EP0408752A1

EP0408752A1 - Process for producing silver halide grains

Info

Publication number: EP0408752A1
Application number: EP89908859A
Authority: EP
Inventors: Shigeharu Urabe
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1988-08-05
Filing date: 1989-07-28
Publication date: 1991-01-23
Also published as: EP0408752A4; JPH0244335A; WO1990001462A1; JPH0782208B2

Abstract

The invention relates to a process for producing silver halide grains, which comprises providing a mixer outside a reaction vessel which contains an aqueous solution of protective colloid and in which nucleation of silver halide grains is to be caused, feeding an aqueous solution of silver nitrate and an aqueous solution of a water-soluble halide into the mixer, vigorously stirring the two solutions in the mixer to form fine grains of silver halide, and immediately introducing the fine grains into the reaction vessel. The fine silver halide grains introduced into the reaction vessel readily dissolve due to their fineness of grain size to again form silver and halide ions, which in turn deposit on a minute amount of remaining fine grains to form silver halide nucleus grains, thus providing silver halide grains which have a completely uniform halide composition within the crystal and are free of a difference in halide distribution between grains.

Description

FIELD OF THE INVENTION

This invention relates to a method for producing silver halide grains and, more particularly, to preparation of silver halide grains having a completely uniform halide composition in each silver halide crystal and having no difference in halide distribution among the grains.

BACKGROUND OF THE INVENTION

The formation of silver halide grains mainly comprises two .steps, i.e., nucleus formation and growth. In T.H. James, The Theory of The Photographic Process, 4th ed., Macmillan Publishing Co., Ltd. (1977), there are the descriptions that nucleus formation is the process in which there is a population explosion of the number of crystals when entirely new crystals are created; growth is the addition of new layers to crystals that are already present; besides the above-described nucleus formation and growth, two additional processes occur under some conditions of photographic emulsion precipitation, Ostwald ripening and recrystallization; Ostwald ripening occurs predominatly at higher temperatures in the presence of solvents when there is a wide distribution of grain size; and recrystallization is the process in which the composition of crystals changes. That is, there is no increase in number of grains during the growth process in the formation of silver halide grains, because crystal nuclei are formed at the initial stage and, in the growth process subsequent thereto, the growth occurs only on the nuclei which are already present.
In general, silver halide grains are produced by the reaction of aqueous solutions of a silver salt and a halide in an aqueous colloidal solution placed in a reaction vessel. There have been known a single-jet method in which a protective colloid, such as gelatin, and an aqueous halide solution are placed and stirred in a reaction vessel, to which an aqueous silver salt solution is added over a certain period of time, and a double-jet method in which an aqueous gelatin solution is placed in a reaction vessel, to which an aqueous halide solution and an aqueous silver salt solution are added over certain periods of time.. In comparison between these two methods, the double-jet method has the advantages that silver halide grains having a narrow size distribution can be obtained, and further the halide composition of grains can, be freely changed during the growth of grains.
Nucleus formation of silver halide grains is known to be-greatly influenced by a silver (or halogen) ion concentration in the reaction solution, a concentration of a silver halide solvent, a degree of supersaturation, a reaction temperature, and so on. In particular, non-uniformity in silver or halogen ion concentration upon addition of aqueous solutions of a silver salt and a halide into a reaction vessel brings about uneven distributions of supersaturation degree and dissolution degree inside the reaction vessel depending on their respective concentrations, so that the nucleation speed varies in localities, resulting in non-uniformity in the crystal nuclei formed.
In order to attain uniform concentrations of silver or halide ions in the reaction vessel it is necessary that the aqueous solutions of a silver salt and a halide are mixed rapidly and homogeneously in an aqueous colloidal solution. However, in a conventional method in which the aqueous solutions of a silver salt and a halide are added to the surface of an aqueous colloidal solution placed in a reaction vessel, it is difficult to obtain silver halide grains with uniformity because of locally higher concentrations of halide and silver ions in the vicinities where the reactant solutions are fed, respectively. Methods for eliminating such local non-uniformity of the concentrations in the reaction vessel are disclosed in U.S. Patent 3,415,650, British Patent 1,323,464 and U.S. Patent 3,692,283 are known. In these methods, a reaction vessel is filled with an aqueous solution of a colloid. The reaction vessel is equipped with a rotary convex cylindrical hollow mixer having slits in the wall thereof (filled with an aqueous solution of a colloid, preferably composed of an upper chamber and a lower chamber partitioned by a disc in the vessel). The axis of rotation of the mixer is vertical. The aqueous solution of the halide and the aqueous solution of the silver salt are supplied into the mixer, which is rotating at a high speed, at the top and bottom open ends through feed pipes so that they are rapidly mixed and reacted with each other. (If there are two chambers in the mixer, the two aqueous solutions supplied into the respective chamber are first diluted with an aqueous solution of the colloid present therein, and then they are rapidly mixed and reacted with each other in the vicinity of the outlet slits.) The silver halide grains thus formed are then introduced into the aqueous solution of the colloid in the reaction vessel by the centrifugal force produced by the rotation of the mixer.
On the other hand, JP-B-55-10545 discloses a method for eliminating an uneven concentration distribution to prevent non-uniform growth of grains. In this method, an aqueous solution of a halide and an aqueous solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of the colloid in a reaction vessel filled with an aqueous solution of the colloid from the bottom open end of the mixer through feed pipes. These reaction solutions are rapidly agitated and mixed with each other by a lower agitator (turbine impeller) provided in the mixer to effect the growth of silver halide. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from the upper open end of the mixer by an upper agitator provided above the lower agitator.
JP-A-57-92523 discloses a preparation method which is intended to eliminate such a non-uniformity in concentration. In this method, an aqueous solution of a halide and an aqueous solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of a colloid in a reaction vessel filled with an aqueous solution of the colloid from a lower open end of the mixer. The two reaction solutions are diluted with the aqueous solution of the colloid and then rapidly mixed with each other by a lower agitator provided in the mixer. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from an upper open end of the mixer. In this method and apparatus therefor, the two reaction solutions which have been diluted with the aqueous solution of the colloid are passed through the clearance between the inner wall of the mixer and the tip of the agitator without being passed through the gaps between the impellers so that they are rapidly mixed and reacted with each other under a shearing force in the clearance to form silver halide grains.
These methods and apparatus can thoroughly eliminate the uneven distribution of concentration of silver ions and halogen ion in the reaction vessel. However, an uneven concentration distribution still exists in the mixer. In particular, a relatively large uneven concentration distribution exists in the vicinity of the nozzle through which the aqueous solution of the silver salt and the aqueous solution of the halide are supplied of the portion under the agitator and of the portions agitated. Furthermore, the silver halide grains supplied into the mixer together with the protective colloid are passed through these portions having an uneven concentration distribution. In should be particularly noted that the silver halide grains rapidly grow in these portions. In other words, these preparation methods and apparatus therefor are disadvantageous in that an uneven concentration distribution exists in the mixer, and the growth of grains takes place rapidly in the mixer, failing to accomplish the object of allowing uniform growth of the silver halide under conditions free of a concentration distribution difference.
In order to accomplish a more efficient mixing so as to eliminate the uneven concentration distribution of silver ions and halogen ions, additional attempts have been made. For example, a reaction vessel and a mixer are independently provided. An aqueous solution of a silver salt and an aqueous solution of a halide are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a preparation method and apparatus disclosed in JP-A-53-37414 and JP-B-48-21045, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. A mixer is provided in the course of the circulation system. An aqueous solution of a silver salt and an aqueous solution of a halogen are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a method disclosed in U.S. Patent 3,897,935, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of a halide and an aqueous solution of a silver salt are pumped into the course of the circulation system. In a preparation method and apparatus disclosed in JP-A-53-47397, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of an alkali metal halide is first introduced into the circulation system. The aqueous solution of an alkali metal halide is diffused into the system until the system becomes uniform. Thereafter, an aqueous solution of a silver salt is introduced into and mixed with the system to form silver halide grains. These methods enable independent altering of the rate at which the aqueous solutions flow from the reaction vessel to the circulation system and the agitation efficiency of the mixer, making it possible to effect growth of grains under a condition of a more uniform concentration distribution. However, these methods are still disadvantages in that the crystalline silver halide which has been delivered from the reaction vessel together with the protective colloid is subject to rapid growth at the inlet portion from which the aqueous solution of the silver salt and the aqueous solution of the halide are introduced into the system. Therefore, in these methods, it is impossible, in principle, to eliminate such a concentration distribution difference in the mixing portion or in the vicinity of the inlet portion. That is, the object of allowing uniform growth of silver halide under a condition free of concentration distribution cannot be accomplished.

DISCLOSURE OF THE INVENTION

An object of this invention is to solve the problem in conventional preparation methods and apparatuses that nucleus formation of silver halide grains proceeds under uneven distribution of concentration (of silver ion and halogen ion), resulting in formation of silver halide grains having lack of uniformity (in grain size, crystal habit, halogen distribution among grains and inside the individual grains, and distribution of reduced silver nuclei among grains and inside the individual grains).
The above-described object of this invention is attained by a method for producing silver halide grains, which comprises providing a mixer disposed outside a reaction vessel, supplying an aqueous solution of a water-soluble silver salt and an aqueous solution of a water-soluble halide to the mixer, mixing them in the mixer to form fine particles of silver halide, and immediately thereafter, introducing the fine particles into the reaction vessel to perform nucleation of silver halide grains.
The term "nucleus" as used herein means grains in the stage wherein the number of silver halide crystals fluctuates during the formation of emulsion grains. Grains in the stage wherein the number of silver halide crystals does not change and only the growth on the nuclei takes place are referred to as "grains only under growth". In the nucleation process, the production of new nuclei, the elimination of existing nuclei and the growth of nuclei take place at the same time.
In the present invention, it is important to avoid addition of an aqueous solution of a silver salt and an aqueous solution of a halide in the reaction vessel during the nucleus formation. It is also important to avoid circulation of the aqueous solution from the reaction vessel to the mixer. In these aspects, the present invention is quite different from conventional methods and is a novel and unique method for producing uniform silver halide grains.
The nucleation process according to the present invention is illustrated in Fig. 1.
In Fig. 1, a reaction vessel 1 contains an aqueous solution of protective colloid 2. The aqueous solution of protective colloid 2 is agitated by a propeller 3 mounted on a rotary shaft. An aqueous solution of silver salt, an aqueous solution of halide and, if desired, an aqueous solution of protective colloid are introduced into a mixer 7 provided outside the reaction vessel through addition systems 4, 5 and 6, respectively. (In this case, the aqueous solution of protective colloid may be added in the form of a mixture with the aqueous solution of halide and/or aqueous solution of silver salt.) These solutions are rapidly and strongly mixed with each other in the mixer 7. The mixture is immediately introduced into the reaction vessel 1 through a system 8.
Fig. 2 is a detailed view of the mixer 7. The mixer 7 comprises a reaction chamber 10 provided therein. In the reaction chamber 10, an agitator 9 mounted on a rotary shaft 11 is provided. An aqueous solution of silver salt, an aqueous solution of halide and an aqueous solution of protective colloid are charged into the reaction chamber 10 from two inlets 4 and 5 and another inlet (not shown). When the rotary shaft is rotated at a high speed, these reaction solutions are rapidly and thoroughly mixed with each other, and the resulting solution containing extremely fine particles is immediately discharged from an outlet 8. The extremely fine particles thus formed by the reaction in the mixer can be easily dissolved in the emulsion in the reaction vessel due to its extremely fine size to provide silver ions and halogen ions again which cause the growth of uniform grains. The halide composition of the extremely fine particles is adjusted to equal that of the desired silver halide grains. The extremely fine particles thus introduced into the reaction vessel are scattered in the reaction vessel by the agitation in the reaction vessel. At the same time, individual extremely fine particles release halogen ions and silver ions of the desired halide composition. The particles produced in the mixer are extremely fine, and their number is very large. Since silver ions and halogen ions (in the case of the growth of mixed crystal, silver ions and halogen ions of the desired halogen ion composition) are released from a relatively large number of particles, and this takes place throughout the protective colloid in the reaction vessel, the growth of completely uniform grains can be achieved. It should be noted that silver ions and halogen ions must not be charged into the reaction vessel in the form of an aqueous solution. It should also be noted that the protective colloid solution must not be circulated from the reaction vessel to the mixer. In this respect, the present process is quite different from the conventional process. In accordance with the present process, a surprising effect can be achieved in the uniform growth of silver halide grains.
The finely divided particle formed in the mixer exhibit a relatively high solubility due to their fine particle size. Therefore, when charged into the reaction vessel, the finely divided particles are dissolved in the solution therein to become silver ions and halogen ions again which are then deposited on existing grains in the reaction vessel to effect the growth of grains. The larger the size of the finely divided particles charged into the reaction vessel is, the lower is the solubility thereof. This retards the solution of the particles in the reaction vessel, remarkably lowering the rate of growth of grains. In some cases, the particles are no longer dissolved and are not effectively consumed for the nucleation.
In the present invention, the above described problems can be solved by the following three methods.

(i) Finely divided particles are formed in a mixer, and the resulting finely divided particles are immediately charted into a reaction vessel.

As described hereinafter, it has been known that a fine grain emulsion is prepared through the step of forming fine grains in advance, the grains are dissolved again in the emulsion, and then the resulting emulsion is added to a reaction vessel in which silver halide grains to become nuclei are retained in the presence of a silver halide solvent, whereby nucleation is carried out. In such a process, however, the very fine grains having once formed suffer Ostwald ripening in the steps of grain formation, washing, redispersion and redissolution, so that the grain size increases.
In the present invention, a mixer is provided close to the reaction vessel and the retention time of the solutions charged in the mixer is shortened. Accordingly, by immediately charging the resulting finely divided particles into the reaction vessel, Ostwald ripening can be avoided. Specifically, the retention time t of the solutions charged in the mixer can be represented by the following equation:

wherein v: volume (ml) of the reaction chamber in the mixer;
a: amount (ml/min) of the aqueous silver nitrate solution added;
b: amount (ml/min) of the aqueous solution added; and
c: amount (ml/min) of the protective colloidal solution added

In the present preparation process, t is in the range of 10 minutes or less, preferably 5 minutes or less, more preferably 1 minute or less, particularly 20 seconds or less but 2 seconds or more. Thus, the finely divided particles formed in the mixer are immediately charged (within 10 seconds) into the reaction vessel without increasing their particle size.

(ii) A vigorous and efficient agitation is achieved in the mixer.

T.H. James, The Theory of the Photographic Process, 4th Ed., pp. 93, Macmillan 1977 states "Another type of grain growth that can occur is coalescence. In coalescence ripening, an abrupt change in size occurs when pairs or larger aggregates of crystals are formed by direct contact and welding together of crystals that were once widely separated. Both Ostwald and coalescence ripening may occur during precipitation, as well as after precipitation has stopped". Coalescence ripening as. referred to herein tends to take place when the grain size is very small, particularly when the agitation is insufficient. In some extreme cases, gross lumps of grains are formed. In the present invention, a closed type mixer is used as shown in Fig. 2. Therefore, the agitator in the reaction chamber can be rotated at a high speed. Thus, a vigorous and efficient agitated mixing, which cannot be accomplished by the conventional open type reaction vessel, can be achieved. (In such an open type reaction vessel, when the agitator is rotated at a high speed, the solution is scattered by the centrifugal force. This high speed rotation also involves foaming of the material. Therefore, this high speed rotation in the open type reaction vessel is not practical.) Furthermore, the above described coalescence ripening can be prevented. As a result, finely divided particles having a relatively small particle size can be obtained. In the present invention, the number of revolutions of the agitator is 1,000 to 10,000 r.p.m. preferably 2,000 r.p.m. or more, particularly preferably 3,000 r.p.m. or more. (iii) An aqueous solution of protective colloid is charged into a mixer.
The above described coalescence ripening can be markedly prevented by the use of a protective colloid. In the present invention, the charging of the aqueous solution of protective colloid into the mixer is accomplished in the following manner.

(a) An aqueous solution of protective colloid is singly charged into the mixer.

The concentration of the protective colloid is in the range of 0.2 to 10% by weight, preferably 0.5% by weight or more. The flow rate at which the aqueous solution of protective colloid is charged into the-mixer is 20 to 300_%, preferably at least 50%, more preferably 100_% or more, of the sum of the flow rate of the aqueous solution of silver salt and the aqueous halide solution.

(b) As protective colloid is incorporated in an aqueous halide solution.

The concentration of the protective colloid is 0.2 to 10% by weight, preferably 0.5% by weight or more.

(c) As protective colloid is incorporated in an aqueous solution of silver salt.

The concentration of the protective colloid is 0.2 to 10_% by weight, preferably 0.5_% by weight or more. When gelatin is used for the purpose, a silver nitrate solution and a gelatin solution should be mixed just before their use, because silver ion and gelatin form gelatin silver which is converted to colloidal silver through photolysis and pyrolysis.
In addition, the (a) to (c) may be employed independently, or in combination. While gelatin is generally used as the protective colloid of this invention, other hydrophilic colloids can also be used. Examples thereof are described in Research Disclosure, vol. 176, No. 17643, Item IX (Dec. 1978).
Size of grains obtained in the above manners (i) to (iii) can be measured by putting the grains on meshes and observing with a transmission electron microscope. A suitable magnification of the observation is from 20,000 to 40,000. The size of the fine grains of this invention is within the range of 0.001 to 0.06 µm, preferably from 0.005 to 0.03 µm, and more preferably 0.01 µm or less.
Although U.S. Patent 2,146,938 and JP-A-57-23932 disclose crystal growth effected by adding a fine grain emulsion to an emulsion of grains to be grown, a previously prepared fine grain emulsion is used for the purpose and this technique deals only with the crystal growth. Therefore, they differ entirely from this invention.
In T.H. James, The Theory of The Photoqraphic Process, 4th edition, Lippmann Emulsion is described as fine grains and has an average grain size of 0.05 um. It is possible to prepare fine grains with a grain size of 0.05 µm or less but such fine grains are.so unstable and subject to Ostwald ripening so that the grain size increases. The Ostwald ripening can be prevented to some extent by adsorbing an agent to the fine grains as disclosed in JP-A-57-23932. However, dissolution of the fine grains becomes slow, contrary to the feature of this invention.
JP-A-58-113927 (p. 207), discloses that a silver salt, a bromide and an iodide can be introduced at the initial stage or the growth stage in the form of fine silver halide grains suspended in a dispersion medium; in other words silver 'bromide, silver iodide and/or silver iodobromide grains can be introduced.
However, this teaches nothing but general use of a fine grain emulsion in silver halide formation, and the method and the system of this invention are not taught or suggested.
In the conventional methods as described above, a fine grain emulsion is prepared in advance, subjected to re-dissolution, and then used. Consequently, grains with a very small size cannot be obtained. Such being the case, these relatively large grains do not rapidly dissolve in a reaction vessel, taking a very long time for completion of dissolution or requiring a large quantity of silver halide solvent. Under such a situration_; grains to grow in the vessel come to undergo nucleation in a very slight supersaturation state. As a result, the size distribution of nuclei formed is markedly broad, and that of the finished grains also becomes broad, resulting in deterioration of photographic properties, such as lowering of photographic gradation, lowering of sensitivity due to uneven chemical sensitization (since optimal chemical sensitization for both grains with a larg-e size and a small size cannot be accomplished at the same time), increase of fog, deterioration of granularity and so on. Moreover, the conventional methods involve several steps such as grain formation, washing, dispersing, cooling, storage and redissolution, and entail high production cost. Moreover, the addition of emulsion is accompanied many limitations, as compared to the addition of solution. These problems are solved by the method of this invention. More specifically, very fine grains which have high solubility are introduced into a reaction vessel, and hence have high dissolution speed. Under these circumstances, grains to grow in the reaction vessel undergo nucleation in a high supersaturation state. Consequently, the finished nuclei come to have a narrow size distribution. In addition, there is no problem in production cost because the fine grains formed in the mixer is added to the reaction vessel as they are.
If a silver halide solvent is added in advance to the reaction vessel of this invention, higher dissolution speed and higher nucleation speed can be achieved.
Examples of a silver halide solvent include water-soluble bromides, water-soluble chlorides, thiocyanates, ammonia, thioethers, thioureas and the like.
More specifically, thiocyanates as disclosed in U.S. Patents 2,222,264, 2,448,534 and 3,320,069, etc., ammonia, thioether compounds as disclosed in U.S. patents 3,271, 157, 3,574,628, 3,704,130, 4,297,439 and 4,276,345, etc., thione compounds as disclosed in JP-A-53-144319, JP-A-53-82408 and JP-A-55-77737, etc. amine compounds as disclosed in JP-A-54-100717, etc., thiourea derivatives as disclosed in JP-A-55-2982, etc., imidazoles as disclosed in JP-A-54-100717, etc., substituted mercaptotetrazoles as disclosed in JP-A-57-202531, etc. and so on can be instanced.
In the method of this invention, the feeding speed of silver ion and halide ion to the mixer can be freely selected. The feeding speed may be constant, but is preferably increased as described in U.S. Patent 3,650,757 and JP-B-52-16364. A halide composition of growing grains can also be freely controlled. In case of silver iodobromide, for example, it becomes feasible to keep the iodide content constant, to increase or decrease the iodide content continuously, or to change the iodide content at a certain point of time.
A suitable reaction temperature in the mixer is not higher than 60°C, preferably 50°C or less, and more preferably from 0 to 40°C.
At reaction temperature below 35°C, conventional gelatin is liable to coagulate. Therefore low molecular weight gelatins (with a number average molecular weight of 1,000 to 30,000, preferably 10,000 or less) are desirably used.
Low molecular weight gelatins which can be used in this invention are generally prepared in the following manner: conventional gelatin with an average molecular weight of 100,000 is dissolved in water, and a gelatin decomposing enzyme is added thereto to effect the enzymatic decomposition of gelatin. For this treatment, reference can be made to R.J. Cox, Photographic Gelatin II, pp. 233 to 251 and pp. 335 to 346, Academic Press, London (1976). This treatment has the advantage that it can provide low molecular weight gelatins with a relatively narrow distribution of molecular weights because specific bonding sites are attacked by the enzyme. For the longer enzymatic decomposition time, the lower molecular weight gelatin can be obtained. Alternatively, gelatin may be hydrolyzed by heating at a low pH (pH 1 to 3) or a high pH (pH 10 to 12).
A temperature of the protective colloid in the reaction vessel is 40°C or higher, preferably 50°C or higher, and more preferably 60°C or higher.
While in this invention, aqueous solutions of a silver salt and a halide are not added to the reaction vessel during the nucleation, they may be added for the purpose of controlling pAg of a solution in the reaction vessel prior to the nucleation. Further, they may also be added (occasionally or continuously) to the reaction vessel during the nucleation for the purpose of controlling pAg of the reaction solution in the reaction vessel wherein the nucleation proceeds. Furthermore, an aqueous halide solution or an aqueous silver salt solution may be added, in accordance with a so-called pAg-controlled double jet method, to keep the pAg inside the reaction vessel constant, if needed.
The method of this invention is very effective for preparation of various kinds of emulsions.
When. conventional methods are employed in nucleation of mixed crystals of silver halides, namely silver iodobromide, silver iodobromochloride, silver iodochloride or silver chlorobromide, the mixed crystal nuclei obtained are not microscopically uniform in halide distribution. This phenomenon cannot be avoided even when an aqueous halide solution having a constant halide composition and an aqueous silver salt solution are added to a reaction vessel so as to realize a uniform halide distribution. This microscopic non-uniformity of halide distribution can be easily confirmed from transmission images of silver halide grains under a transmission electron microscope.
For instance, such a non-uniformity can be directly observed under low temperatures with a transmission electron microscope as described in J.F. Hamilton, Photographic Science and Engineering, vol. 11, p. 57 (1967), and Takekimi Shiosawa, Nippon Shashin Gakkai, vol. 35, No. 4, p. 213 (1972). More specifically, silver halide grains are taken out under safelight such that the emulsion grains are not exposed to form a latent image, are mounted on an electron microscope observation mesh. Observation is carried out using the transmission electron microscopy while cooling the grains .with liquid. nitrogen or helium so as to prevent damage from electron beam (e.g., exposure to form latent image and the like).
Here, the higher the accelerating voltage of the electron microscope, the clearer the image obtained, and this is generally 200 Kvolt for a grain thickness up to 0.25 um, and 1,000 Kvolt for larger grain thicknesses. It is preferable to cool the sample with liquid helium rather than liquid nitrogen since damage to the grain by the irradiated electron beam increases with the higher accelerating voltage.
The magnification can be appropriately adjusted in accordance with the sample grain size, and it is from 20,000 to 40,000 times.
Naturally, silver halides composed of a single halide should not exhibit non-uniformity in halide distribution, and, so their transmission electron micrographs merely have flat images. In the case of mixed crystals composed of plural halides, on the other hand, very fine year-ring-shaped striped pattern are observed on their transmission electron micrographs. Intervals in these striped patterns are as narrow as 100 A or less, and these striped patterns imply the presence of microscopic non-uniformity in halide distribution.
It can be verified in various ways that the very fine striped pattern show the non-uniformity of halide distribution. In a direct way, for instance, the grains to be examined are annealed under such a condition that iodine ions can migrate throughout the individual silver halide grains (e.g., at 250°C for 3 hours), resulting in complete disappearance of the very fine striped pattern. This result clearly supports the above saying. As for the phenomena of this kind, there are detailed descriptions in Japanese Patent Application Nos. 63-78512, 63-8752 and 63-7853. However, these patent applications, however, relate to the crystal growth of silver halide grains. In analogy with the crystal growth process, the above-described phenomenon in the nucleation process has been found in this invention.
An iodide content in silver iodobromide or iodochlorobromide phase contained in the emulsion grains prepared in accordance with the method and the apparatus of this invention ranges from 2 to 45 mol%, preferably 5 to 35 mol%. The total iodide content is within the range of 2 to 45 mol%, more effectively 5 mol% or more, more preferably 7 mol% or more, and particularly preferably 12 mol% or more.
The method of this invention is useful in the preparation of silver chlorobromide grains, and can provide silver chlorobromide grains with a completely uniform distribution of bromide (or chloride). A chloride content in such grains is within the range of 10 to 90 mol%, preferably 20 mol% or more.
In addition, the method of this invention is very effective in preparing pure silver bromide and pure silver chloride. According to conventional methods, it is unavoidable that uneven distributions of silver ions and halogen ions are present in a reaction vessel, respectively. By passing through such a localized non-uniform zone, a part of the silver halide grains in the reaction vessel come to be under circumstances different from other- grains present in the other uniform zone. Thus, non-uniformity of crystal growth is resulted and further, for example, reduced silver or fogged silver is generated in the zone of higher silver ion concentration. Though uneven distribution of halide does not occur, in the preparation of silver bromide or silver chloride, another type of non-uniformity as described above occur. According to the method of-this invention, this problem can be perfectly solved.
The silver halide nuclei obtained in accordance with this invention are grown into silver halide grains with desired size and halide composition in the subsequent crystal growth process. The crystal growth can be effected in a conventional manner, in which an aqueous silver salt solution and an aqueous halide solution are added to the reaction vessel after completion of the nucleation, but it is preferred that the growth process be performed in the same manner as in the nucleation process of this invention. It is also desirable that a fine grain emulsion prepared in advance be added to the reaction vessel to grow the silver halide nuclei. Details of the growth process are disclosed in Japanese Patent Application Nos. 63-7851, 63-8752 and 63-7853. The halide distribution of the thus obtained silver halide grains is "perfectly uniform" in- both nucleus part and grown phase and, what is more, the grain size distribution thereof is very narrow.
The completely uniform silver halide emulsion grains thus formed have no particular limitation with respect to their size, but it is preferably within the range of 0.3 to 5 µm, and more preferably at least 0.8 um. The silver halide grains of this invention may have a regular crystal form, such as hexahedron, octahedron, dodecahedron, tetradecahedron, a tetracosahedron or octatetracosahedron; an irregular crystal form, such as sphere, potato-like form or so on; ' or any form having at least one twinning plane, particularly a hexagonal or triagonal tabular form having two or three parallel twinning planes.
Photographic materials using the silver halide photographic emulsion of this invention have no particular limitations with respect to constitution additives, processing methods and so on. For details of additives, reference can be made to JP-A-63-123042, JP-A-63-106745, JP-A-63-100749, JP-A-63-100445, JP-A-63-71838, JP-A-63-85547, Research Disclosure, vol. 176, Item 17643, and ibid. vol. 187, Item 18716.
The above-cited Research Disclosure includes description as to additives at the portion indicated below.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 schematically illustrates the method of this invention, wherein
Fig. 2 schematically illustrates the cross section of the mixer of this invention, wherein

BEST EMBODIMENTS OF INVENTION

This invention will now be illustrated in greater detail by reference to the following examples.

EXAMPLE 1

Silver Iodobromide Fine Grain Emulsion 1-A:

2.6 liter of a 2.0 wt_% gelatin solution containing 0.126 M or potassium bromide was stirred, and thereto were added over 15 minutes 1,200 ml of an aqueous solution containing 1.2 M of silver nitrate and 1,200 ml of an aqueous solution containing 0.9 M of potassium bromide and 0.3 M of potassium iodide according to a double jet method. During the addition, the gelatin solution was kept at 35°C. The resulting emulsion was washed using a conventional flocculation process, and then 30 g of gelatin was further added and dissolved. Furthermore, the emulsion was adjusted to pH 6.5 and pAg 8.6. The thus obtained silver iodobromide fine emulsion grains (iodide content: 25_%) had an average grain size of 0.05 µm.

Octahedral Silver Iodobromide Emulsion 1-B (Comparison)

1.2 liter of a 1.5 wt% of gelatin solution containing 0.05 M of potassium bromide was placed in a reaction vessel, and stirred. Thereto, 100 ml of a 0.05% 3,6-dithiaoctane-l,8-diol was added and then, the solution inside the reaction vessel was kept at 75°C. Then, an emulsion obtained by mixing 270 ml of water with 100 g of the silver iodobromide fine grain emulsion 1-A (containing silver in an amount corresponding to 10 g of silver nitrate) was added to the reaction vessel over 10 minutes to effect nucleation. The thus obtained octahedral silver iodobromide emulsion grains had an average grain size of 0.5 µm. Successively, the resulting emulsion in the reaction vessel was stirred and kept at 75°C, to which 800 ml of a 1.5 M silver nitrate solution and 800 ml of a halide solution containing 0.375 M of potassium iodide and 1.13 M of potassium bromide were added simultaneously over 100 minutes in accordance with a double jet method. Thereafter, the emulsion was cooled to 35°C, and washed by a conventional flocculation process. After addition of 70 g of gelatin, the resulting emulsion was adjusted to pH 6.2_' and pAg 8.8. The thus obtained emulsion grains comprised octahedral silver iodobromide grains (iodide content: 25 mol%) having an average projection area diameter of 1.7 microns (the term projection area diameter used herein refers to the diameter of the circle having the same area as the projected area of the grain).

Octahedral Silver Iodobromide Emulsion I-C (Invention):

1.2 liter of a 1.5 wt% of gelatin solution containing 0.05 M of potassium bromide was placed in a reaction vessel, and stirred. Then 50 ml of a 0.05% 3,6-dithia-octane-1,8-diol was added to the reaction vessel and the solution inside the reaction vessel was kept at 75°C. To a mixer disposed near the reaction vessel, were added 100 ml of an aqueous solution containing 0.59 M of silver nitrate, 100 ml of an aqueous solution containing 0.44 M of potassium bromide and 0.148 M of potassium iodide and 300 ml of a 2 wt% aqueous gelatin solution over 5 minutes in accordance with a triple jet method. The reaction chamber in the mixer was 20°C, and the agitator was rotated at 1,000 r.p.m.. The thus formed fine grains was 0.01 µm in size when observed directly with a transmission electron microscope, of 20,000 magnifications. The fine grains produced in the mixer were successively introduced into the reaction vessel kept at 75°C. The thus formed silver iodobromide nuclei were octahedral, and had an iodide content of 25 mol% and a size of 0.5 µm. Further, the grain growth was performed at 75°C in the same manner as in the preparation of Emulsion 1-B, and the resulting emulsion was washed, and adjusted to the same pH and the same pAg as Emulsion 1-B. The resulting silver iodobromide emulsion grains were octahedron, and had an average projected area diameter of 1.7 µm and an iodide content of 25 mol_%.
Emulsion 1-B and Emulsion 1-C each was subjected to optimum chemical sensitization using sodium thiosulfate, potassium chloroaurate and potassium thiocyanate. Variation coefficient of the grain size distribution in the finished emulsion was 13% in the case of Emulsion 1-B, whereas it was 8% in the case of Emulsion l-C. Evidently, the grain size distribution of Emulsion 1-C was narrower.
The ingredients set forth below were added to each of the emulsions 1-B and I-C, and coated on a triacetylcellulose film support provided with a subbing layer.

(1) Emulsion Layer

Emulsion shown in Table 1

Coupler illustrated below:
Tricresyl phosphate

Sensitizing dye (Sodium 5-chloro-5'-phenyl-4-ethyl-3,3'-(3-sulfopropyl)oxacarbocyanine)

Stabilizing agent (4-Hydroxy-6-methyl-l,3,3a,7-tetrazaindene)

Coating aid (Sodium dodecylbenzenesulfonate)

(2) Protective Layer

Sodium salt of 2,4-dichloro-6-hydroxy-s-triazine Gelatin

These samples each was subjected to sensitometric exposure, and then to the following photographic processing.
The density measurement of the processed samples each was carried out through a green filter. The thus obtained data concerning photographic properties are shown in Table 1.
The photographic processing employed herein was carried out at 38°C under the following condition.

1. Color development 2 min. 45 sec.
2. Bleaching 6 min. 30 sec.
3. Washing 3 min. 15 sec.
4. Fixation 6 min. 30 sec.
5. Washing 3 min. 15 sec.
6. Stabilization 3 min. 15 sec.

Compositions of the processing baths used in the above-described steps, respectively, are described below.
Emulsion 1-C prepared in accordance with this invention had higher sensitivity than Emulsion 1-B. In addition, Emulsion 1-C was superior in granularity to Emulsion 1-B.

EXAMPLE 2

Octahedral Silver Iodobromide Emulsion 2-A (Comparison):

Silver iodobromide nuclear grains having a size of 0.5 µm were formed by performing nucleation in the same manner as in Emulsion 1-B. Then, 800 ml of a 1.5 M silver nitrate solution, 800 ml of an aqueous solution containing 0.375 M of potassium iodide and 1.13 M of potassium bromide and 800 ml of a 10 wt% aqueous solution of low molecular weight gelatin (number average molecular weight: 10,000) were added to a mixer disposed near a reaction vessel over 100 minutes in accordance with a triple jet method. The reaction chamber in the mixer was kept at 20°C, and the agitator was rotated at 3,000 r.p.m. The thus formed fine grains was 0.008 µm in size when measured in the same manner as described above. The fine grains produced in the mixer were introduced successively into the reaction vessel kept at 75°C. Thereafter, the emulsion was washed in the same manner as Emulsion 1-B, and adjusted to pH 6.2 and pAg 8.8. The thus obtained silver iodobromide emulsion grains were octahedron and had a size of 1.7 µm and an iodide content of 25 mol%.

Octahedral Silver Iodobromide Emulsion 2-B (Invention):

Nucleation was performed in the same manner as in Emulsion 1-C to obtain octahedral silver iodobromide nuclear grains having a size of 0.5 µm, and successively the nuclear grains were grown at 75°C under the same condition as in the grain growth of Emulsion 2-A. The resulting emulsion was washed, and adjusted to the same pH and pAg as Emulsion 2-A. The thus formed emulsion grains were octahedral silver iodobromide grains of 1.7 µm in size (iodide content: 25 mol%).
Variation coefficient of the grains size distribution among the finished emulsion grains was 14% in the case of Emulsion 2-A, whereas it was 9_% in the case of Emulsion 2-B. Evidently, the grain size distribution of Emulsion 2-B was narrower. Emulsion 2-A and Emulsion 2-B each was subjected to optimum chemical sensitization using sodium thiosulfate, potassium chloroaurate and potassium thiocyanate. Thereafter, coating compositions were prepared in the same manner as in Example 1. The results of sensitometry carried out in the same manner as in Example 1 are shown in Table 2.
Emulsion 2-B of this invention had higher sensitivity than Emulsion 2-A, and further was superior in granularity to Emulsion 2-A.

EXAMPLE 3

Silver Iodobromide Fine Grain Emulsion 3-A:

2.6 liter of a 2.0 wt% gelatin solution containing. 0.1 M of potassium bromide was stirred, to which were added over 15 minute 1,200 ml of a 1.2 M silver nitrate solution and 1,200 ml of an aqueous solution containing 1.08 M of potassium bromide and 0.12 M of potassium iodide according to a double jet method. During the addition, the gelatin solution was kept at 35°C. The resulting emulsion was washed using a conventional flocculation process,, and then 30 g of gelatin was further added and dissolved. Furthermore, the emulsion was adjusted to pH 6.5 and pAg 8.6. The thus obtained silver iodobromide fine grains (iodide content: 10_%) were the mixture of tabular fine grains of about 0.1 µm in projected area diameter and spherical fine grains having a size of 0.06 µm.

Tabular Silver Iodobromide Emulsion 3-B (Comparison):

1.1 liter of a 2 wt% gelatin solution containing 0.1 M of potassium bromide was placed in a reaction vessel, and stirred. Then, 70 ml of a 0.5% 3,6-dithiaoctane-l,8-diol was added to the reaction vessel, and the solution inside the reaction vessel was kept at 75°C. An emulsion obtained by adding 200 ml of water to 100 g of the silver iodobromide fine grain emulsion 3-A (containing silver in an amount corresponding to 8 g of silver nitrate) and heating the mixture at 40°C to convert it into a solution was added to the reaction vessel over 15 minutes to effect the nucleation of tabular grains. The thus obtained silver iodobromide tabular nuclei had an average projected area diameter of 0.7 µm. Then, 900 ml of a 1 M silver nitrate solution, 900 ml of an aqueous solution containing 0.9 M of potassium bromide and 0.1 M of potassium iodide and 900 ml of a 2 wt% aqueous gelatin solution were added to a mixer disposed near the reaction vessel over 90 minutes in accordance with a triple jet method. The reaction chamber in the mixer was kept at 15°C, and the agitator was rotated.at 6,000 r.p.m. The thus formed fine grains was 0.008 µm in size. The fine grains produced in the mixer were introduced successively into the reaction vessel kept at 75°C to effect the grain growth. Thereafter, the emulsion was washed in the same manner as described above, and adjusted to pH 6.4 and pAg 8.6. The thus obtained silver iodobromide emulsion grains were a crystal form of a tablet and had an average projected area diameter of 2.0 µm and an iodide content of 10 mol%.

Tabular Silver Iodobromde Emulsion 3-C (Invention):

1.1 liter of a 2 wt% gelatin solution containing 0.1 M of potassium bromide was placed in a reaction vessel, and stirred, to which was added 30 ml of a 0.5% 3,6-dithiaoctane-l,8-diol. Then, the solution inside the reaction vessel was kept at 75°C. 100 ml of a 0.47 M silver nitrate solution, 100 ml of an aqueous solution containing 0.57 M of potassium bromide and 0.047 M of potassium iodide and 200 ml of a 2 wt% aqueous gelatin solution were added to a mixer disposed near the reaction vessel over 8 minutes in accordance with a triple jet method. The reaction chamber in the mixer was 25°C, and the agitator was rotated at 6,000 r.p.m.. The thus formed fine grains was 0.02 µm in size. According to careful observation under a transmission electron microscope, the formed fine grain emulsion comprised two kinds of grains, namely fine hexagonal or triagonal twin crystals, and spherical grains. The fine produced in the mixer were introduced successively into the reaction vessel kept at 75°C. The tabular silver iodobromide nuclear grains obtained in the thus performed nucleation had a diameter of 0.8 µm and an iodide content of 10 mol%. Further, the grain growth was performed at 75°C in the same manner as used in Emulsion 3-B, and the resulting emulsion was washed, and. adjusted to pH 6.4 and pAg 8.6. The thus obtained emulsion grains were tabular silver iodobromide (iodide content: 10 mol%) emulsion grains of 2.0 µm in an average projected area diameter.
Variation coefficient of the grains size distribution among the finished emulsion grains was 24% in the case of Emulsion 3-B, whereas it was 19% in the case of Emulsion 3-C. Evidently, the grains size distribution of Emulsion 3-C was narrower. Emulsion 3-B Emulsion 3-C each was subjected to optimum chemical sensitization using sodium thiosulfate, potassium chloroaurate and potassium thiocyanate. Thereafter, coating compositions were prepared in the same manner as in Example l. The results of sensitometry carried out in the same-manner as in Example 1 are shown in Table 3.
Emulsion 3-C of this invention had higher sensitivity than Emulsion 3-B, and further was superior in granularity to Emulsion 3-B.

Claims

A method for producing silver halide grains, which comprises providing a mixer disposed outside a reaction vessel, supplying an aqueous solution of a water-soluble silver salt and an aqueous solution of water-soluble halide(s) to the mixer, mixing them in the mixer to form fine particles of silver halide, and immediately thereafter introducing the fine particles into the reaction vessel to perform nucleation of silver halide grains.