductor manufacturing processes due to their high plasma resistance. It has been devised to be applied to a member requiring the most plasma resistance in a silicon wafer plasma etching furnace, such as a ring boat or a ceiling board. However, the CaF 2 single crystal is extremely expensive, and no report has been made that it has been used in an actual production line. CaF 2 single crystal, lithium fluoride (LiF), or aluminum fluoride (AlF 3) single crystal was rarely used as a shield for neutron radiation, which is one of the radiation.
- LiF lithium fluoride
- AlF 3 aluminum fluoride
- Neutron rays that are considered to have the highest penetrating power among the radiation are further classified according to energy level.
- An example is shown below.
- the parentheses indicate the energy levels of various neutron beams, and the larger the value, the greater the transmission power.
- low-temperature neutrons ⁇ 0.002 eV
- thermal neutrons ⁇ 0.025 eV
- epithermal neutrons ⁇ 1 eV
- slow neutrons (0.03 to 100 eV
- medium-speed neutrons (0. 1 to 500 keV)
- fast neutrons 500 keV or more.
- the energy values in parentheses are not exact, and there are various theories on the classification of neutron beams. For example, there is a theory that describes the energy of epithermal neutrons of 40 keV or less that falls within the above-mentioned medium speed neutron energy range.
- neutron radiation A typical example of effective use of neutron radiation is in the medical field.
- radiotherapy that destroys malignant tumor cells by irradiating them with neutrons has been rapidly spreading.
- neutron beams With a certain amount of energy.
- a high energy neutron beam is irradiated, it is not possible to avoid an influence on a part other than the affected part (healthy part) of the patient, and there are side effects. Therefore, at present, radiation therapy is limited to severe patients.
- IMRT intensity-modulated radiation therapy
- moving body tracking radiation that irradiates radiation in accordance with body movements such as patient breathing and heart movement.
- Treatment method or “Particle beam therapy” that intensively irradiates heavy particle beam or proton beam with high therapeutic effect.
- boron is reacted with a patient's malignant tumor cells to form a reaction product (boron compound) in the tumor portion, and the reaction product is subjected to neutron radiation (mainly heat, which has little effect on the healthy part). Irradiation with external neutrons and thermal neutrons.
- neutron radiation mainly heat, which has little effect on the healthy part.
- a nuclear reaction is caused only in a very fine range where the boron compound is formed, and only tumor cells are killed.
- a desirable medical particle beam can be formed by increasing the ratio of neutron beams having a high effect (for example, low-energy portions of medium-speed neutron beams and epithermal neutron beams).
- the low-energy part of epithermal neutrons and medium-speed neutrons has a high level of penetration into the patient's body tissue, for example, no craniotomy is required even for brain tumors, or laparotomy is performed in other major organs. Even if it is not easy, it does not require laparotomy, and effective irradiation to the affected area is possible. On the other hand, when extremely low energy neutron beams such as thermal neutron beams are used, craniotomy or laparotomy is required due to low depth of penetration, resulting in a heavy burden on the patient.
- Non-patent Document 1 and Non-Patent Document 2 boron neutron capture therapy
- This method is not attached to an existing nuclear reactor, a dedicated cyclotron accelerator is provided as a neutron beam generator, and a medical dedicated neutron beam generator is employed.
- the moderator selected as the radiation shield includes lead (Pb), iron (Fe), aluminum ( In addition to Al) and polyethylene, polyethylene containing calcium fluoride (CaF 2 ) and lithium fluoride (LiF) is used.
- the neutron beam obtained after decelerating with a combination of these moderators obtains the necessary dose of epithermal neutrons most suitable for treatment with BNCT When the conditions were set, it was configured to contain a large amount of fast neutrons that adversely affect the healthy tissue.
- the thickness of the moderator becomes considerably thick.
- the speed reduction device becomes large, and the entire device cannot be sufficiently downsized. there were.
- downsizing of the entire device is an essential requirement, and in order to reduce the size of the accelerator and the speed reduction system, it is urgent to develop a moderator with excellent deceleration performance. It was.
- the standard of the epithermal neutron beam and the thermal neutron dose required when the irradiation time is about 1 hour is said to be approximately 1 ⁇ 10 9 [n / cm 2 / sec].
- Be beryllium
- approximately 5 to 10 MeV is required as the energy of the emitted beam from the accelerator that is the source of neutron beam generation.
- the beam emitted from the accelerator collides with the target (Be) and generates high-energy neutrons such as fast neutrons mainly by nuclear reaction.
- a fast neutron beam decelerating method first, Pb or Fe having a large inelastic scattering cross section is used, and decelerating while suppressing attenuation of the neutron beam.
- These two kinds of moderators will decelerate to a certain extent (up to about 1 MeV), and then decelerate and optimize according to the neutron beam energy required for the irradiation field.
- Aluminum oxide (Al 2 O 3 ), aluminum fluoride (AlF 3 ), CaF 2 , graphite, heavy water (D 2 O), or the like is used as a moderator for the neutron beam after being moderated to some extent. It is demanded that a neutron beam decelerated to the vicinity of 1 MeV is incident on these moderators to decelerate to an epithermal neutron energy region (4 keV to 40 keV) suitable for BNCT.
- polyethylene containing Pb, Fe, Al, CaF 2 , polyethylene, and LiF is used as a moderator.
- polyethylene and LiF-containing polyethylene are shielding moderators that are provided so as to cover the outside of the apparatus in order to prevent leakage to places other than the irradiation field of high-energy neutron rays.
- LiF-containing polyethylene used in the latter half of the stage with CaF 2 is used to cover the entire surface other than the neutron beam exit on the treatment room side, and is installed to prevent the whole body from being exposed to fast neutrons to the patient. It is not used as a moderator at the exit.
- the polyethylene as the moderator in the first half is used to cover the entire outer surface of the device other than the treatment room side in the same way as the LiF-containing polyethylene in the second half, thereby preventing leakage of fast neutrons around the device. Is installed. Therefore, instead of CaF 2 in the latter half of the stage, it is hoped to develop an excellent moderator that can block and slow down high-energy neutron beams while suppressing the attenuation of neutron beams at the medium energy level required for treatment. It was rare.
- the present inventors mainly use epithermal neutron rays that are expected to have the highest therapeutic effect from the neutron rays ( ⁇ 1 MeV) moderated to a certain extent, and have optimal energy (4 keV to 40 keV) for treatment.
- MgF 2 sintered body As a moderator for obtaining a neutron beam having a distribution, attention was paid to the MgF 2 sintered body.
- MgF 2 has been used for a neutron moderator.
- MgF 2 -based sintered bodies including MgF 2 sintered bodies and MgF 2 -CaF 2 binary sintered bodies have been adopted as neutron moderators.
- MgF 2 sintered body is a colorless crystal called a melting point of 1248 ° C., a boiling point of 2260 ° C., a density of 3.15 g / cm 3 , a cubic system, and a rutile structure.
- the single crystal is highly transparent and can be used mainly as a window material for excimer lasers because of its high light transmission in a wide wavelength range of approximately 0.2 to 7 ⁇ m and its wide band gap and high laser resistance.
- MgF 2 is deposited on the surface of the lens and used for optical applications such as internal protection and irregular reflection prevention.
- JP-A-2000-86344 As an application example of a sintered body based on MgF 2 to a plasma-resistant member, there is JP-A-2000-86344 (Patent Document 1 below).
- the claims include a fluoride of at least one alkaline earth metal selected from the group consisting of Mg, Ca, Sr and Ba, and the total amount of metal elements other than the alkaline earth metal is 100 ppm in terms of metal.
- the average particle diameter of the fluoride crystal particles is 30 ⁇ m or less and the relative density is 95% or more.
- the substances listed in the list of the examples of this publication are calcined using the respective metal fluorides of the four alkaline earth metals (that is, MgF 2 , CaF 2 , SrF 2 , BaF 2 ) as raw materials. There is no description that the mixture of these raw materials was fired.
- the firing temperature of the case evaluated as appropriate is MgF 2.
- the temperatures are 850 ° C., 950 ° C., and 1050 ° C., and the relative density of the sintered body is 95% or more.
- the temperature is 950 ° C., 1040 ° C., and 1130 ° C., and it is described that the relative density of each sintered body is 97% or more.
- both MgF 2 and CaF 2 exhibit sublimation from a temperature equal to or lower than these firing temperatures, and severe foaming occurs at the above firing temperatures. It was found that it was impossible to obtain a relative density of 95% or more with MgF 2 and a relative density of 97% or more with CaF 2 .
- the sintered body was produced at a firing temperature higher than the temperature at which the sublimation starts.
- active foaming occurs inside the sintered body during the firing process in which the raw powder is sintered, and sintering is performed under conditions where it is difficult to obtain a dense sintered body. .
- the present inventors have studied such a phenomenon and studied a method for reducing the influence of the sublimation phenomenon as much as possible in the sintering process, and developed an excellent sintering method capable of stably obtaining a dense sintered body. developed.
- MgF 2 simple sintered body has a drawback of weak mechanical strength, it contains at least one non-alkali metal material whose average linear thermal expansion coefficient is lower than MgF 2 such as Al 2 O 3, AlN, SiC, MgO.
- a method for compensating for the drawback of the mechanical strength of the MgF 2 simple sintered body being weak is disclosed.
- the sintered body of such mixtures when used in moderator of the neutron, the influence of the non-alkaline metal to be mixed with MgF 2, differ significantly from that the reduction performance of the MgF 2 plain, baked of such mixture It was foreseen that it would be difficult to apply the moderator to the moderator material.
- Patent Document 3 JP-A-2000-302553
- the biggest drawback of sintered fluoride ceramics such as MgF 2 , CaF 2 , YF 3 , and LiF is that the mechanical strength is weak.
- these fluorides and Al 2 O 3 It is said that it is the sintered compact which compounded these by the predetermined ratio.
- the corrosion resistance and the mechanical strength of the sintered body produced by this method are merely obtained with any combination of the properties of both fluoride and Al2O3.
- a compound that exceeds both characteristics by compounding has not been obtained.
- the present invention has been made in view of the above problems, and is a moderator used to decelerate the energy of a neutron beam when effectively using a neutron beam which is a kind of radiation, and is a high-purity simple substance.
- Neutron beams that do not become as expensive as crystals and that can provide an effective deceleration effect, resulting in an increase in therapeutic effect and a reduction in the size of the treatment device It aims at providing the fluoride sintered compact for moderators, and its manufacturing method.
- the present inventors first made basic considerations regarding selection of a substance (metal or compound) having a sufficient moderating effect on high-energy neutron beams.
- BNCT high-energy neutron rays that are harmful during treatment are reduced as much as possible, and on the other hand, in order to obtain a large therapeutic effect, neutron rays mainly composed of epithermal neutron rays and slightly including thermal neutron rays are used. It is important to irradiate the affected area.
- a guideline for the extra-thermal and thermal neutron dose required when the irradiation time is about 1 hour is approximately 1 ⁇ 10 9 [n / cm 2 / sec]. Therefore, it is said that the emitted beam energy from the accelerator, which is a neutron beam generation source, requires approximately 5 to 10 MeV when beryllium (Be) is used as a neutron beam generation target.
- Be beryllium
- the beam emitted from the accelerator collides with the target (Be), and mainly generates high-energy neutron rays (fast neutron rays) by nuclear reaction.
- the fast neutron beam is decelerated to some extent while suppressing attenuation of the neutron beam with Pb or Fe having a large inelastic scattering cross section.
- the moderator for the neutron beam that has been decelerated to some extent is optimized according to the amount of neutron energy required for the irradiation field.
- Al 2 O 3 aluminum oxide
- AlF 3 aluminum fluoride
- CaF 2 calcium fluoride
- D 2 O heavy water
- the present inventors selected two types of fluorides, MgF 2 and CaF 2 , as potential moderator candidates for neutron beams that have been moderated to some extent from various compounds, and investigated the moderation effect described below. went.
- Fig. 4 the neutron beam decelerated to the vicinity of 1 MeV is incident on the moderator made of MgF 2 , so that the fast neutron beam harmful to BNCT can be almost completely removed, and the energy region most suitable for the treatment. It has been found that epithermal neutron beams (4 keV to 40 keV) can be obtained.
- the manufacturing method includes a crystal method, a single crystal method, and a polycrystal method (that is, a sintering method). Crystals produced by the crystal method generally have segregation in crystal orientation, and are likely to segregate with respect to impurities, and when used as a moderator, moderation performance tends to vary depending on the site. Therefore, it is considered unsuitable for the moderator.
- a single crystal produced by the single crystal method requires high control accuracy in production, is inferior in stability of quality, and is extremely expensive. Therefore, it must be said that it is not suitable for a moderator. Therefore, this time, the present inventors have completed the present invention by researching and developing a moderator production method by a polycrystal method (hereinafter referred to as a sintering method).
- the sintering reaction is mainly due to grain growth by solid-phase reaction (hereinafter referred to as solid-phase sintering), and in the main sintering process, sintered body formation (hereinafter referred to as solid solution formation reaction) is performed mainly in the solid solution generation temperature range.
- solid solution sintering Solid solution sintering
- melt sintering formation reaction
- the moderator must be resistant to damage during handling, such as being installed in a deceleration system, and to be resistant to dust generation due to neutron irradiation impact. That is, it is required that the material has excellent mechanical strength.
- the mechanical strength of the sintered body is the defoamed state such as the micro-strength of the joint part between the particles, the size, shape, distribution, number of bubbles, in other words, the thickness of the joint part (matrix) of the joint part and the original particle.
- the length is determined by the degree of brittleness caused by the shape such as length (denseness of the sintered body) and the crystal structure (single crystal, polycrystal, etc.) of the matrix.
- MgF 2 which is a raw material for forming a high-density sintered body by suppressing foaming in the sintering process and reducing large residual bubbles, tends to cause a vaporization (sublimation) phenomenon in the sintering process, generates fluorine gas, Many fine bubbles are likely to be generated inside the sintered body. Foaming due to this vaporization contradicts the reduction of voids due to the progress of the original sintering process, and foaming was suppressed as much as possible.
- the fluoride material When the fluoride material is heated at a high temperature, a part of the material is vaporized.
- the temperature at which vaporization starts varies depending on the composition, and in the case of a composition mainly composed of MgF 2 , vaporization starts from about 800 ° C., and the vaporization starts fairly actively from about 850 ° C.
- fluorine gas When vaporized, fluorine gas is generated, and fine bubbles are generated in the sintered body.
- the shape of the generated bubble is almost spherical, and when the fracture surface of the sintered body is observed with an electron microscope (SEM), the cross section of the bubble appears to be a circle close to a perfect circle.
- the small one is several ⁇ m and the large one is about 20 to 40 ⁇ m.
- the shape of small ones of several ⁇ m is almost circular, and the shape of large ones is rare, and most of them are elongated, square, or irregular. From these shapes, it is considered that small bubbles are just generated, and large bubbles are a collection of some of the generated bubbles.
- the fluoride sintered body (1) for a neutron moderator according to the present invention is made of MgF 2 having a dense polycrystalline structure and has a bulk density of 2.90 g / cm 3 or more. It is characterized by that.
- the fluoride sintered body for neutron beam moderator (1) since it is a dense polycrystalline MgF 2 sintered body having a bulk density of 2.9 g / cm 3 or more, the structure of the sintered body Is uniform, the difference between the inner and outer positions is small, and the amount of solid solution produced can be suppressed to suppress crystal growth, the generation of brittle portions can be reduced, and the strength of the sintered body can be increased. Therefore, cracks and chips are not easily generated in the processing steps during the production of the sintered body and in handling between the steps.
- the fluoride sintered body for neutron beam moderator (2) according to the present invention is a machine having a bending strength of 10 MPa or more and a Vickers hardness of 71 or more in the above-mentioned fluoride sintered body for neutron beam moderator (1). It has a characteristic strength.
- the sintered body for neutron beam moderator (2), the sintered body has extremely excellent mechanical strength, and causes cracking or the like during machining when used as a moderator. In addition, it is possible to have sufficient impact resistance against neutron irradiation impact irradiated during use as a moderator.
- the method (1) for producing a fluoride sintered body for a neutron moderator comprises grinding a high-purity MgF 2 raw material to a median diameter of about 1 to 2 ⁇ m and sintering aid.
- a step of adding 0.1 to 1 wt.% Of an agent and blending a step of molding the compounded raw material using a uniaxial press molding machine as a starting material at a molding pressure of 5 MPa or more, and cold isostatic pressing of the uniaxial molded product
- CIP molding
- CIP is a uniaxial molded product placed in a bag sealed with a plastic bag so as not to come into direct contact with the water, and the degassed product is placed in a pressure vessel, and water is poured into the vessel.
- a pressure forming method in which a predetermined water pressure is applied.
- the foaming start temperature refers to a temperature at which a part of the fluorine compound starts to decompose, generates fluorine gas, and starts generating fine bubbles.
- a pre-sintered body formed by heat treatment at 550 ° C. for 6 hours in an air atmosphere is pulverized, and this pulverized product is used as a test sample for a differential thermal analyzer.
- the change and the change of heat absorption and heat generation were investigated. A slight decrease in weight was observed from about 800 ° C, but this was due to weak bonding, for example, fluorine attached to the base material of the pre-sintered body or fluorine dissolved in the base material first dissociated and decomposed first.
- the temperature at the inflection point that is, about 850 ° C. is referred to as the foaming start temperature.
- the temperature range immediately below the foaming start temperature specifically refers to a temperature range of 750 to 840 ° C.
- the temperature range where the solid solution starts to be generated refers to a temperature range around 980 ° C., which is the temperature at which the solid solution starts to form in the MgF 2 —CaF 2 binary phase diagram shown in FIG.
- the sintered body manufactured using the method (1) for manufacturing a fluoride sintered body for a neutron moderator according to the present invention has a strong bonding force between particles, and a mechanical strength ( Micro strength) is high.
- the bending strength and impact resistance, which were the problems, were remarkably improved, and a neutron moderator that could be used without any problem in practice was obtained.
- the sintered body to be manufactured has a higher density by selecting the purity of MgF 2 , the heating atmosphere, the heating temperature pattern, and the like. Further, since it is a sintered body, its crystal structure becomes polycrystalline, and the brittleness is remarkably improved as compared with a single crystal.
- the sintered body manufactured using the method (1) for manufacturing a fluoride sintered body for a neutron moderator according to the present invention is cut, ground, and polished for a moderator in a BNCT moderator system.
- it has sufficient mechanical strength for handling such as forming and the like, and handling such as installation in the speed reduction system apparatus, and can be constructed without problems. Further, even when irradiated with neutron beams, it can be used without problems with respect to the irradiation impact, and the neutron beam decelerating performance was extremely excellent.
- the method (2) for producing a fluoride sintered body for a neutron beam moderator according to the present invention is the same as the method for producing a fluoride sintered body for a neutron beam moderator (1) in the main sintering step.
- the active gas atmosphere is characterized by comprising one kind of gas selected from nitrogen, helium, argon, and neon, or a mixture of plural kinds of gases.
- nitrogen (N 2 ), helium (He), argon (Ar), or neon (Ne) can be used as the inert gas.
- FIG. I is a table showing changes in neutron species after reduction and the relative density of MgF 2 sintered body. It is a table
- Embodiments of a fluoride sintered body for a neutron moderator and a method for producing the same according to the present invention will be described below with reference to the drawings.
- high-purity (purity 99.9 wt.% Or more) MgF 2 powder is used, and, for example, carboxymethylcellulose (sintering aid) CMC) solution was added to 0.03 to 0.5 wt.% (Outer coating) of the powder 100 and kneaded to obtain a starting material.
- the material After filling the raw material in a mold with a predetermined size, the material is compressed with a molding pressure of 5 MPa or more using a uniaxial press, and the molded product is further molded using a cold isostatic pressing (CIP) machine. Molding was performed at a pressure of 5 MPa or more.
- CIP cold isostatic pressing
- This CIP molded product is pre-sintered by heating in a temperature range of 550 to 600 ° C. in the air atmosphere, and the pre-sintered product is subjected to foaming start temperature (in a differential thermal analyzer in the atmosphere or in an inert gas atmosphere). For 4 to 16 hours in the temperature range (750 to 840 ° C.) immediately below the temperature determined in the measurement of (about 850 ° C.). By heating, the sintering proceeds more uniformly, and then heated in the vicinity of the temperature range where the solid solution starts to form, that is, in the temperature range of 900 to 1100 ° C. for 0.5 to 3 hours, and then cooled to form a dense structure. An MgF 2 sintered body is manufactured.
- the temperature at which the solid solution starts to be generated is in the temperature range of about 980 ° C., but the present inventors actually sintered. From observation of the cross section of the sintered body, it was estimated that in the case of MgF 2 alone, there is a high possibility that a solid solution is generated at a temperature several tens of degrees lower than the display temperature of 980 ° C. in this phase diagram. Therefore, the vicinity of the temperature range where the solid solution starts to be generated is 900 ° C. or higher, and it is considered that the solid solution is generated even when heated at less than 980 ° C.
- the raw material MgF 2 was pulverized by filling a ball for ball mill in a pot mill, filling 3 kg of the raw material therein, and kneading and pulverizing for one week.
- the pot mill was made of alumina and had an inner diameter of 200 mm and a length of 250 mm.
- the filled balls were made of alumina and had a diameter of 5: 1800 g, 10: 1700 g, 20: 3000 g, and 30: 2800 g.
- the particle size of the pulverized raw material was measured with a laser diffraction / scattering particle size distribution analyzer LA-920 manufactured by Horiba. The median diameter was approximately 1.2 to 1.3 ⁇ m.
- the sintering aid two types of CMC and calcium stearate were selected, and the respective addition ratios were changed, and a test for examining the effect of each sintering aid was conducted. For comparison, a test without using a sintering aid was also performed.
- the mixing of the sintering aids two types of sintering aids are added at a blending ratio of 0 to 2 wt.%, And the ball mill balls are filled in the pot mill and kneaded all day and night in the same manner as the pulverization of the raw materials. Was done.
- the CIP compact was pre-sintered in an air atmosphere at various temperatures within a range of 500 to 700 ° C. for 3 to 18 hours with various heating conditions. After observing the appearance of this temporary sintered body, the temperature was raised from room temperature to 550 ° C. over 6 hours at a constant rate in a nitrogen gas atmosphere, and kept at that temperature for 8 hours. Thereafter, the temperature was raised to 950 ° C. over 2 hours at a constant rate, maintained at the same temperature for 1 hour, and then cooled to 100 ° C. over 20 hours. The appearance of the sintered body taken out and the state of densification inside were observed, and proper blending, processing conditions and pre-sintering conditions were investigated.
- the shape maintaining performance of the uniaxial press-molded product was inferior, and there were many types of deformation during handling to the next CIP molding step. .
- the mixing ratio of the sintering aid is 0.03 wt.% Or more, the above-mentioned mold distortion is not observed, and when the mixing ratio exceeds 0.6 wt.%, The sintering aid remains in the temporary sintered body or the sintered body. Coloring that seems to be a thing may be recognized. For these reasons, the appropriate range of the mixing ratio of the sintering aid is set to 0.03 to 0.5 wt.
- the bulk density of the sintered body in the test for optimizing the heating conditions for preliminary sintering and main sintering is 2% in all tests compared to the case where the molding pressure is 5 MPa or more. It was lower than that.
- the compacting pressure is 10 MPa
- the sintered body sintered under the same sintering conditions has a bulk density of 2.95 g / cm 3
- the compacted body has a compacting pressure of 4.8 MPa.
- the bulk density was 2.86 g / cm 3 and was 3% lower.
- the molding pressure was gradually increased from 5 MPa to 20 MPa, the bulk density of the sintered body after sintering tended to increase little by little.
- the test was performed up to 50 MPa by further gradually increasing the molding pressure.
- the molding pressure was 20 MPa or higher, the increase in the bulk density of the temporary sintered body and the sintered body was only a slight increase, and no linear improvement was observed between 5 and 20 MPa.
- the appropriate value of the molding pressure is set to 5 MPa or more, desirably 20 MPa or more.
- the pre-sintering condition of the molded body in the air atmosphere is that the shrinkage is small compared to the size of the molded body when the heating temperature is less than 550 ° C., and the shrinkage is higher than 610 ° C. Large and difficult to control shrinkage. For this reason, the appropriate range of the pre-sintering temperature was set to 550 to 600 ° C.
- the appropriate value of the heating time was optimal at 8 to 9 hours from the evaluation of the shrinkage rate at 550 ° C., and it was determined that 4 to 10 hours were appropriate. At 600 ° C., 6 to 8 hours was optimum, and 4 to 10 hours could be judged appropriate. From these results, the heating condition for pre-sintering was set to 550 to 600 ° C. for 4 to 10 hours in an air atmosphere.
- the “primary agglomeration process” is the first half stage of sintering, and in the initial stage, the interval between the particles gradually decreases and the gap between the particles also decreases. Furthermore, the contact portion between the particles becomes thick, and the gap between them becomes small. However, the majority of the voids are open pores and communicate with the surrounding atmosphere. Such a stage is referred to as “primary aggregation process”.
- the gaps between the particles of the temporary sintered body are small and the gaps are gathered by fine pulverization of raw materials, particle size adjustment, kneading of sintering aid, uniaxial press molding, CIP molding, temporary sintering, etc. It was confirmed that the particles were dispersed almost uniformly without any treatment (the first half of the primary aggregation process).
- the heating temperature is gradually raised in the temperature raising process of the next main sintering step, and the aggregation of particles starts from around the pre-sintering temperature range (550 to 600 ° C.). Subsequently, the solid solution starts to form from 980 ° C. However, the reaction between the solid phases starts from a considerably low temperature range, and the aggregation of the particles progresses accordingly, the distance between the particles becomes shorter, and the void becomes smaller. However, in the case of heating for a short time at a relatively low temperature (a temperature close to 550 ° C.) such as pre-sintering, most of the voids still remain in the open pore state (the second half stage of the primary aggregation process).
- the reaction between solid phases generally starts from a temperature range of about 10% or more from that temperature. From the observation of the cross-section of the sintered body in the preliminary test of the present inventors, it is considered that the reaction between solid phases starts from a temperature range lower than the generally called temperature, and starts from about 500 ° C. It was. The reason for this is that the sintering has already proceeded considerably at 550 ° C., which is the lower limit of the proper pre-sintering temperature, and the pre-sintered body shrinks considerably as compared with the molded body. In this preliminary test, the bulk volume contracted by about 10 to 20 vol.%. It was considered that the reaction proceeded at a slow reaction rate in this temperature range, and that the reaction rate was considerable in the temperature range of about 700 ° C. or higher and below 980 ° C.
- the point to be noted here is the behavior of fine bubbles (foaming gas) generated when a part of the raw material is vaporized in a temperature range of about 850 ° C. or higher.
- fine bubbles foaming gas
- the heating temperature is less than 980 ° C.
- the reaction between the solid phases proceeds, and as time passes, the voids gradually decrease and become closed pores.
- the gas components in the closed pores diffuse into the bulk (matrix) of the sintered body, and defoaming proceeds, resulting in a dense sintered body with few bubbles (secondary aggregation process).
- heating for a considerably long time is required, and productivity is lowered, which is not economical.
- the difference in appearance between this foaming gas and the air bubbles that closed during the sintering process and could not be removed (hereinafter referred to as residual air bubbles) is described.
- the size of the foamed gas generated by heating for a relatively short period of time is about several ⁇ m in diameter, and the shape is almost spherical.
- the size of residual bubbles is large, medium, and small, and the shape is not a perfect sphere, but an indeterminate shape, and it is possible to distinguish both from the difference in shape.
- the foaming gases or the residual bubbles and the foaming gas are aggregated to form a large irregular shape. It grows into bubbles and it becomes difficult to distinguish the origin.
- the voids between the particles become smaller, and all or most of the voids are surrounded by particles or a bridge portion of the sintered body to form closed pores (bubbles).
- the gas is degassed through voids (open pores), or the gas component in the bubbles penetrates into the bulk of the particles or the bridge portion of the sintered body to degas and the bubbles disappear (defoaming phenomenon).
- the voids between the particles remain as closed pores (bubbles) or whether they disappear as a result of degassing and do not remain as bubbles determines the degree of densification of the sintered body and thus the characteristics of the sintered body It is a big factor.
- the preliminary sintering process corresponding to the first half of the primary agglomeration process and the main sintering process corresponding to the second half of the primary agglomeration process and the secondary agglomeration process are performed separately. It is easy to proceed uniformly as a whole. However, it does not make sense if the heating conditions are not appropriate just because the sintering process is divided into two in the preliminary sintering and the main sintering. For example, heating at a high temperature exceeding the proper range in the preliminary sintering process, rapid heating at the temperature rising stage of the main sintering process, or holding temperature in the main sintering process at a high temperature exceeding the proper range.
- the density was higher than 2.90 g / cm 3 , but in the case of a sintering temperature of 850 ° C. or lower, conversely, in the case of a sintering temperature of 1150 ° C. or higher, The bulk density was less than 2.90 g / cm 3 .
- the bridge width of the sintered portion was narrow when it was 800 ° C. or lower, and it was judged that the progress of sintering was insufficient, and at 850 ° C., there were slight open pores. .
- At 1100 ° C. some amorphous bubbles were observed inside, and at 1150 ° C.
- a porous pumice-like structure was formed in which countless amorphous bubbles were generated inside.
- Innumerable fine bubbles having a substantially spherical shape with a diameter of several to several tens of ⁇ m and irregular bubbles with a diameter of 10 ⁇ m or more were observed innumerably on the entire cross section of the sintered body. From this shape, it was possible to determine that this spherical bubble was a foamed gas, and this irregularly shaped bubble was also an aggregated bubble from its shape.
- This sublimation generates fine bubbles throughout the sintered body as described above.
- the behavior of whether the generated fine bubbles (foaming gas) are defoamed or remain as bubbles is determined depending on the degree of progress of the sintering process and at which part of the sintered body.
- the entire sintered body is still mainly composed of open pores, most of the bubbles are degassed through the open pores, and few remain as bubbles.
- the sintered body is mainly composed of closed pores, a large amount of foaming gas cannot be defoamed and remains as bubbles. Basically, it can be said that promptly completing the sintering in the secondary agglomeration process is a direction in which residual bubbles can be reduced.
- the transition from the primary agglomeration process to the secondary agglomeration process should be made as small as possible in the entire sintered body.
- the present inventors perform lower heating in a temperature range immediately below the foaming start temperature (about 850 ° C.) for a relatively long time to complete the primary aggregation process and the first half of the secondary aggregation process, and then a solid solution is formed.
- the second half of the secondary agglomeration process was completed by heating for a relatively short time in the vicinity of the starting temperature (980 ° C.) region. It has been found that this is an excellent sintering method capable of matching the degree of sintering progress in the entire sintered body and generating less foaming gas.
- the size of the temporary sintered body is a rectangular parallelepiped shape of approximately 212 mm ⁇ 212 mm ⁇ t72 mm.
- the heating atmosphere was changed to a nitrogen gas atmosphere, and a preliminary test was performed in three cases of heating time and temperature lowering conditions of 3, 6, and 9 hours, respectively. As a result, small cracks occurred in the sintered body in 3 hours, and the others were good, so the time was set to 6 hours.
- the heating atmosphere is changed to a nitrogen gas atmosphere.
- the heating temperature is changed in the range of 700 to 1250 ° C.
- the holding time is set to 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18 hours. It was carried out in 11 cases.
- the temperature was 750 ° C. or lower, the densification was insufficient regardless of the holding time.
- the holding time was 4 hours or less, densification was insufficient at temperatures other than 1100 ° C.
- the sintered state was good when the holding time was 8 hours or more, and slightly under-sintered after 6 hours. In the case of 900 ° C., it was good at 5 hours or longer, slightly sintered at 4 hours or shorter, and could not be judged at 16 hours or longer. In the case of 950 ° C., 5 to 14 hours were good, the sintering was slightly insufficient in 4 hours or less, and the pass / fail judgment was impossible in 15 hours or more. When the temperature was 1000 ° C., 5 to 12 hours were good, sintering was slightly insufficient at 4 hours or less, and foaming was large at 14 hours or more.
- the main sintering process is divided into two stages. Since there is a kneading step, it was decided that the evaluation in the previous main sintering step was good.
- the same temporary sintered body as described above is used and the holding time is made constant for 6 hours.
- the heating temperature was changed in the range of 600 to 1300 ° C.
- the heating temperature is 900 ° C.
- the bulk density is approximately 2.90 g / cm 3
- the sintered body having a bulk density higher than this is likely to be broken by handling in the subsequent process, as in the result shown in FIG. There was no trouble and it was judged that densification was sufficient.
- the heating temperature is 850 to 1100 ° C.
- the holding time is 3 to 14 hours (high temperature within this range, heating for a short time, It was judged that heating at a low temperature for a relatively long time was an appropriate condition.
- the inventors of the present application as a basic policy of the main sintering process, suppresses foaming as much as possible, and further proceeds the sintering reaction sufficiently, and is good in the subsequent machining process. It was decided to produce a sintered body having processability.
- the basic policy is to prevent foaming as much as possible in the first stage of the main sintering process, and to proceed with sintering slowly and to minimize the difference in the degree of sintering between the sintered body and its outer periphery. did.
- the heating temperature and the holding time range were set in the range of 800 to 1100 ° C. as described above. Since the temperature at which foaming becomes prominent is about 850 ° C., the temperature is 840 ° C. or lower, that is, the heating temperature in the first stage of the sintering process is 750 to 840 ° C., and the holding time is 5 to 12 hours.
- the heating at the stage of enhancing the sintering reaction of the next sintered body was performed in a temperature range of around 980 ° C., that is, a temperature at which the solid solution starts to be generated within the above appropriate conditions, that is, 900 to 1100 ° C.
- the holding time is as short as possible in order to enhance the sintering reaction and suppress foaming, and 0.5 hours using the results of FIG. 3 and examples and comparative examples described later as judgment materials. If the ratio is less than 4, the sintering reaction is not sufficiently enhanced, and if it is 4 hours or more, foaming becomes excessive, so that the holding time is 0.5 to 3 hours.
- the holding time is 6 hours using the same temporary sintered body as described above.
- the heating temperature was varied in the range of 600 to 1300 ° C. The result is the same as in the case of nitrogen gas, and the bulk density is approximately 2.90 g / cm 3 at a heating temperature of 900 ° C. It was judged that the densification was sufficient without being broken by handling. On the other hand, when the heating temperature is 1110 ° C. or higher, the mass loss TG becomes ⁇ 0.8% or higher, and the yield is significantly reduced. Occurred.
- the heating temperature was 900-1100 ° C. and the holding time 0.5-2.5 hours were the proper conditions. Furthermore, when the heating temperature is 950 to 1050 ° C. and the holding time is 0.5 to 3 hours, defects such as cracks are difficult to occur when subjected to machining, and the machinability is also good, so that the desired heating temperature and holding are maintained.
- the time was judged to be 950 to 1050 ° C. and 0.5 to 3 hours. Therefore, the proper heating conditions for the main sintering step in the helium gas atmosphere are the same as in the nitrogen gas atmosphere described above.
- the initial heating in the main sintering step is a holding time of 750 to 840 ° C. for 5 to 12 hours.
- the subsequent heating was performed under the proper conditions of 900 to 1100 ° C. and a holding time of 0.5 to 3 hours.
- the inert gas is not limited to nitrogen and helium, and the same effect can be obtained with argon or neon.
- argon or neon since the sintered body is expected to have high solubility or diffusivity in the same manner as helium, the defoaming phenomenon is further promoted, and the same or further improvement as helium is expected.
- the final state of the sintered body is always dense, and there are obvious defects such as locally large voids and cracks found in general ceramic sintered bodies. Was not found in these sintered bodies.
- Non-Patent Document 3 The total thickness of the second-half moderator to be evaluated was constant at 320 mm, and the types of moderators were MgF 2 and CaF 2 . Further, a case where MgF 2 and CaF 2 were superposed in two layers (total thickness was fixed at 320 mm) was also evaluated.
- the contents to be evaluated here are how many fast neutrons remaining in the neutron beam decelerated by the moderator are likely to adversely affect the patient.
- the result is shown in FIG.
- MgF 2 and CaF 2 here, a dense sintered body having a relative density (100 ⁇ (actual density) / (true density), unit%)) of 95 ⁇ 2% was used.
- MgF 2 is an excellent moderator.
- the relative density of MgF 2 (i.e., compactness) was investigated the effect that on the deceleration capability.
- the moderator only MgF 2 sintered body with a relative density of 90 to 97% was used.
- High purity MgF 2 raw material (average particle size 20 ⁇ m, purity 99.9 wt.% Or more) is pulverized using the pot mill and the balls made of alumina described in the above “Mode for Carrying Out the Invention” to obtain high purity MgF 2 powder (average particle size 1.2 ⁇ m, purity 99.9 wt.% Or more).
- a carboxymethyl cellulose (CMC) solution as a sintering aid was added to this powder at a rate of 0.2 wt.% With respect to the MgF 2 powder 100 and mixed for 12 hours in a pot mill as a starting material.
- This starting material was filled into a mold (mold size 220 mm ⁇ 220 mm ⁇ H 150 mm) using a uniaxial press, and compressed and molded by applying a uniaxial press pressure of 10 MPa.
- This press-molded body (dimensions: about 220 mm ⁇ 220 mm ⁇ t85 mm) is placed in a thick plastic bag, degassed and sealed, and the molded part of a cold isostatic pressing (CIP) machine (inside dimensions: inner diameter 350 mm ⁇ H120 mm).
- CIP cold isostatic pressing
- isotropic pressure is applied at a molding pressure of 20 MPa, and the molded body by CIP molding (dimensions of about 215 mm x 215 mm x t75 mm) ).
- This molded body was pre-sintered at 600 ° C. for 5 hours in an air atmosphere to obtain a pre-sintered body having a size of about 208 mm ⁇ 208 mm ⁇ t 72 mm.
- This pre-sintered body was heated from room temperature to 830 ° C. over 6 hours at a constant rate in a nitrogen gas atmosphere, and kept at the same temperature for 6 hours. Thereafter, the temperature was raised to 1000 ° C. over 2 hours at a constant rate, and kept at the same temperature for 1 hour. Thereafter, the heating was stopped, and the mixture was naturally cooled (furnace cooling) over about 20 hours to 100 ° C., which was set to the removal temperature, and then removed.
- the bulk density of the sintered body was calculated to be 3.05 g / cm 3 (relative density 96.8%) from the approximate dimensions (193 mm ⁇ 193 mm ⁇ t62 mm) and weight.
- the sintered state was good.
- the "bulk density” here refers to the appearance of the sintered body in a plan view and a square shape, so the bulk volume is calculated from the two sides and thickness of the square, and the weight measured separately The method obtained by dividing by the bulk volume was adopted. Hereinafter, it carried out similarly.
- Example 2 Using the same starting materials as in Example 1 above, uniaxial press forming and cold isostatic pressing (CIP) were performed, followed by temporary sintering at 550 ° C. for 10 hours in an air atmosphere. Thus, a temporary sintered body of 208 mm ⁇ 208 mm ⁇ t 73 mm was obtained.
- the preliminary sintered body was heated from room temperature to 750 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 9 hours. Thereafter, the temperature was raised to 920 ° C. over 2 hours at a constant rate, maintained at the same temperature for 2 hours, and then cooled in the furnace to 100 ° C., which was set as the removal temperature, and then taken out.
- the approximate dimensions of the sintered body were 195 mm ⁇ 195 mm ⁇ t64 mm, bulk density 2.90 g / cm 3 (relative density 92.1%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 3 Using the same starting materials as in Example 1 above, similarly to a uniaxial press molding and the same cold isostatic pressing (CIP), pre-sintering at 600 ° C. for 8 hours in an air atmosphere The preliminary sintered body of 206.5 mm ⁇ 207 mm ⁇ t71 mm was obtained. This pre-sintered body was heated from room temperature to 840 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, and kept at the same temperature for 12 hours. Thereafter, the temperature was raised to 1080 ° C. over 2 hours at a constant rate, maintained at the same temperature for 1 hour, and then cooled to 100 ° C., which was set to the removal temperature, and then taken out.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 192 mm ⁇ 192 mm ⁇ t61 mm, the bulk density was 3.00 g / cm 3 (relative density 95.2%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 4 Using the same starting materials as in Example 1 above, the raw materials are filled into a uniaxial press-molding mold, compressed and molded by applying a uniaxial press pressure of 70 MPa, and then cold isostatic pressing (CIP) Using a machine, the molding pressure was set to 40 MPa, and molding was performed to obtain a molded body (dimensions of about 213 mm ⁇ 214 mm ⁇ t 74 mm).
- This molded body was pre-sintered at 600 ° C. for 10 hours in an air atmosphere to obtain a pre-sintered body of 204.5 mm ⁇ 205 mm ⁇ t 70 mm.
- the pre-sintered body was heated from room temperature to 830 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere and held at that temperature for 12 hours. Thereafter, the temperature was raised to 1080 ° C. over 2 hours at a constant rate, kept at the same temperature for 1 hour, then cooled to 100 ° C., which was set to the take-out temperature, and then taken out.
- the approximate dimensions of the sintered body were 190.5 mm ⁇ 191 mm ⁇ t60 mm, the bulk density was 3.07 g / cm 3 (relative density 97.5%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 5 Using the same starting materials as in Example 1 above, pre-sintering was carried out at 580 ° C. for 10 hours in an air atmosphere on a molded body that was similarly subjected to uniaxial press molding and cold isostatic pressing (CIP). As a result, a temporary sintered body of 206 mm ⁇ 206 mm ⁇ t 70.5 mm was obtained. This pre-sintered body was heated from room temperature to 800 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, and kept at the same temperature for 12 hours. Thereafter, the temperature was raised to 920 ° C.
- CIP cold isostatic pressing
- Example 6 Using the same starting materials as in Example 1 above, pre-sintering was carried out in an air atmosphere at 580 ° C. for 7 hours on a compact that was similarly subjected to uniaxial press molding and cold isostatic pressing (CIP). As a result, a temporary sintered body of 207 mm ⁇ 207 mm ⁇ t 71.5 mm was obtained. The pre-sintered body was heated from room temperature to 830 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere and held at that temperature for 12 hours. Thereafter, the temperature was raised to 1000 ° C.
- CIP cold isostatic pressing
- Example 7 Using the same starting materials as in Example 1 above, pre-sintering was carried out at 580 ° C. for 10 hours in an air atmosphere on a molded body that was similarly subjected to uniaxial press molding and cold isostatic pressing (CIP). As a result, a temporary sintered body of 206 mm ⁇ 206 mm ⁇ t 70.5 mm was obtained. This pre-sintered body was heated from room temperature to 840 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, and kept at that temperature for 8 hours. Thereafter, the temperature was raised to 980 ° C.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 193 mm ⁇ 193.5 mm ⁇ t 62.5 mm, bulk density 2.96 g / cm 3 (relative density 94.0%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 8 Using the same starting materials as in Example 1 above, uniaxial press forming and cold isostatic pressing (CIP) were performed, and tentative sintering was performed at 560 ° C. for 8 hours in an air atmosphere. Thus, a temporary sintered body of 207 mm ⁇ 206 mm ⁇ t 70.5 mm was obtained. This pre-sintered body was heated from room temperature to 840 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 5 hours. Thereafter, the temperature was raised to 920 ° C. over 2 hours at a constant rate, maintained at the same temperature for 3 hours, and then cooled in the furnace to 100 ° C., which was set to the removal temperature, and then removed.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 194.5 mm ⁇ 194.5 mm ⁇ t 64 mm, bulk density 2.91 g / cm 3 (relative density 92.4%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 9 Using the same starting materials as in Example 1 above, pre-sintering was carried out at 580 ° C. for 10 hours in an air atmosphere on a molded body that was similarly subjected to uniaxial press molding and cold isostatic pressing (CIP). Thus, a temporary sintered body of 205 mm ⁇ 205 mm ⁇ t 70.5 mm was obtained. This pre-sintered body was heated from room temperature to 840 ° C. at a constant rate over 6 hours in a helium gas atmosphere and held at that temperature for 8 hours. Thereafter, the temperature was raised to 980 ° C.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 192.5 mm ⁇ 192.5 mm ⁇ t62 mm, the bulk density was 3.00 g / cm 3 (relative density 95.2%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 10 Using the same starting materials as in Example 1, uniaxial press molding and cold isostatic pressing (CIP) were performed, and tentative sintering was performed at 560 ° C. for 6 hours in an air atmosphere. , 207 mm ⁇ 207 mm ⁇ t 70.5 mm pre-sintered body was obtained. This pre-sintered body was heated from room temperature to 770 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere and held at that temperature for 10 hours. Thereafter, the temperature was raised to 900 ° C. over 2 hours at a constant rate, maintained at the same temperature for 3 hours, and then cooled to 100 ° C., which was set to the take-out temperature, and then taken out.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 194.5 mm ⁇ 194.5 mm ⁇ t64 mm, bulk density 2.90 g / cm 3 (relative density 92.1%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 11 Using the same starting materials as in Example 1, uniaxial press molding and cold isostatic pressing (CIP) were performed, and calcination was performed at 550 ° C. for 8 hours in an air atmosphere. , A pre-sintered body of 207 mm ⁇ 207 mm ⁇ t 70 mm was obtained. This pre-sintered body was heated from room temperature to 790 ° C. over 6 hours at a constant rate in a nitrogen gas atmosphere and kept at the same temperature for 6 hours. Thereafter, the temperature was raised to 940 ° C. over 2 hours at a constant rate, maintained at the same temperature for 1.5 hours, and then cooled in the furnace to 100 ° C., which was set to the removal temperature, and then taken out.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body were 194.5 mm ⁇ 194.5 mm ⁇ t64 mm, bulk density 2.91 g / cm 3 (relative density 92.4%), and the sintered state was good. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results were all good.
- Example 1 Using the same starting materials as in Example 1 above, uniaxial press forming and cold isostatic pressing (CIP) were performed, followed by temporary sintering at 550 ° C. for 10 hours in an air atmosphere. Thus, a temporary sintered body of 208 mm ⁇ 208 mm ⁇ t 73 mm was obtained.
- the preliminary sintered body was heated from room temperature to 750 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 9 hours. Thereafter, the temperature was raised to 920 ° C. over 2 hours at a constant rate, kept at the same temperature for 2 hours, and then cooled in the furnace to 100 ° C. which was set to the removal temperature, and then taken out.
- the approximate dimensions of the sintered body were 195 mm ⁇ 195 mm ⁇ t64 mm, bulk density 2.90 g / cm 3 (relative density 92.1%), and the sintered state was good.
- the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- Example 2 Using the same starting materials as in Example 1 above, pre-sintering was carried out at 530 ° C. for 5 hours in an air atmosphere on a molded body that was similarly subjected to uniaxial press molding and cold isostatic pressing (CIP). As a result, a temporary sintered body of 209 mm ⁇ 209 mm ⁇ t76 mm was obtained. This pre-sintered body was heated from room temperature to 740 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 4 hours. Thereafter, the temperature was raised to 890 ° C.
- CIP cold isostatic pressing
- the approximate dimensions of the sintered body are 198 mm x 198 mm x t68 mm, bulk density 2.80 g / cm 3 (relative density 88.9%), and the sintered state is clearly porous, which is problematic in handling. It was inconvenient to come.
- the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is so low that it cannot be measured, which also has a problem.
- Example 3 Using the same starting materials as in Example 1 above, uniaxial press forming and cold isostatic pressing (CIP) were performed, followed by temporary sintering at 550 ° C. for 10 hours in an air atmosphere. Thus, a temporary sintered body of 208 mm ⁇ 208 mm ⁇ t 73 mm was obtained.
- the preliminary sintered body was heated from room temperature to 750 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 9 hours. Thereafter, the temperature was raised to 880 ° C. over 2 hours at a constant rate, maintained at the same temperature for 1.5 hours, and then cooled in the furnace to 100 ° C., which was set to the removal temperature, and then taken out.
- the approximate dimensions of the sintered body were 197 mm ⁇ 196 mm ⁇ t 67 mm, and the bulk density was 2.88 g / cm 3 (relative density 91.4%).
- the sintered state was good in appearance, a phenomenon in which the grinding fluid was absorbed in the sintered body was observed at the stage of grinding to finish the sintered body with a grinding machine. Therefore, the microstructure in the sintered body was examined in detail. As a result, it was found that many open pores were formed and sintering was insufficient.
- the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- Example 4 Using the same starting materials as in Example 1 above, a uniaxial press molding and a cold isostatic pressing (CIP) molded body were pre-sintered at 600 ° C. for 10 hours in an air atmosphere. Thus, a temporary sintered body of 208 mm ⁇ 208 mm ⁇ t 73 mm was obtained. This pre-sintered body was heated from room temperature to 840 ° C. at a constant rate over 6 hours in a nitrogen gas atmosphere, and kept at that temperature for 8 hours. Thereafter, the temperature was raised to 1150 ° C.
- CIP cold isostatic pressing
- the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- Example 5 Using the same starting material as in Example 1, this material was filled into a mold (mold size 220 mm ⁇ 220 mm ⁇ H 150 mm) using a uniaxial press, and compressed and molded by applying a uniaxial pressing pressure of 4 MPa. .
- This press-molded body (dimensions: about 220 mm ⁇ 220 mm ⁇ t85 mm) is placed in a thick plastic bag, degassed and sealed, and the molded part of a cold isostatic pressing (CIP) machine (inside dimensions: inner diameter 350 mm ⁇ H120 mm).
- CIP cold isostatic pressing
- isotropic pressurization with a molding pressure of 4 MPa is performed to form a molded body by CIP molding (dimensions of about 218 mm ⁇ 218 mm ⁇ t75 mm ).
- the molded body was pre-sintered at 550 ° C. for 5 hours in an air atmosphere to obtain a pre-sintered body having dimensions of about 211 mm ⁇ 211 mm ⁇ t 73 mm.
- This pre-sintered body was heated from room temperature to 740 ° C. over 6 hours at a constant rate in a nitrogen gas atmosphere and kept at the same temperature for 6 hours. Thereafter, the temperature is raised to 900 ° C. over 2 hours at a constant rate and maintained at the same temperature for 1 hour. Thereafter, the heating is stopped, and natural cooling (furnace cooling) is performed over about 20 hours up to 100 ° C., which is set to the take-out temperature. ) And then removed.
- the bulk density of the sintered body was calculated as 2.86 g / cm 3 (relative density 90.8%) from the approximate dimensions (199 mm ⁇ 199 mm ⁇ t68 mm) and weight, and the sintered state was porous.
- Table 2 the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- This press-molded body (dimensions: about 220 mm ⁇ 220 mm ⁇ t85 mm) is placed in a thick plastic bag, degassed and sealed, and the molded part of a cold isostatic pressing (CIP) machine (inside dimensions: inner diameter 350 mm ⁇ H120 mm).
- CIP cold isostatic pressing
- isotropic pressure is applied at a molding pressure of 20 MPa, and the molded body by CIP molding (dimensions of about 215 mm x 215 mm x t75 mm) ).
- the molded body was pre-sintered at 500 ° C. for 4 hours in an air atmosphere to obtain a pre-sintered body having dimensions of about 211 mm ⁇ 211 mm ⁇ t 72 mm.
- the pre-sintered body was heated from room temperature to 730 ° C. in a nitrogen gas atmosphere at a constant rate over 6 hours and held at the same temperature for 5 hours. Thereafter, the temperature is raised to 900 ° C. over 2 hours at a constant rate and maintained at the same temperature for 1 hour. Thereafter, the heating is stopped, and natural cooling (furnace cooling) is performed over about 20 hours up to 100 ° C., which is set to the take-out temperature. ) And then removed.
- the bulk density of the sintered body was calculated to be 2.85 g / cm 3 (relative density 90.5%) from the approximate dimensions (198 mm ⁇ 198 mm ⁇ t68 mm) and weight, and the sintered state was insufficient and porous. .
- Table 2 the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- Example 7 Using the same starting material as in Example 1, this material was filled into a mold (mold size 220 mm ⁇ 220 mm ⁇ H 150 mm) using a uniaxial press, and compressed and molded by applying a uniaxial pressing pressure of 4 MPa. .
- This press-molded body (dimensions: about 220 mm ⁇ 220 mm ⁇ t85 mm) is placed in a thick plastic bag, degassed and sealed, and the molded part of a cold isostatic pressing (CIP) machine (inside dimensions: inner diameter 350 mm ⁇ H120 mm).
- CIP cold isostatic pressing
- isotropic pressurization with a molding pressure of 4 MPa is performed to form a molded body by CIP molding (dimensions of about 218 mm x 218 mm x t75 mm). ).
- the molded body was pre-sintered at 550 ° C. for 5 hours in an air atmosphere to obtain a pre-sintered body having dimensions of about 211 mm ⁇ 211 mm ⁇ t 72.5 mm.
- This pre-sintered body was heated from room temperature to 740 ° C. over 6 hours at a constant rate in a helium gas atmosphere and held at that temperature for 6 hours. Thereafter, the temperature is raised to 900 ° C. over 2 hours at a constant rate and maintained at the same temperature for 1 hour. Thereafter, the heating is stopped, and natural cooling (furnace cooling) is performed over about 20 hours up to 100 ° C., which is set to the take-out temperature. ) And then removed.
- the bulk density of the sintered body was calculated as 2.89 g / cm 3 (relative density 91.7%) from the approximate dimensions (198 mm ⁇ 198.5 mm ⁇ t 67.5 mm) and weight, and the sintered state was porous. It was. As shown in Table 2, the neutron beam deceleration performance and various characteristics evaluation results show that there are many fast neutron beams that can adversely affect the body in the neutron beam bundle after deceleration. It was not done, and it left a problem. Moreover, the mechanical strength is low, and this also has a problem.
- This raw material was filled into a mold (mold size 220 mm ⁇ 220 mm ⁇ H 150 mm) using a uniaxial press, and compressed and molded by applying a uniaxial press pressure of 10 MPa.
- This press-molded body (dimensions: about 220 mm ⁇ 220 mm ⁇ t85 mm) is placed in a thick plastic bag, degassed and sealed, and the molded part of a cold isostatic pressing (CIP) machine (inside dimensions: inner diameter 350 mm ⁇ H120 mm).
- CIP cold isostatic pressing
- isotropic pressure is applied at a molding pressure of 20 MPa, and the molded body by CIP molding (dimensions of about 215 mm x 215 mm x t75 mm) ).
- the molded body was pre-sintered at 600 ° C. for 6 hours in an air atmosphere to obtain a pre-sintered body having a size of about 208 mm ⁇ 208 mm ⁇ t 72 mm.
- This pre-sintered body was heated from room temperature to 870 ° C. over 6 hours at a constant rate in a nitrogen gas atmosphere and kept at the same temperature for 6 hours. Thereafter, the temperature is raised to 1100 ° C. over 2 hours at a constant rate and held at the same temperature for 1 hour. Thereafter, the heating is stopped, and natural cooling (furnace cooling) is carried out over about 20 hours until the temperature is set to 100 ° C. ) And then removed.
- the bulk density of the CaF 2 sintered body is 3.05 g / cm 3 (relative density 95.9%.
- the true density of CaF 2 is 3.18 g / cm 3 ) based on the approximate dimensions (193 mm ⁇ 193 mm ⁇ t62 mm) and weight. Calculated and the sintered state was good.
- Table 2 a sintered body in a dense sintered state was obtained, and the mechanical strength was sufficient.
- the speed reduction performance with respect to neutron beams has a large amount of remaining fast neutron beams and has left a major problem. This result showed that even if the CaF 2 sintered body was sufficiently dense, the properties as a moderator were inferior to those of the MgF 2 sintered body.
- It can be used as a moderator for suppressing the radiation speed of various types of radiation such as neutrons.
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Abstract
Description
放射線のひとつである中性子線の遮蔽物用としてCaF2単結晶体、フッ化リチウム(LiF)あるいはフッ化アルミニウム(AlF3)単結晶体が稀に使用される程度であった。
放射線は、アルファ(α)線、ベータ(β)線、ガンマ(γ)線、エックス(X)線および中性子線などに分けられ、この順番で物質を透過する能力(透過力)が徐々に大きくなる。
透過力の小さい方から順に、低温中性子(~0.002eV)、熱中性子(~0.025eV)、熱外中性子(~1eV)、低速中性子(0.03~100eV)、中速中性子(0.1~500keV)、高速中性子(500keV以上)に分類される。ただし、括弧内のエネルギー値は厳密なものではなく、中性子線の分類には諸説が存在する。例えば、熱外中性子のエネルギーとして、上記の中速中性子のエネルギー領域に入る40keV以下を記す説などもある。
医療効果の面から見ると、健全な身体組織に悪影響を与える高エネルギーの中性子線を除去し、また、医療効果の少ない極低エネルギーの中性子線(例えば、熱中性子線と低温中性子線)を減らし、同効果の高い中性子線(例えば、中速中性子線の内の低エネルギー部分と熱外中性子線)の割合を高めることにより、望ましい医療用粒子線を形成することができる。
一方、熱中性子線などの極低エネルギーの中性子線を使用すると、深達性が低いが故に、開頭または開腹手術が必要となり、患者への負担が大きなものとなる。
サイクロトロンなどの加速器で発生させた中性子線の大部分は高エネルギー中性子線であり、これを減速材を用い、まずは身体に悪影響を及ぼすレベルの高エネルギー中性子線(例えば、高速中性子線と中速中性子線の内の高エネルギー部分など)を極力除外することである。
BNCTにおいては、高速中性子線などの高エネルギー中性子線を除去し、熱外中性子線を主体とし、熱中性子線を少量含む中性子線を患部に照射することが必要とされている。
加速器から出射されたビームはターゲット(Be)に衝突し、核反応により主として高速中性子線などの高エネルギー中性子線を発生させる。高速中性子線の減速方法として、まずは非弾性散乱断面積の大きいPbやFeなどを使用し、中性子線の減衰を抑えながら減速する。これら2種類の減速材である程度(~1MeV程度)まで減速し、その後、照射場に必要な中性子線エネルギーに応じて減速・最適化してゆく。
この内、ポリエチレンと、LiF含有ポリエチレンとは高エネルギー中性子線の照射場以外への漏洩防止のため、装置外部を覆うように設けられる遮蔽用の減速材である。
その原因は、後半段階の減速材のうち、CaF2の高エネルギー中性子線に対する遮断性能が十分でなく、一部が遮断されずに透過してしまうことにあった。
LiF含有ポリエチレンと同様に、治療室側以外の装置外周の全面を覆うように使用され、装置周囲への高速中性子線の漏洩を防ぐために設置されている。
このため、後半段階のCaF2に代わり、治療に必要とされる中エネルギーレベルの中性子線の減衰を抑えながら、高エネルギー中性子線を遮断し、減速させることが出来る優れた減速材の開発が望まれていた。
MgF2系焼結体には、MgF2焼結体のほか、MgF2-CaF2二元系焼結体、MgF2-LiF二元系焼結体、MgF2-CaF2-LiF三元系焼結体なども含まれる。これまでに中性子線の減速材用としてMgF2が使用されたという報告は見当たらない。ましてや、MgF2焼結体やMgF2-CaF2二元系焼結体をはじめとするMgF2系焼結体が中性子線減速材に採用された例は報告されていない。
MgF2は、理化学辞典によると、融点1248℃、沸点2260℃、密度3.15g/cm3、立方晶系、ルチル構造と称される無色の結晶である。その単結晶体は透明度が高く、おおよそ波長0.2~7μmの広範囲の波長域で高い光透過性が得られることと、バンドギャップが広くレーザー耐性が高いことから主としてエキシマレーザー用窓材として使用されている。また、MgF2はレンズの表面に蒸着されて内部保護や乱反射防止用など、いずれも光学用途に使用されている。
他方、MgF2焼結体は多結晶構造のため、光透過性に乏しく、透明度が低いことから光学用途には不向きである。
単結晶体、焼結体ともにMgF2が光学用途以外に使用されたケースは極めて少なく、以下に記述する耐プラズマ性部材用に焼結体が使用された例が2,3ある程度である。
MgF2単味の焼結体は機械的強度が弱い欠点があるため、Al2O3、AlN、SiC、MgOなどの平均線熱膨張係数がMgF2よりも低い、非アルカリ金属系物質を少なくとも1種混合し、MgF2単味の焼結体の機械的強度が弱い欠点を補う方法が開示されている。このような混合物の焼結体を、上記中性子線の減速材に使用すると、MgF2に混合する非アルカリ金属の影響で、MgF2単味の減速性能と大きく異なることとなり、この種混合物の焼結体では、減速材用途への適応は困難であることが予見された。
BNCTにおいては、上記したように、治療時に有害となる高エネルギー中性子線を極力少なくし、他方、大きな治療効果を得るために、熱外中性子線を主体とし、熱中性子線をわずかに含む中性子線を患部に照射することが重要となる。具体的には、照射時間を1時間程度とした場合に必要とされる熱外及び熱中性子線量の目安は、おおよそ1×109[n/cm2/sec]である。そのための中性子線の発生源である加速器からの出射ビームエネルギーは、中性子線生成のターゲットにベリリウム(Be)を使用する場合、おおよそ5~10MeVが必要と言われている。
加速器から出射されたビームはターゲット(Be)に衝突し、核反応により主として高エネルギー中性子線(高速中性子線)を発生させる。高速中性子線の減速方法としては、まずは非弾性散乱断面積の大きいPbやFeなどで中性子線の減衰を抑えながらある程度まで減速する。ある程度(~1MeV)まで減速された中性子線に対する減速材は、照射場に必要な中性子エネルギー量に応じて最適化させていく。
結晶法で製造した結晶は、一般的に、結晶方位に偏析があり、不純物に関しても偏析を生じやすく、減速材として使用した場合、その部位により減速性能にばらつきを生じ易い。従って、減速材には不向きと考えられる。
そこで、今回は、多結晶法(以下、焼結法と記す)による減速材の製造方法について研究、開発し、本発明を完成するに至った。
(1)減速材としての性能確保のための製品純度の確保
MgF2減速材としての性能の確保のためには、まずは製品純度の確保が重要となる。純度確保のためには、原料レベルでの純度の確保、及び製造過程における不純物混入の阻止が重要と考え、これらを考慮することで減速性能を確保した。
市販品のMgF2原料の純度レベルには2N(99.0%)、3N(99.9%)、4N(99.99%)の3種類があり、予め小規模な試験でこれら3種類の純度の原料を使用し、焼結性の状態を評価した。
原料粒子の微粉化により、焼結過程における粒子間の反応界面を増加させて脱泡の進行を促進し、焼結部位毎の焼結反応の進行を均一化した。
焼結工程を仮焼結と本焼結(本焼結をさらに分割すると効果が増す傾向にある)とに分割することで、仮焼結工程では焼結反応を主として固相間反応による粒成長(以後、固相焼結と記す)によるものとし、本焼結工程では固溶体生成温度域で主として固溶体生成反応による焼結体形成(以後、固溶体焼結と記す)、あるいは溶融体生成反応による焼結体形成(以後、溶融焼結と記す)によるものとした。このことにより、上記(2)の原料の微粉化による効果と相俟って、焼結部位毎の焼結反応の進行を均一化し、焼結体を強固な粒子間結合力を有するものとすることができた。
ここで、固溶体を生成し始める温度域とは、図1に示すMgF2-CaF2二元系状態図における固溶体を生じ始める温度である980℃前後の温度域のことをいう。
このように、不活性ガスとしては、窒素(N2)、ヘリウム(He)、アルゴン(Ar)、ネオン(Ne)が使用され得る。
実施の形態に係る中性子線減速材用に好適なフッ化物焼結体の製造には、高純度(純度99.9wt.%以上)のMgF2粉末を用い、焼結助剤として例えばカルボキシメチルセルロース(CMC)溶液を前記粉末100に対し、0.03~0.5wt.%添加(外掛け)し、混練したものを出発原料とした。
焼結助剤の混合は、焼結助剤二種類をおのおの0~2wt.%の配合比で添加し、前記原料の粉砕と同様に、ポットミル中にボールミル用のボールを充填して一昼夜混練することにより行った。
焼結工程の進行度を表現する用語である“一次凝集過程”、“二次凝集過程”の定義について記述する。“一次凝集過程”とは、焼結の前半段階であり、その初期段階では粒子と粒子との間隔が徐々に狭まり、粒子同士の間の空隙も狭まってくる。さらには、粒子同士の接触部分が太くなり、その間の空隙は小さくなる。ただし、その空隙の大多数は開気孔で周りの雰囲気と通じている。この様な段階までを“一次凝集過程”と称する。
粉砕した原料であるMgF2に、焼結助剤としてCMCを0.2wt.%添加した配合原料に、一軸プレス成形とCIP成形を施し、550℃で6時間の仮焼結を実施した仮焼結体を用いた。いずれも加熱時間を一定の6時間にして焼結温度を600℃から1200℃までの間で、50℃毎にそれぞれ変更させた場合の焼結体の嵩密度を測定した。おおよそ900℃から1100℃の範囲の場合は、2.90g/cm3を超える高密度となったが、850℃以下の焼結温度の場合、逆に1150℃以上の焼結温度の場合はいずれも嵩密度が2.90g/cm3を下回った。それらの焼結体の断面を観察すると800℃以下のものの場合、焼結部分のブリッジ幅が細く、如何にも焼結進行不足と判断でき、850℃ではわずかではあるが開気孔が認められた。1100℃では内部に幾つかの不定形の気泡が見られ、さらに1150℃以上では内部に不定形の気泡が無数に発生したようなポーラスな軽石状の組織となっていた。また、焼結体全体に直径数~10数μmのほぼ真球状の微細な気泡と径10μm以上の不定形の気泡が観察した断面の全面に無数に認められた。この真球状の気泡はその形状から発泡気体、また、この不定形の気泡は同じくその形状から集合気泡であると判断できた。
900℃の場合、5時間以上で良好であり、4時間以下ではやや焼結不足であり、16時間以上では良否判定不能であった。
950℃の場合、5~14時間が良好で、4時間以下ではやや焼結不足となり、15時間以上では良否判定不能であった。
1000℃の場合、5~12時間が良好で、4時間以下ではやや焼結不足となり、14時間以上では発泡が多いものとなった。
1100℃の場合、3~8時間が良好で、10時間以上では発泡が多いものとなった。
1150℃の場合、いずれの保持時間でも発泡が多く見られた。
1200℃の場合、3時間以下では焼結不足となり、4時間以上では、良否判定不能か、溶け過ぎなどの不良なものであった。
最初に、実施例の中で焼結体について行う代表的な特性評価試験方法を説明しておく。
中性子線の減速性能を評価するには、まず、加速器から出射されたビームをターゲットであるBeに衝突させ、核反応により主として高エネルギーの中性子線(高速中性子線)を発生させる。次に、これを前半の減速材としての非弾性散乱断面積の大きいPbとFeとを用い、中性子数の減衰を抑えながらある程度(おおよそ、~1MeV)まで減速する。次に、これを評価したい減速材(後半の減速材)に照射し、減速させたあとの中性子線を調べることにより評価する。中性子線の内容(以下、「中性子束」と称す)の測定は、本発明者らが考案した方法(前記の非特許文献3)に準じて行った。評価する後半の減速材のトータル厚さは320mmの一定とし、減速材の種類はMgF2、CaF2の2種類とした。
さらに、MgF2とCaF2とを2層に重ね合わせたケース(トータル厚さは320mmの一定)の評価も行った。
硬度 = 0.18909 × P/(d)2
ここで、P:荷重(N)、 d:圧痕対角線長さ(mm)
高純度のMgF2原料(平均粒径20μm、純度99.9wt.%以上)を、上記の「発明を実施するための形態」中で説明したポットミルとアルミナ製ボールを用いて粉砕し、高純度のMgF2粉末(平均粒径1.2μm、純度99.9wt.%以上)とした。この粉末に焼結助剤としてカルボキシメチルセルロース(CMC)溶液を前記MgF2粉末100に対し、0.2wt.%の割合で添加し、ポットミルで12時間混合したものを出発原料とした。
この出発原料を一軸プレス機を用いて型枠(型寸法220mm×220mm×H150mm)内に充填し、一軸のプレス圧を10MPa掛けて圧縮、成形した。
この仮焼結体を窒素ガス雰囲気中で室温から830℃まで6時間掛けて一定速度で昇温させ、同温度に6時間保持した。この後、1000℃まで2時間掛けて一定速度で昇温させ、同温度に1時間保持した。この後、加熱を停止し、取り出し温度に設定した100℃まで約20時間かけて自然冷却(炉冷)し、その後、取り出した。
中性子線の減速性能は、比較材であるCaF2と比べて熱外中性子線量の減少はわずかに少ない程度であったが、患者に悪影響を与える可能性が高い高速中性子線量は約1/4に低減され、優れた減速性能を有するものであることが分かった。
また、同じく表2に示したように、その他の機械的強度は問題のない良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で550℃、10時間の仮焼結を実施し、208mm×208mm×t73mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から750℃まで6時間掛けて一定速度で昇温させ、同温度に9時間保持した。この後、920℃まで2時間掛けて一定速度で昇温させ、同温度に2時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、195mm×195mm×t64mm、嵩密度2.90g/ cm3(相対密度92.1%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、同じ冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で600℃、8時間の仮焼結を実施し、206.5mm×207mm×t71mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から840℃まで6時間掛けて一定速度で昇温させ、同温度に12時間保持した。その後、1080℃まで2時間掛けて一定速度で昇温させ、同温度に1時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、192mm×192mm×t61mm、嵩密度3.00g/ cm3(相対密度95.2%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したように、いずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、この原料を一軸プレス成形の型枠内に充填し、一軸プレス圧を70MPa掛けて圧縮、成形し、その後、冷間等方圧力成形(CIP)機を用いて成形圧を40MPaに設定して成形を行い、成形体(寸法約213mm×214mm×t74mm)を得た。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で580℃、10時間の仮焼結を実施し、206mm×206mm×t70.5mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から800℃まで6時間掛けて一定速度で昇温させ、同温度に12時間保持した。その後、920℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、191.0mm×191.5mm×t62mm、嵩密度3.02g/ cm3(相対密度95.9%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で580℃、7時間の仮焼結を実施し、207mm×207mm×t71.5mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から830℃まで6時間掛けて一定速度で昇温させ、同温度に12時間保持した。その後、1000℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、192.5mm×192.5mm×t63mm、嵩密度2.99g/ cm3(相対密度94.9%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で580℃、10時間の仮焼結を実施し、206mm×206mm×t70.5mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から840℃まで6時間掛けて一定速度で昇温させ、同温度に8時間保持した。その後、980℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、193mm×193.5mm×t62.5mm、嵩密度2.96g/ cm3(相対密度94.0%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で560℃、8時間の仮焼結を実施し、207mm×206mm×t70.5mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から840℃まで6時間掛けて一定速度で昇温させ、同温度に5時間保持した。その後、920℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、194.5mm×194.5mm×t64mm、嵩密度2.91g/ cm3(相対密度92.4%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で580℃、10時間の仮焼結を実施し、205mm×205mm×t70.5mmの仮焼結体を得た。この仮焼結体をヘリウムガス雰囲気中で室温から840℃まで6時間掛けて一定速度で昇温させ、同温度に8時間保持した。その後、980℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、192.5mm×192.5mm×t62mm、嵩密度3.00g/ cm3(相対密度95.2%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で560℃、6時間の仮焼結を実施し、207mm×207mm×t70.5mmの仮焼結体を得た。
この仮焼結体を窒素ガス雰囲気中で室温から770℃まで6時間掛けて一定速度で昇温させ、同温度に10時間保持した。その後、900℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、194.5mm×194.5mm×t64mm、嵩密度2.90g/ cm3(相対密度92.1%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で550℃、8時間の仮焼結を実施し、207mm×207mm×t70mmの仮焼結体を得た。
この仮焼結体を窒素ガス雰囲気中で室温から790℃まで6時間掛けて一定速度で昇温させ、同温度に6時間保持した。その後、940℃まで2時間掛けて一定速度で昇温させ、同温度に1.5時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、194.5mm×194.5mm×t64mm、嵩密度2.91g/ cm3(相対密度92.4%)であり、焼結状態は良好であった。
中性子線の減速性能および各種特性評価結果は、表2に示したようにいずれも良好なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で550℃、10時間の仮焼結を実施し、208mm×208mm×t73mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から750℃まで6時間掛けて一定速度で昇温させ、同温度に9時間保持した。その後、920℃まで2時間掛けて一定速度で昇温させ、同温度に2時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、195mm×195mm×t64mm、嵩密度2.90g/ cm3(相対密度92.1%)であり、焼結状態は良好であった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で530℃、5時間の仮焼結を実施し、209mm×209mm×t76mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から740℃まで6時間掛けて一定速度で昇温させ、同温度に4時間保持した。その後、890℃まで2時間掛けて一定速度で昇温させ、同温度に2時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、198mm×198mm×t68mm、嵩密度2.80g/ cm3(相対密度88.9%)であり、焼結状態は明らかにポーラスなものになっており、取扱いに問題を来す不都合なものであった。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で550℃、10時間の仮焼結を実施し、208mm×208mm×t73mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から750℃まで6時間掛けて一定速度で昇温させ、同温度に9時間保持した。その後、880℃まで2時間掛けて一定速度で昇温させ、同温度に1.5時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、197mm×196mm×t67mm、嵩密度2.88g/ cm3(相対密度91.4%)であった。焼結状態は外観上は良好であったが、焼結体を研削機で仕上げる研削加工する段階において、焼結体内に研削液を吸収する現象が認められた。そのため、焼結体内のミクロ組織を詳細に調べた。その結果、開気孔が多数出来ており、焼結が不十分であることが判明した。
上記実施例1の場合と同じ出発原料を用い、同様に一軸プレス成形、冷間等方圧力成形(CIP)を施した成形体に、大気雰囲気中で600℃、10時間の仮焼結を実施し、208mm×208mm×t73mmの仮焼結体を得た。この仮焼結体を窒素ガス雰囲気中で室温から840℃まで6時間掛けて一定速度で昇温させ、同温度に8時間保持した。その後、1150℃まで2時間掛けて一定速度で昇温させ、同温度に3時間保持し、この後、取り出し温度に設定した100℃まで炉冷し、その後、取り出した。焼結体の概略寸法は、196.5mm×197mm×t68mm、嵩密度2.87g/ cm3(相対密度91.1%)であった。焼結状態はポーラスであった。焼結体内のミクロ組織を調べたところ、組織が疎になっており、激しい発泡により多孔質化した跡が観察された。
上記実施例1の場合と同じ出発原料を用い、この原料を一軸プレス機を用いて型枠(型寸法220mm×220mm×H150mm)内に充填し、一軸のプレス圧を4MPa掛けて圧縮、成形した。
このプレス成形体(寸法約220mm×220mm×t85mm)を厚手のビニール袋内に入れ、脱気、封入したものを冷間等方加圧成形(CIP)機の成形部(内寸法:内径350mm×H120mm)に装填した。このプレス成形体が入った前記ビニール袋と前記CIP機成形部との隙間に上水を満たしてから成形圧4MPaの等方加圧を行い、CIP成形による成形体(寸法約218mm×218mm×t75mm)とした。
中性子線の減速性能および各種特性評価結果は、表2に示したように、減速後の中性子線束において身体に悪影響を及ぼす恐れがある高速中性子線が多く残存しており、十分な減速効果が得られておらず、問題を残したものとなっていた。しかも、機械的強度が低く、このことも問題がある結果となっていた。
上記実施例1の場合と同じ出発原料を用い、この原料を一軸プレス機を用いて型枠(型寸法220mm×220mm×H150mm)内に充填し、一軸のプレス圧を10MPa掛けて圧縮、成形した。
中性子線の減速性能および各種特性評価結果は、表2に示したように、減速後の中性子線束において身体に悪影響を及ぼす恐れがある高速中性子線が多く残存しており、十分な減速効果が得られておらず、問題を残したものとなっていた。しかも、機械的強度が低く、このことも問題がある結果となっていた。
上記実施例1の場合と同じ出発原料を用い、この原料を一軸プレス機を用いて型枠(型寸法220mm×220mm×H150mm)内に充填し、一軸のプレス圧を4MPa掛けて圧縮、成形した。
このプレス成形体(寸法約220mm×220mm×t85mm)を厚手のビニール袋内に入れ、脱気、封入したものを冷間等方加圧成形(CIP)機の成形部(内寸法:内径350mm×H120mm)に装填した。このプレス成形体が入った前記ビニール袋と前記CIP機成形部との隙間に上水を満たしてから成形圧4MPaの等方加圧を行い、CIP成形による成形体(寸法約218mm×218mm×t75mm)とした。
中性子線の減速性能および各種特性評価結果は、表2に示したように、減速後の中性子線束において身体に悪影響を及ぼす恐れがある高速中性子線が多く残存しており、十分な減速効果が得られておらず、問題を残したものとなっていた。しかも、機械的強度が低く、このことも問題がある結果となっていた。
高純度のCaF2原料(平均粒径20μm、純度99.9wt.%以上)を、上記ポットミルとアルミナ製ボールを用いて粉砕し、高純度のCaF2粉末(平均粒径1.4μm、純度99.9wt.%以上)とした。この粉末に焼結助剤としてカルボキシメチルセルロース(CMC)溶液を前記CaF2粉末100に対し、0.2wt.%の割合で添加し、ポットミルで12時間混合したものを出発原料とした。
このプレス成形体(寸法約220mm×220mm×t85mm)を厚手のビニール袋内に入れ、脱気、封入したものを冷間等方加圧成形(CIP)機の成形部(内寸法:内径350mm×H120mm)に装填した。このプレス成形体が入った前記ビニール袋と前記CIP機成形部との隙間に上水を満たしてから成形圧20MPaの等方加圧を行い、CIP成形による成形体(寸法約215mm×215mm×t75mm)とした。
評価結果は、表2に示したように緻密な焼結状態の焼結体が得られており、機械的強度は十分なものであった。しかしながら、中性子線に対する減速性能は高速中性子線の残存量が多く、大きな問題を残すものとなっていた。この結果は、CaF2焼結体においては十分に緻密なものであっても、減速材としての特性は、MgF2焼結体に比べると劣るものであることを示していた。
Claims (4)
- 緻密な多結晶構造のMgF2からなり、嵩密度が2.90g/cm3以上であることを特徴とする中性子線減速材用フッ化物焼結体。
- 曲げ強度が10MPa以上、ビッカース硬度が71以上の機械的強度を有することを特徴とする請求項1記載の中性子線減速材用フッ化物焼結体。
- MgF2 焼結体からなる中性子線減速材用フッ化物焼結体の製造方法であって、
高純度のMgF2原料を微粉砕し、焼結助剤を0.1~1wt.%添加して混合する工程、
一軸プレス機を用いて成形圧5MPa以上で成形する工程、
冷間等方加圧成形(CIP)機を用いて成形圧5MPa以上で成形する工程、
大気雰囲気中、550~600℃の温度範囲、4~10時間の条件で仮焼結させる工程、
不活性ガス雰囲気中で、750~840℃の温度範囲で、5~12時間加熱する工程、
前工程と同じ雰囲気中で900~1100℃の温度範囲で、0.5~3時間加熱して緻密な構造のMgF2焼結体を形成する本焼結工程、
を含むことを特徴とする請求項1記載の中性子線減速材用フッ化物焼結体の製造方法。 - 前記本焼結工程における不活性ガス雰囲気が、窒素、ヘリウム、アルゴン、及びネオンの中から選択される1種類のガス、または複数種類のガスを混合させたものからなることを特徴とする請求項3記載の中性子線減速材用フッ化物焼結体の製造方法。
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WO2017014230A1 (ja) * | 2015-07-21 | 2017-01-26 | 日本軽金属株式会社 | フッ化マグネシウム焼結体、フッ化マグネシウム焼結体の製造方法、中性子モデレータ及び中性子モデレータの製造方法 |
US10343951B2 (en) | 2015-07-21 | 2019-07-09 | Nippon Light Metal Company, Ltd. | Magnesium fluoride sintered compact, method for manufacturing magnesium fluoride sintered compact, neutron moderator, and method for manufacturing neutron moderator |
CN107848895A (zh) * | 2015-07-21 | 2018-03-27 | 日本轻金属株式会社 | 氟化镁烧结体、氟化镁烧结体的制造方法、中子减速剂和中子减速剂的制造方法 |
CN107848895B (zh) * | 2015-07-21 | 2021-04-09 | 日本轻金属株式会社 | 氟化镁烧结体、氟化镁烧结体的制造方法、中子减速剂和中子减速剂的制造方法 |
JP2022164529A (ja) * | 2021-04-16 | 2022-10-27 | 国立大学法人 筑波大学 | 放射線遮蔽材用焼結体、放射線遮蔽材及びその製造方法 |
JP7165339B2 (ja) | 2021-04-16 | 2022-11-04 | 国立大学法人 筑波大学 | 放射線遮蔽材用焼結体、放射線遮蔽材及びその製造方法 |
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US10961160B2 (en) | 2021-03-30 |
CN104640824B (zh) | 2018-03-20 |
TWI496744B (zh) | 2015-08-21 |
EP3214058A1 (en) | 2017-09-06 |
JPWO2015005006A1 (ja) | 2017-03-02 |
JP5813258B2 (ja) | 2015-11-17 |
EP2865658A4 (en) | 2016-03-30 |
US20160002116A1 (en) | 2016-01-07 |
CN104640824A (zh) | 2015-05-20 |
EP3214058B1 (en) | 2021-04-07 |
CN107082642A (zh) | 2017-08-22 |
EP2865658A1 (en) | 2015-04-29 |
CN107082642B (zh) | 2021-11-16 |
TW201502073A (zh) | 2015-01-16 |
EP2865658B1 (en) | 2017-10-11 |
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