CN114262223A - In + Ta co-doped TiO2Mega dielectric ceramic, preparation method and application thereof - Google Patents
In + Ta co-doped TiO2Mega dielectric ceramic, preparation method and application thereof Download PDFInfo
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Abstract
The invention relates to the technical field of dielectric ceramic preparation, In particular to an In + Ta co-doped TiO2The composition (In) is prepared by flash firing method0.5Ta0.5)0.05Ti0.95O2Of TiO 22During fast sintering, the sample is set inside a tubular furnace and two ends of the sample are connected to DC power supply to provide electric field and heated at the heating rate of 10 deg.c/minWhen the temperature of the sample reaches 1200 ℃, keeping the temperature for 5min, applying an electric field of 450-600V/cm, setting the initial preset current to be 0.5A, increasing the current by 0.1A every 5min until the limit current reaches 0.7A when the electric field is applied, then closing a direct current power supply, and cooling the sample to room temperature to obtain the In + Ta co-doped TiO2Compared with conventional sintering, the rapid sintering method can shorten the processing time by 15 times, reduce the sintering temperature by 200 ℃, and ensure that the grain size is obviously smaller and the structure is more uniform.
Description
Technical Field
The invention relates to the technical field of dielectric ceramic preparation, In particular to an In + Ta co-doped TiO2Giant dielectric ceramic, preparation method and application thereof.
Background
The development of a high-efficiency high-performance giant dielectric material with better frequency and frequency stability, giant dielectric constant and low dielectric loss can realize the miniaturization and high-energy density storage application of a microelectronic device, and the (In + Nb) co-doped TiO2Has high dielectric constant (more than or equal to 10)4) And low dielectric loss (less than or equal to 0.05), and good temperature and frequency stability. The huge dielectric Constant (CP) performance results from the electron pinned defective dipole Effect (EPDD) in which electrons generated by Nb doping are bound, which is a couple of co-doped TiO2Plays an important role in the giant dielectric Constant (CP). The good dielectric properties of such materials have stimulated a great deal of research into related material systems. The co-doped TiO is prepared by the traditional sintering method2Giant dielectric ceramic. However, on the one hand, the high temperature (. gtoreq.1400 ℃) and the long holding time(s) of conventional sintering>4h) Resulting in a large energy consumption. On the other hand, a longer sintering time at a high temperature may cause abnormal growth of grains, resulting in deterioration of final electrical properties of the sintered sample. Therefore, it is necessary to find a new sintering method to reduce the sintering temperature and shorten the processing time.
Flash Firing (FS) is a novel and promising sintering method that can reduce the sintering temperature of 3YSZ ceramics from 1450 ℃ to 850 ℃ and greatly reduce the sintering time to around 5 s. It has been extensively studied in a number of materials, including ionic conductors (3YSZ), CeO2Electric conductor (TiO)2) Ferroelectric (BaTiO)3) Semiconductors and pressure sensitive ceramics (ZnO), perovskites (SrTiO)3) And an insulator (Al)2O3And Y2O3) Since flash firing has the advantage of shortening the sintering time (from a few hours to a few seconds) and reducing the furnace temperature. Although flash firing is used in many materials, the mechanism is still controversial. To explain the phenomenon of flash firing, three main mechanisms have been proposed, including joule heating, frenck nucleation, and electrochemical reactions induced by the application of voltage.
Rutile type TiO2Is a mixed conductor and is an ideal research system because of its excellent temperature stability at high temperatures. In recent years, there have been some references to rutile TiO2Research report of flash firing, based on different diffusion mechanisms of different electric fields, on rapidly sintering TiO2In ceramics, ionic conductivity dominates under low electric fields and conductivity dominates under high electric fields.
King et al in The effect of The aerosol on The flash-sintering of nanoscopic titanium dioxide ceramics (script Material 199 (2021)) 113894 propose an applied electric field of 400V/cm, atmosphere (in air and argon) to titanium dioxide (TiO) (TiO 2)2) The effect of the flash-burning of the nanopowder, the effect of sintering conditions (including electric field, current density limitations and retention time) on the microstructure and defect structure of flash-burned titanium dioxide as proposed by Wang et al in Staged micro structural study of flash-burned titanium dioxide (Materialia 8(2019)100451), and the effect of firing conditions on flash sintering of TiO in Effects of phase and doping on flash sintering of TiO, the Effects of sintering conditions in Effect of phase and sintering of flash sintering of TiO, are proposed by Wen et al2(Journal of the Ceramic Society of Japan 124(2016)296-300) proposes different doping (V or N) and initial powder phases (anatase and rutile initial powders) for TiO2Influence of fast sintering of ceramics, however, co-doped TiO prepared by flash firing2Of giant dielectric ceramicsPreparation and electrical properties have been rarely studied. On the one hand, rapid sintering can refine grains and reduce dielectric loss. In addition, it can form defects and make an electron pinning defective dipole cluster (EPDD) more easily formed, thereby improving dielectric properties. Therefore, flash firing is used to prepare co-doped TiO2Giant dielectric ceramics are of great significance.
In view of the above-mentioned drawbacks, the inventors of the present invention have finally obtained the present invention through a long period of research and practice.
Disclosure of Invention
The invention aims to solve the problem that the final electrical property of a sintered sample is poor due to abnormal growth of crystal grains caused by long sintering time at high temperature, and provides an In + Ta co-doped TiO2Giant dielectric ceramic, preparation method and application thereof.
In order to realize the purpose, the invention discloses an In + Ta co-doped TiO2The preparation method of the giant dielectric ceramic comprises the following steps:
s1: mixing oxide powders in ZrO2Ball-milling the medium in ethanol for 24 hours, and then drying the medium in an oven at 80 ℃ for 24 hours to obtain pre-calcined powder;
s2: calcining the pre-calcined powder obtained in the step S1 to obtain a calcined powder;
s3: carrying out secondary ball milling on the calcined powder obtained in the step S2 under the same ball milling condition as the pre-calcined powder, and then drying for 24h at 80 ℃;
s4: adding a binder to the dried powder obtained in step S3, and uniaxially pressing the mixed powder into a disk under a pressure of 310 MPa;
s5: baking the disc obtained in the step S4 at 650 ℃ for 2h to remove the adhesive;
s6: placing the sample obtained in the step S5 in a tube furnace, wherein a direct current power supply is arranged at two ends of the sample to provide an electric field, and the sample is heated at a heating rate of 10 ℃/min;
s7: when the temperature of the sample reaches 1200 ℃, keeping for 5min, then applying an electric field of 450V/cm-600V/cm, initially presetting the current to be 0.5A, when applying the electric field, increasing the current by 0.1A every 5min until the limiting current reaches 0.7A, then closing the direct current power supply, and cooling the sample to the room temperature.
In the step S2, the calcining temperature is 1000 ℃, and the calcining time is 5 h.
The binder in step S4 was 5 wt.% polyvinyl alcohol.
And in the step S4, the disc is a green disc with the thickness of 2mm and the diameter of 7 mm.
The temperature increase rate in step S5 is 2 ℃/min.
The invention also discloses the In + Ta co-doped TiO prepared by the preparation method2Mega-based dielectric ceramic and In + Ta co-doped TiO2Use of a base dielectric ceramic in a microelectronic device.
Compared with the prior art, the invention has the beneficial effects that: the In + Ta co-doped TiO of the invention2The preparation method of the base dielectric ceramic can shorten the processing time by 15 times, reduce the sintering temperature by 200 ℃, ensure that the grain size is obviously smaller and the tissue is more uniform, ensure that the rapid sintering has the electrical property similar to the traditional sintering, ensure that the rapid sintering sample has the best dielectric property when the electric field is 550V/cm, wherein epsilonr>104And tan delta 0.36 at 103Hz, while two dielectric anomalies were observed, the dielectric anomaly near room temperature being attributed to Ti3+And Ti4+Which contributes to their large dielectric properties.
Drawings
FIG. 1 shows the electric field (In)0.5Ta0.5)0.05Ti0.95O2Changes in electric field and current of the sample with time, (a) FS450, (b) FS500, (c) FS550, (d) FS 600;
FIG. 2 is (In)0.5Ta0.5)0.05Ti0.95O2The power density of the sample under the action of the electric field and the estimated temperature of the sample along with the time, (a) FS450, (b) FS500, (c) FS550, (d) FS 600;
FIG. 3 is XRD spectra of a conventional solid phase sintered sample and a flash sintered sample under different electric fields;
FIG. 4 shows conventional sintering and flash sintering (In)0.5Ta0.5)0.05Ti0.95O2SEM images of samples under different electric fields, (a) CS1400, (b) FS450, (c) FS500, (d) FS550, (e) FS 600;
FIG. 5 shows the dielectric properties at room temperature of ceramic samples in different sintering modes, (a) dielectric constant (. epsilon.r), (b) dielectric loss (tan. delta.);
FIG. 6 shows (In)0.5Ta0.5)0.05Ti0.95O2The dielectric properties of the ceramic sample under different sintering modes and flash sintering parameters at room temperature of 1 kHz;
FIG. 7 shows (In) at different frequencies0.5Ta0.5)0.05Ti0.95O2Changes in dielectric constant and dielectric loss with temperature of ceramic samples, (a) CS1400, (b) FS 550;
FIG. 8 is a plot of dielectric loss and dielectric modulus as a function of temperature for FS550 ceramic samples at different frequencies, showing an Arrhenius fit for this component, (a) tan δ, (b) M ";
FIG. 9 shows conventional sintering and flash sintering (In) under different electric fields0.5Ta0.5)0.05Ti0.95O2E-J curves of the samples.
Detailed Description
The above and further features and advantages of the present invention are described in more detail below with reference to the accompanying drawings.
First, disc preparation
Prepared by flash firing the composition (In)0.5Ta0.5)0.05Ti0.95O2Of TiO 22A giant dielectric ceramic. Mixing the oxide powders in a mixture containing ZrO2The medium was ball milled in ethanol for 24h and then dried in an oven at 80 ℃ for 24 h. The dried sample was calcined at 1000 ℃ for 5h to reduce the activity of the powder and remove internal moisture and impurities. The calcined powder was ball milled a second time under the same conditions as the pre-calcined powder to increase the activity of the powder. Then dried again at 80 ℃ for 24 h. The dried powder was weighed, poured slowly into an agate mortar, and then the binder (5 wt.% polyvinyl alcohol (PVA)) was added with a rubber-tipped dropper. Uniaxially pressing the mixed powder to a thickness of-310 MPaA disc of 2mm and 7mm diameter. The sample was baked at 650 ℃ for 2h at a rate of 2 ℃/min to remove the binder.
Second, sintering process
Preparing TiO by respectively adopting flash firing and conventional sintering processes2A giant dielectric ceramic. During conventional sintering, the green body was held at 1400 ℃ for 4h, and the sample was labeled CS 1400. In the flash burning process, a sample is placed in a tube furnace, and two ends of the sample are connected with a direct current power supply to provide an electric field. The cylindrical blank was heated in a tube furnace at a heating rate of 10 deg.C/min. When the temperature of the sample reaches 1200 ℃, keeping for 5min and then applying an electric field of 450V/cm to 600V/cm. The initial preset current is set to 0.5A to reduce power consumption and avoid thermal budget during flash. When the electric field is applied, the current increases by 0.1A (limiting current 0.7A) every 5 min. Finally, the dc power supply was turned off and the sample was cooled to room temperature. The samples prepared by flash firing at different electric fields of 450V/cm to 600V/cm are labeled FS450, FS500, FS550 and FS600, respectively.
Third, characterize
Measurement of Co-doped TiO by Archimedes method2Relative density of the giant dielectric ceramic. And coating silver paste on the surface of the sintered cylindrical sample, and keeping the surface at 650 ℃ for 20min to form a silver electrode. Prepared (In)0.5Ta0.5)0.05Ti0.95O2The phase analysis of the ceramics was carried out using an X-ray diffractometer (D8 Advanced, Germany) manufactured by Bruck. The prepared ceramics were subjected to microscopic morphology analysis using a scanning electron microscope (Nano SEM430, usa) manufactured by FEI corporation. Non-ohmic characteristics (threshold field (V)T) Leakage current (I)L) And non-linearity (α)) and dielectric properties were measured using a piezo-resistive dc parametric meter (CJP CJ1001, CHINA) and Agilent HP4294A impedance analyzer, respectively.
Fourthly, the result
FIG. 1 shows (In)0.5Ta0.5)0.05Ti0.95O2The ceramic is subjected to heat preservation for 5min after the samples reach constant furnace temperature in different electric fields (450V/cm, 500V/cm, 550V/cm and 600V/cm), so that the samples are heated uniformly in the sintering process. Notice the first stage of flash under different electric fieldsThe interval is almost 0. This indicates that the electric field has a large influence on the time of the first stage of flash. The purpose of increasing the current in a gradient way is to ensure that the sample is sintered uniformly and avoid the phenomenon of local sintering of the sample. After different electric fields (450V/cm, 500V/cm, 550V/cm, 600V/cm) are applied, the electric fields drop sharply to 165V/cm, 189V/cm, 306V/cm, 361V/cm. When the preset current is reached, the electric field is maintained at about 80V/cm.
Figure 2 shows the power density of the flash-burned samples at different electric fields and the estimated sample temperature as a function of time. Notably, the power density of the sample is consistent with the predicted temperature trend. Flash sintering has two distinct characteristics: the sintering time is short, and the temperature rise speed is high. Therefore, it is very important to measure the temperature of the sample. The black body radiation model (BBR) was used to predict the temperature of the samples, ignoring the loss of heat conduction and convection. The temperature of the sample at different electric fields can be calculated from the following formula (1):
in the formula T0Is the furnace temperature, and the unit is K; w is the electric energy dissipated by the sample and the unit is W; a is the sample surface area m2(ii) a Zeta is the Stefan-Boltzmann constant with a value of 5.67X 10-8 W.m-2·K-4. The equation shows that the emissivity of the sample is 1. Most oxide ceramics have emissivity greater than 0.9, which results in a calculated sample temperature that is higher than the sample temperature calculated using true emissivity. Generally, the calculated sample temperature will be 100-200 ℃ higher.
The results of flash firing are summarized in table 1.
Table 1 summary of sintering results
When the ceramic samples with electric fields of 450V/cm, 500V/cm, 550V/cm and 600V/cm are subjected to flash firing, the corresponding peak power densities are respectively 0.19W/mm3、0.22W/mm3、0.36W/mm3And 0.42W/mm3. It can be seen that as the electric field increases, the corresponding peak power density of the ceramic sample also increases. When flash firing occurs, ceramic samples with electric fields of 450V/cm, 500V/cm, 550V/cm and 600V/cm correspond to predicted temperatures of 1666 ℃, 1700 ℃, 1804 ℃ and 1848 ℃ of the sample, respectively. The predicted sample temperature increases with increasing electric field. This is consistent with the trend of peak power density with electric field. The relative density of the ceramic samples increased from 50% to 91.8%, 94.4%, 95.2%, 92.4% of the flash sintered samples, respectively. It is clear that as the electric field increases, the relative density increases and then decreases. The low density of the sample with an electric field of 600V/cm may be due to the fact that the furnace temperature is high during flash firing, the sample is not uniformly sintered and is easy to crack. The FS550 sample had a higher relative density relative to the conventional sintered sample, as shown in table 1.
Fig. 3 shows XRD spectra of the conventional sintered sample and the flash sintered sample at different electric fields. All samples exhibited the typical rutile structure (PDF # 76-0320). In terms of the ionic radii of In and Ta, the ionic radii of Ti are close to those of In
The mismatch ratio principle is satisfied, and In and Ta can replace the position of Ti to form a substitutional solid solution. Meanwhile, the formation of the solid solution can promote the densification process of the ceramic, and the electron pinning defect dipole model (EPDD) is satisfied. However, except for rutile TiO2In addition to the diffraction peaks, additional diffraction peaks were detected at about 38 ° in FS500, FS550 and FS600 samples, identified as Ta2And O a second phase. May be the result of instability in the flash process.
Figure 4 shows SEM micrographs of ceramic samples prepared by different sintering methods. The average grain size of the ceramic samples was measured using a linear intercept method. The average grain sizes of the CS1400, FS450, FS500, FS550 and FS600 ceramic samples were 14.99 μm, 3.28 μm, 1.31 μm, 1.47 μm and 1.32 μm, respectively (see Table 1). Apparently, codoped TiO prepared by conventional sintering2The ceramic is used for a long timeAbnormal growth easily occurs in high-temperature sintering. The reduction in average grain size under the influence of the electric field may be due to the influence of interfacial energy. On the one hand, the energy caused by entropy is reduced because the grain boundary has a certain degree of local heating; meanwhile, this is because of the interaction between the electric field and the boundary charge. Therefore, rapid sintering may play an important role in refining grains, as compared to conventional sintering.
Fig. 5 shows the dielectric properties of ceramic samples at room temperature under different sintering methods. As shown in fig. 5(a), the samples prepared by rapid sintering achieved large dielectric constants over a wide frequency range. The best electrical properties were prepared by flash firing at an electric field of 550V/cm, with a dielectric constant of 10319, a dielectric loss of 0.36 and a frequency of 1 kHz. The dielectric constant gradually decreases as the frequency increases. FS600 samples at 104～106The frequency range in Hz shows a gradient dip, corresponding to the dielectric loss peak in fig. 5(b), which indicates a relaxation process related to space charge polarization. The dielectric loss peak of the FS600 ceramic sample shifted towards high frequencies compared to conventional sintering, indicating that application of an electric field results in a reduction of the polarization response time. The FS450 sample had a lower dielectric constant, which may be due to lower density and lower electric field. In general, the ceramic sample prepared by flash firing can still maintain good electrical properties at lower temperature and shorter time, and is codoped with TiO2The giant dielectric ceramic opens up a new preparation technology.
FIG. 6 shows (In)0.5Ta0.5)0.05Ti0.95O2The dielectric constant and dielectric loss of the giant dielectric ceramic at 1 kHz. As can be seen, the ceramic samples obtained by both conventional sintering and flash firing achieved enormous dielectric constants. The ceramic sample prepared by conventional sintering has high dielectric constant, but the dielectric loss is high, so that the ceramic sample is difficult to be applied to practical production. Ceramic samples prepared by flash firing have an optimum dielectric loss compared to conventional sintering, with a value of about 0.36 being obtained at FS550, which may be associated with having a finer grain size. For the flash-fired sample, the dielectric constant of the ceramic sample gradually increased with increasing electric field, at FS550 the sampleThe best dielectric properties are obtained, wherein epsilonr10319 and tan δ ≈ 0.36.
FIG. 7 shows (In) at different frequencies0.5Ta0.5)0.05Ti0.95O2The dielectric constant and dielectric loss of the ceramic samples change with temperature. As can be seen, the dielectric constant increases with increasing temperature, which may be due to increased thermal motion of space charge at the interface at high temperatures. The greater the space charge content, the greater the probability of crossing the crystal barrier, resulting in a higher dielectric constant. In addition, the dielectric constant gradually decreases with increasing frequency, which may be caused by polarization attenuation. As shown in fig. 7(a), the dielectric loss peak shifts to higher temperatures with increasing frequency in the temperature range of 20-250 ℃ for samples prepared by conventional sintering, indicating the presence of thermal activation behavior. In contrast to conventional sintering, the FS550 sample did not exhibit significant thermal activation in this temperature range, which may be related to the sintering process. In general, TiO prepared using flash firing technology2The gigantic dielectric ceramic also has good dielectric temperature stability.
Fig. 8(a) shows the variation of dielectric loss with temperature at different frequencies. In all frequency ranges, the dielectric loss gradually increases with temperature. Electrical modulus is generally considered to be a more effective form of defect relaxation in polycrystalline ceramics. (In)0.5Ta0.5)0.05Ti0.95O2The imaginary part of the complex modulus of the ceramic sample is shown in FIG. 8 (b). The complex modulus can be expressed as:
wherein i, ε ', M ' and M ' are each
Complex permittivity, real and imaginary parts of complex permittivity, and real and imaginary parts of complex modulus. The dielectric relaxation peak can be clearly seen in the figure. The kinetic energy can be calculated from the Arrhenius formula:
wherein f, f0、kB、EaAnd T is the frequency value of the relaxation peak, the pre-factor, the Boltzmann constant, the activity performance, and the Kelvin temperature. The activity energy calculated by this equation was 0.453eV, similar to the previous results. The relaxation process can be described by Maxwell-Wagner relaxation, which may be described by Ti4+And Ti3+By charge transfer therebetween.
FIG. 9 shows the E-J curves of ceramic samples under different sintering methods. It is clear that all samples have piezoresistive properties, but the nonlinear coefficients of these samples are small. The nonlinear coefficient is a parameter of the voltage dependent ceramic which is an important performance of the voltage dependent resistor. Mainly, when the potential gradient and the leakage current are close, the larger the nonlinear coefficient is, the better the performance of the piezoresistor is. The threshold voltage, leakage current and nonlinear coefficient of the ceramic samples prepared by the different sintering methods are shown in table 2.
TABLE 2 varistor Properties of ceramic samples prepared by different sintering methods
| VT(V/mm) | IL(μA) | α | |
| FS450 | 60.96 | 451 | 2.7 |
| FS500 | 15.63 | 401 | 3.2 |
| FS550 | 17.16 | 281 | 3.4 |
| FS600 | 7.00 | 477 | 2.5 |
| CS1400 | 32.27 | 499 | 2.1 |
Furthermore, the samples prepared by flash firing have higher nonlinear coefficients than those prepared by conventional sintering, which may be due to the generation of grain boundary barriers under doping conditions. In the flash-firing process, the nonlinear coefficient of the sample increases and then decreases with increasing electric field. In contrast, the leakage current decreased first and then increased with the increase of the electric field, which is probably due to the higher sample density of the FS550, as shown in table 1. This is consistent with previous reports. The FS550 sample showed the best varistor performance with a breakdown field of 17.16V/mm, a nonlinear coefficient of 3.4 and a leakage current of 281 uA. Albeit pure TiO2Has no piezoresistance characteristic, but is codoped with TiO In (In + Ta)2The giant dielectric ceramic shows the piezoresistance characteristic, and provides a good research direction for developing a capacitor-piezoresistance device by a flash firing method.
The foregoing is merely a preferred embodiment of the invention, which is intended to be illustrative and not limiting. It will be understood by those skilled in the art that various changes, modifications and equivalents may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (7)
1. In + Ta co-doped TiO2The preparation method of the giant dielectric ceramic is characterized by comprising the following steps of:
s1: mixing oxide powders in ZrO2Ball-milling the medium in ethanol for 24 hours, and then drying the medium in an oven at 80 ℃ for 24 hours to obtain pre-calcined powder;
s2: calcining the pre-calcined powder obtained in the step S1 to obtain a calcined powder;
s3: carrying out secondary ball milling on the calcined powder obtained in the step S2 under the same ball milling condition as the pre-calcined powder, and then drying for 24h at 80 ℃;
s4: adding a binder to the dried powder obtained in step S3, and uniaxially pressing the mixed powder into a disk under a pressure of 310 MPa;
s5: baking the disc obtained in the step S4 at 650 ℃ for 2h to remove the adhesive;
s6: placing the sample obtained in the step S5 in a tube furnace, wherein a direct current power supply is arranged at two ends of the sample to provide an electric field, and the sample is heated at a heating rate of 10 ℃/min;
s7: when the temperature of the sample reaches 1200 ℃, keeping for 5min, then applying an electric field of 450V/cm-600V/cm, initially presetting the current to be 0.5A, when applying the electric field, increasing the current by 0.1A every 5min until the limiting current reaches 0.7A, then closing the direct current power supply, and cooling the sample to the room temperature.
2. The In + Ta co-doped TiO of claim 12The preparation method of the Mesoxazole dielectric ceramic is characterized in that the calcination temperature in the step S2 is 1000 ℃, and the calcination time is 5 h.
3. The In + Ta co-doped TiO of claim 12The method for preparing the gigantic dielectric ceramic is characterized in that the step S4The binder was 5 wt.% polyvinyl alcohol.
4. The In + Ta co-doped TiO of claim 12The preparation method of the gigantic dielectric ceramic is characterized in that in the step S4, the disc is a green disc with the thickness of 2mm and the diameter of 7 mm.
5. The In + Ta co-doped TiO of claim 12The preparation method of the gigantic dielectric ceramic is characterized in that the temperature rise rate in the step S5 is 2 ℃/min.
6. In + Ta co-doped TiO prepared by the preparation method of any one of claims 1-52A giant dielectric ceramic.
7. The In + Ta co-doped TiO of claim 62Use of a base dielectric ceramic in a microelectronic device.
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