GB2053499A - Photographic silver halide emulsion and process for preparing same - Google Patents

Abstract

There are provided a photographic silver halide emulsion which comprises composite silver halide crystals consisting essentially of multi- faceted silver iodide crystals and silver bromide or silver iodobromide crystals being combined with the multi- faceted silver iodide crystals through epitaxial junction, at least half of the facets of the multi-faceted silver iodide crystals being substantially free of the epitaxial silver halide, and the epitaxial silver halide being limited to not more than 75 mole % based on the total silver halide forming the composite crystals, and a process for preparing the same.

Description

SPECIFICATION Photographic silver halide emulsion and process for preparing same This invention relates to improvements in a photographic silver halide emulsion and a process for preparing the same. More particularly, this invention relates to a photographic silver halide emulsion comprising epitaxial composite silver halide crystals which shows excellent stability during storage, and a process for preparing the same.

It is heretofore known that two or more than two kinds of silver halides are employed in combination so as to make the best use of each of the constitutional silver halides combined in the structure of the single silver halide grain.

For instance, British Patent No. 1027146 discloses composite silver halide grains which comprise silver halide nuclei (nuclear grains) coated with a single layer or several layers of silver halide. If silver chloride is caused to precipitate on facets of silver bromide in the manner as described in the British Patent, the spectral sensitivity of the silver bromide and the developing characteristics of the silver chloride are both given to the so obtained crystals.

U.S. Patent No. 3,505,068 discloses an art comprising preparing a lower sensitive emulsion layer to be used together with a highly sensitive emulsion layer by applying the art disclosed in the abovementioned British Patent, and obtaining a lower contrast on a dye image by employing the so prepared emulsion layer. Silver halide grains employed in the low sensitive emulsion layer comprise both of cores consisting of silver iodide or silver iodohalide and shells consisting of silver bromide, silver chloride or silver chiorobromide.

There has recently been developed a photographic silver halide emulsion containing composite silver halide crystals that are prepared through the epitaxial junctions of crystals of silver chloride or the like with crystals of silver iodide for combining the radiation-sensitivity of silver iodide and the rapid developability of silver chloride. The epitaxial composite silver halide crystals are composed of multifaceted silver iodide crystals and silver chloride crystals formed on the multi-faceted crystals through epitaxial junction, as disclosed in U.S. Patent No. 4,096,484 (referred to hereinafter as "Reference 1").

According to this disclosure, at least half of the facets of the silver iodide crystal are substantially free of epitaxial silver chloride, and silver chloride is limited to not more than 75 mole %, based on the total silver halide forming the above-mentioned composite crystals. A photographic silver halide emulsion containing the epitaxial composite silver halide crystals is capable of liberating relatively large quantities of iodide ion upon development, and this enables achievement of preferred photographic effects by the liberation of the iodide ion. These effects are considered to be great characteristics of the emulsion of that type.Other characteristics of the photographic silver halide emulsion reside in that the emulsion exhibits favorable interimage and edge effects; that the emulsion produces, upon development, a heterogeneous catalyst image, i.e., a silver image, for use in a redox amplification reaction; that photographic images, both silver and dye images, of reduced graininess and granularity are obtained; and that the emulsion can be selectively developed so that silver chloride is developed or so that both silver chloride and silver iodide are developed. In the last way, development conditions can be selected to control the graininess and granularity of photographic images, control iodide ion liberation and control maximum image densities obtained.

However, the silver halide emulsion comprising composite silver halide crystals composed of the silver chloride combined through epitaxial junction has drawbacks in that the sensitivity lowers and that fog occurs during storage.

With reference to the art described in the above-mentioned Reference 1, an art involving substitution of silver chloride formed on multi-faceted silver iodide crystals which serve as host crystals through epitaxial junction with silver bromide by a conversion method is disclosed in U.S. Patent No.

4,142,900 (referred to hereinafter as "Reference 2"). This art deserves attention as an art for improving the developing speed by the use of silver halide crystals prepared through the epitaxial junction, and the art has a variety of the same advantageous aspects as stated with respect to the art of the Reference 1.

However, that art is not free from lack of the storage stability due to lattice defect or structural deformation of the crystal, and this disadvantageous lack of the storage stability is the same as for the aforementioned composite silver chloride-silver iodide crystals.

Accordingly, it is a primary object of the present invention to provide a photographic silver halide emulsion comprising epitaxial composite silver halide crystals which is free from the drawbacks of the arts disclosed in the aforementioned Reference 2 and further which is excellent in the storage stability.

Another object of the present invention is to provide a process for preparing such photographic silver halide emulsion comprising epitaxial composite silver halide crystals, particularly a photographic silver halide emulsion comprising epitaxial composite silver halide crystals in which a ratio of area covered by the epitaxial silver halide crystals is relatively low.

The present inventor's photographic silver halide emulsion comprises composite silver halide crystals consisting essentially of multi-faceted silver iodide crystals and silver bromide or silver iodobromide crystals being combined with the multi-faceted silver iodide crystals through epitaxial junction, at least half of the facets of the multi-faceted silver iodide crystals being substantially free of the epitaxial silver halide, and the epitaxial silver halide being limited to not more than 75 mole %, based on the total silver halide forming the composite crystals.

In the present specification and claims, the term "epitaxy" is used to signify the same meaning as described in both of References 1 and 2, that is, the term "epitaxy" means that the crystal orientation of the silver bromide or silver iodobromide deposited on the silver iodide host crystal of the composite silver bromide-silver iodide crystal of the composite silver iodobrnmide-silver iodide crystal is controlled during its growth by the host crystals such as silver iodide. The epitaxial relationship between the epitaxial silver halide such as silver bromide or silver iodobromide in the composite crystal and the silver iodide portion serving as the silver halide host crystal is entirely different from the direct physical contact in other crystals of silver iodide, silver bromide, silver iodobromide, or the like.

One of the main characteristics of the present invention lies in that the silver bromide or silver iodobromide crystal to be deposited on the multi-faceted silver iodide host crystal through epitaxial junction is one formed by directly depositing it on the silver iodide host crystal. In other words, the deposited silver bromide or silver iodobromide is not one produced by depositing, in the first place, a silver chloride crystal on the silver iodide host crystal through epitaxial junction and replacing, in the second place, the epitaxially deposited silver chloride with silver bromide or silver iodobromide by the conversion process.

The formation of the epitaxial composite silver halide crystals according to the present invention can be done, for instance, by the following processes.

[A] A solution containing a water-soluble bromide or a solution containing either of a water-soluble bromide and a water-soluble iodide, and a silver salt solution are simultaneously added to multi-faceted silver iodide crystals so that crystals of silver bromide or silver iodobromide can be deposited on the multi-faceted silver iodide crystals through epitaxial junction.

[B] A silver bromide crystal is deposited on the multi-faceted silver iodide through epitaxial junction, and the so deposited silver bromide crystal is changed to a silver iodobromide crystal through the conversion method using a solution containing a water-soluble iodide.

[C] A solution containing a water-soluble bromide or a solution containing either of a water-soluble bromide and a water-soluble iodide is added to a solution containing the multi-faceted silver iodide host crystals, a water-soluble silver salt and a protective colloid by the single-jet method to deposit crystals of silver bromide or silver iodobromide on the silver iodide host crystals through epitaxial junction.

The present inventors have found the fact that when a silver bromide or silver iodobromide crystal is epitaxially grown on facets of the host crystal under the conditions in which the silver bromide or silver iodobromide crystal is made to substantially have a crystal habit of a (1 1 1) plane, epitaxial composite silver halide crystals, can easily be produced, in which a silver bromide or silver iodobromide crystal is combined with a host crystal through only a relatively small contact area, that is, in which a ratio of area covered by the epitaxial silver halide crystals is relatively low.

On the other hand, when silver bromide or silver iodobromide crystal is epitaxially grown on facets of the host crystal under the conditions in which the produced silver bromide or silver iodobromide crystal is made to have a crystal habit of a (1 0 0) plane, silver halide crystals having the core/shell structure is likely produced.

The basic principle supporting the above-mentioned process according to the present invention has not been clearly revealed. However, a hypothetical explanation can be made as follows. The effect of the present invention is based on the fact that the epitaxial junction between the silver iodide crystal and the silver bromide or silver iodobromide crystal is formed with a (1 0 1 ) plane of p-silver iodide crystal and a (1 0 0) plane of the silver bromide or silver iodobromide crystal.In other words, when the epitaxial growth is carried out under the conditions to give the crystal habit of a (1 1 1) plane to the produced silver bromide or silver iodobromide crystal, the (1 0 0) plane of snver bromide or silver iodobromide crystal is not stably grown and, therefore, this (1 00) plane is combined with a facet of the silver iodide crystal through a small area. Further, since the outer facet of the silver bromide or silver iodobromide crystal forms a (1 1 1) plane, the silver bromide or silver iodobromide crystal predominantly grown on a facet of the silver iodide crystal in a direction perpendicular to the facet of the silver iodide crystal.Accordingly, there is produced an epitaxial composite silver halide crystal, of which silver iodide crystal is covered with an epitaxial silver halide crystal through a relatively small area of the surface of the silver iodide crystal. In contrast, when the epitaxial growth is carried out under the conditions for producing a silver bromide or silver iodobromide crystal having a crystal habit of a (1 0 0) plane, both of the combining plane and outer facet, of the so produced epitaxial silver halide crystal are the (1 0 0) plane, both of the combining plane and outer facet, of the so produced epitaxial silver halide crystal are the (1 00) planes, and the (1 00) plane stably grows. This means that the epitaxial silver halide crystal is apt to grow on the facet of silver iodide in a direction parallel to the facet rather than in a direction perpendicular to the facet. Thus, the crystal having a core/shell structure whose coverage ratio is high is apt to be produced. If more than half of a total area of facets of a multi-faceted silver iodide crystal is covered with an epitaxial silver bromide or silver iodobromide crystal, an emulsion comprising the so formed crystals is called a "core/shell type emulsion". When the core/shell type emulsion is employed, a large number of sensitized nuclei are formed on the facets of the silver bromide or silver iodobromide crystals in the course of a chemical ripening process. The resulting emulsion gives a dispersion to a latent image during sensitization, and the photographic sensitivity is therefore reduced.

Moreover, the fact that the facets of silver iodide crystal are covered with an epitaxial silver bromide or silver iodobromide crystals means that the silver iodide crystal has a number of development initiating points. Under the conditions, it is difficult to stop development at the time when the development of the epitaxial silver bromide or silver iodobromide crystals are just complete. Therefore, an amount of developed silver iodide increases and the granularity worsens.

There are known a great number of references disclosing the conditions for producing silver bromide or silver iodobromide crystals having the stable crystal habit of the (1 1 1) plane. For instance, Photographic Science and Engineering, vol. 6, pp. 159-165 (1962) discloses that the (1 1 1) plane is produced at 500C., in the presence of a 2 N ammonia and at pAg of not lower than 9.0. Photographic Science and Engineering, vol. 17, pp. 289-294 (1973) and J. Photo. Sci., vol.24, pup.8194 (1976) both disclose a range of conditions regarding an ammonia concentration, pAg and a silver iodide content, under which the (1 1 1) plane is produced. Photographic Science and Engineering, vol. 12, pp.

207-212 (1968) shows a variation of the crystal habit observed when the silver iodide content in the silver iodobromide is varied.

Generally, the (1 1 1) plane is hardly produced at a high ammonia concentration and is apt to be produced at high pAg value and high silver iodide content.

Moreover, J. Photogr. Sci.,vol. B, pp.131-135(1970) discloses that growth of the (1 1 1) plane is accelerated in the presence of poly(ethylene oxide).

The conditions for producing the (1 1 1) plane according to the present invention are easily estimated from the above-mentioned references. The (1 1 1) plane can be produced, maintaining a pAg value (a logarithm of an inverse number of the activity of the silver ion) of the protective colloidal solution for silver halide production at not lower than 9, as far as no agent for altering the crystal habit is specifically included. A range of the conditions under which the (1 1 1) plane is produced varies in the presence of ammonia, depending upon the concentration of the ammonia, and this is disclosed in the above-mentioned references.

The crystal habit of a silver brnmide or silver iodobromide crystal is affected by the presence of an additive, for instance, a thioether, a thiourea, imidazole, adenine, a tetraazaindene, or a heavy metal ion such as Pub++, Cd++, iridium ion, as well as ammonia and poly(ethylene oxide). A range of the conditions under which the (1 1 1) plane is produced varies depending upon the kind and the amount of the additives added, and an equivalent crystal habit at selected conditions can be readily ascertained by the following method. The outer facets of silver bromide or silver iodobromide crystals which are produced at the selected conditions in the absence of silver iodide crystals are observed on the property of the facets.Whether the produced crystals are composed of 11 1 1) planes or not can be easily judged by checking the crystal form on an electromicroscopic photograph. The relationship between the crystal form and the outer facet are widely known from a large number of publications. For instance, Photographic Science and Engineering, vol. 6, pp. 159-165 (1962), Photographic Korrespondenz, vol.

99, p. 99-102 (1963) and vol. 100, pp. 57-71(1964), Photographic Science and Engineering, vol.

8, pp.65-69(1964) and J. Photogr. Sci., vol. 12, pp. 242--251 (1964) disclose the relationship between a crystal form and an outer facet of a crystal based on an electromicroscopic photograph.

The outer facets of a crystal can be easily determined from a powder X-ray diffraction (orientation method), as well as from the above-mentioned electromicroscopic photograph. This method is disclosed in Zeitschrift fijr Wissenschaftliche Photographic, vol. 55, pup.97102 (1 961) and Bulletin of the Society of Scientific Photography of Japan, vol. 16, pp. 1-7 (1966).

At least a part of the silver iodide crystals to be employed as host crystals in the present invention are radiation-sensitive multi-faceted silver iodide crystals having the minimum average grain diameter of at least 0.1 y.

According to the process of the present invention, there is easily prepared a silver halide emulsion comprising epitaxial composite silver halide crystals having the multi-faceted silver iodide crystals and silver bromide or silver lodobromide crystals which are deposited on the multi-faceted silver iodide crystals through epitaxial junction and in which at least half of the facets of the multi-faceted silver iodide crystals are substantially free of the epitaxial silver halide and the epitaxial silver halide is limited to not more than 75 mole %, based on the total silver halide forming the composite crystals.

The photographic silver halide emulsion comprising the epitaxial composite silver halide crystals according to the present invention can liberate a relatively great amount of iodide ions during a development process, and therefore the emulsion is advantageous in achievement in a photographic effect based on the liberation of iodide ions.

The photographic emulsion according to the present invention shows preferred interimage and edge effects. When a photographic material comprises, for instance, a photographic emulsion layer provided by the present invention, an iodide ion liberated during development process can be employed for deactivating the surface of a heterogeneous catalyst which is used for the redox amplification reaction between an oxidizing agent such as cobalt hexamine or hydrogen peroxide and a reducing agent for forming dye image such as a color developing agent or a redox color releasing agent used together with an electron transferring agent.

The photographic emulsion according to the present invention can be coated and dried on an appropriate support to prepare a silver halide emulsion layer, exposed imagewise by radiation of visible spectral ray and developed under an appropriate developing condition to yield a photographic silver image. Further, an iodide ion liberated during development can be utilized, even under the redox amplification reaction conditions, for deactivating a silver image serving a redox amplification catalyst.

The photographic emulsion according to the present invention can contain a dye forming coupler so that both of a photographic silver image and a photographic dye image can be obtained. in addition to the abovementioned effect, the so obtained images are advantageous in that the grainess and granularity are at lower levels.

Moreover, a photographic emulsion according to the present invention can be subjected to the selective development process, that is, the photographic emulsion can be so developed as to develop the epitaxial silver halide only, or can be so developed as to develop both of the epitaxial silver halide and silver iodide. Accordingly, a photographic emulsion of the present invention can provide a number of advantageous aspects, that is, the grainess arid granularity of a photographic image can be controlled; the liberation of an iodide ion can be controlled; a development condition for controlling the maximum density of the obtained image can be selected; or the like.

In the composite crystals according to the present invention, the silver iodide crystal serves as the first radiation-receptor. If the photographic emulsion containing the composite crystals according to the present invention are exposed imagewise to a blue light, a developable latent image is formed. If the composite crystal consisting essentially of silver iodide and silver bromide or silver iodobromide according to the present invention is exposed, the whole portion of the composite crystal is made developable. Alternatively, the epitaxial silver halide portion only can be likewise developed.

The epitaxial silver halide portion of the composite crystal according to the present invention does not serve as the first radiation receptor in the composite crystal. Accordingly, a photographic speed of a photographic emulsion according to the present invention is not controlled by radiation given to the epitaxial silver halide.

The sizes of the epitaxial silver halide crystals can be so selected that the silver iodide host crystals cannot be developed, but that the epitaxial silver halide only can be developed. In this case, the grainess and granularity of the photographic image are defined by the sizes (diameters) of the epitaxial silver halide crystals, as far as the solution physical development does not take place. The photographic speed is defined by the sizes of the silver iodide host crystals which have larger diameter than the epitaxial silver halide crystals.

A protective colloid employed in the present invention can be selected from a group of the protective colloids generally employed for preparing a silver halide emulsion. As a protective colloid is advantageously employed a colloid made from gelatin. Other types of hydrophilic colloids, however, can be employed as well.

The amount of the protective colloid to be included ranges from about 0.5 g. to about 100 g., per one liter of a solution (emulsion) containing the produced silver halide.

The water-soluble bromide, iodide and silver salt employed in the present invention can be selected from those generally used in a conventional silver halide emulsion. Examples of the watersoluble bromides include potassium bromide and sodium bromide; examples of the water-soluble iodides include potassium iodide and sodium iodide; and examples of the water-soluble silver salts include silver nitrate.

Apart from the conditions of the pAg during production of silver halide, there are no other limitations on conditions such as pH value and temperature. However, the pH value preferably is within the range from about 2 to about 9, more preferably is not higher than 5, and particularly preferably is not higher than 3. The temperature favorably is within the range from about 300 C. to about 900C., and preferably is within the range from 350C. to 800C. Under the same pAg value conditions, a higher temperature can provide silver halide grains of narrower granularity distribution. The pH value and the temperature may be-varied during production of silver halide grains, but these are preferably kept at constant levels during the production.

For carrying out the process for preparing silver halide grains to be employed for the present invention, reference may be made to, besides the above-mentioned references, U.S. Patent No.

2,222,264, U.S. Patent No. 2,592,250, U.S. Patent No. 3,206,313, U.S. Patent No. 3,447,927, U.S.

Patent No. 3,501,307, British Patent No. 723019, British Patent No. 1027146, The Journal of Photographic Science, Vol. 12, pp. 242-251(1963), ibid., vol. 13, pp. 85-89, and ibid., Vol. 13, pp.

98-107.

There are no specific limitations on conditions for carrying out the chemical ripening procedure for the silver halide emulsion prepared by the process of the present invention, for instance, pH, pAg, temperature, period and additives. For these properties, the conditions generally employed in the art to which the present invention belongs are adopted.

The silver halide emulsion prepared by the process of the present invention can be sensitized with a conventionally employed chemical sensitizing agent such as a sulfur sensitizer, a reduction sensitizer, or a noble metal sensitizer.

To the emulsion obtained upon the chemical ripening procedure is added an additive that is named "coating finals" in a field of art to which the present invention belongs, and the so obtained mixture is coated on a variety of supports and dried to prepare a light-sensitive, silver halide photographic material.

For the purposes of inhibition of fogs appearing on a light-sensitive material in the courses of preparation, storage or photographic processing or for the purpose of stabilization of the photographic performance, a variety of compounds such as antifoggant and a stabilizer can be also included.

The above-mentioned protective colloid employed for the production of silver halide can also be used as a binding agent or a protective colloid for preparing a light-sensitive material which is prepared according to the process of the present invention.

In a photographic emulsion layer or other hydrophilic colloidal layers of the light-sensitive material prepared according to the process of the present invention, a variety of known surface active agents can be incorporated for a variety of purposes such as assisting the coating procedure, improving the slidability, providing emulsifibility and dispersibility, preventing adhesion and improving photographic characteristics such as acceleration of developing process, provision of high contrast and improvement of sensitization.

The emulsion according to the present invention can be sensitized with a methine dye or other dyes. Examples of the dyes to be employed include a cyanine dye, a rnerocyanine dye, a complex cyanine dye, a complex merocyanine dye, a holo-polar cyanine dye, a hemicyanine dye, a styryl dye and a hemioxonal dye. Particularly useful dyes are those belonging to the merocyanine dye or the complex merocyanine dye.

The photographic emulsion according to the present invention may include a dye-image forming coupler, that is, a compound capable of reacting with an oxidation product in an aromatic amine developing agent (generally, it is a primary amine) to form a dye; this compound being referred to, hereinbefore, as "coupler". The couple may have equivalency number of two or four to a silver ion. The photographic emulsion may include colored coupler that provides color correction or a coupler that releases a development inhibitor, with the advance of development (so-called "DIR coupler"). The coupler may be one that yields colorless product upon coupling reaction.

The photographic emulsion according to the present invention is coated on a flexible support such as a plastic material film, paper or cloth, or a rigid support such as glass, ceramics or metal, which is generally employed for preparing a light-sensitive photographic material, through the dipping coating method, the roller coating method, the curtain coating method and the extruding coating method.

Photographic processing of a light-sensitive material prepared by the application of the silver halide emulsion according to the present invention can be carried out in any of known ways. A processing solution can be selected from known processing solutions. A processing temperature generally ranges from 18 to 500 C., but a temperature lower than 1 80C. or higher than 500 C. can be also utilized. The photosensitive material can be subjected either to the black-and-white photographic processing for forming a silver image or to the color photographic processing for forming a dye image, according to a purpose as desired.

A developing solution to be employed for the black-and-white photographic processing can contain a known developing agent. Examples of the developing agents include dihydroxybenzenes such as hydroquinone, 3-pyrazolidones such as 1 -phenyl-3-pyrazolidone, aminophenols such as N-methyl-p aminophenol, 1 -phenyl-3-pyrazolines, ascorbic acid, etc. The solution can be employed alone or in combination. The developing solution may also contain known preservative, alkaline agent, pH buffering agent, anti-foggant, etc.

A fixing agent is composed of a generally adopted composition.

Examples of fixing agents include an organic sulfur compound that is known as being effective as a fixing agent, as well as thiosulfate and thiocyanate.

A color developing solution generally consists of an aqueous alkaline solution containing a color developing agent. The color developing agent may be a known primary aromatic amine developing agent such as a phenylenediamine. Examples of the phenylenediamines include 4-amino-N,N diethylaniline, 3-methyl-4-amino-N,N-diethylaniline, 4-amino-N-ethyl-N--hydrnxyethylaniline, 3- methyl-4-a mino-N-ethyl-N-,3-hydroxyethylaniline 3-methyl-4-amino-N-ethyl-N-- methanesulfoamidoethylaniline, and 4-amino-3-methyl-N-ethyl-N-p-methoxyethylaniline.

Upon color development, a photographic emulsion layer is generally subjected to a bleaching treatment. The bleaching treatment can be carried out separately or simultaneously with a fixing treatment.

A photographic emulsion prepared according to the process of the present invention is preferably applied to a great number of light-sensitive silver halide photographic materials, because the emulsion is high in the light-sensitivity and the contrast, and is low in the fog. The emulsion is particularly advantageous in a field expecting photographic effects by an iodide ion such as an interimage effect, an edge effect, a triger effect, etc.

The photographic emulsion according to the present invention is advantageously applied to a variety of light-sensitive silver halide photographic materials such as thse for X-ray, color image, blackand-white image, transfer process, high contrast photography and photothermography.

The present invention is further illustrated by the following Examples, but these Examples are not understood to limit the present invention.

EXAMPLE 1 A monodispersed silver iodide emulsion was prepared employing the three solutions set forth in Table 1.

TABLE 1 Solution A Ossein gelatin 100.0 g.

Distilled water 3.0 1.

Kl 2.23 g.

Temperature 35o C.

pid 6.0 Solution B 5 molar aqueous solution of Kl 1,000 ml.

Solution C 5 molar aqueous solution of AgNO3 800 ml.

An iodide ion electrode and a doublejunction type silver/silver chloride reference electrode (junction solution: 1 molar KNO3 aqueous solution) sold on the market were immersed in Solution A to measure the potential. The potential (-175 mV) was maintained during addition of Solutions B and C by adjusting the flow rate of Solution B.

Solution C was added at a constant flow rate of 0.5 ml./min. during the initial 6 min. period, and subsequently the flow rate was elevated straightforwardly by a rate of 0.385 ml./min. per every 10 min.

The complete addition of Solution C required 197 min., and the temperature of the solution was kept at 350C. during the physical ripening procedure. When the addition of Solution C was complete, the addition was discontinued. Subsequently washing with water and desalting were carried out in the following manner.

A precipitant (5% aqueous solution of Demo N, produced by Kao Atlas Co., Ltd., Japan) and an aqueous solution of magnesium sulphate (20%) were added in a ratio of 10:9 until a precipitate was produced. The precipitate was sedimented upon allowing to stand quietly, and then the supernatant liquid was removed by decantation. To the sedimented precipitate was added 3,000 ml. of distilled water to disperse again the precipitate. An aqueous solution of magnesium sulphate (20%) was further added to the so obtained disperse system until a precipitate was again produced. After sedimentation of the precipitate was complete, the supernatant liquid was removed by decantation. The aqueous solution of ossein gelatin containing 56.6 g. of gelatin was added to the precipitate, and the precipitate was dispersed under stirring at 350C. for 20 min.Then, distilled water was so added to the disperse system to make the total volume to be 2,270 ml. The so obtained emulsion is referred to hereinafter as EM--1.

The average diameter of the grains contained in the EM-1 emulsion and the standard deviation of the grain diameter was confirmed to be 0.25 m and 20% of the average grain diameter respectively upon observation of the electron microscope photograph. Moreover, the X-ray diffraction analysis indicated that the EM-1 emulsion consisted substantially of ,B-phase silver iodide with little amounts of those of cr- and y-phase.

EXAMPLE 2 An emulsion comprising composite silver halide crystals in which silver bromide crystals were deposite on silver iodide crystals through epitaxial junction were prepared at pAg 10.0, that is, under conditions for producing silver bromide having the outer facet of the (111) plane, employing the three solutions set forth in Table 2. The preparation was not carried out according to the conversion method.

TABLE 2 Solution D 1 molar aqueous solution of AgNO3 Solution E 1 molar aqueous solution of KBr Solution F Ossein gelatin 4.4 g.

pH 6 EM-1 emulsion 160 ml.

Distilled water 840 ml.

To Solution F were added simultaneously 70.6 ml. of Solution D and 70.6 ml. of Solution E at 350C. over 6 min., by the doubiejet method. While the addition was continued, the potential of Solution F was maintained at -11 mV, that is, the pAg value of Solution F was maintained at 10.0, by controlling the flow rate of Solution E.

After the addition was complete, the washing with water and desalting were carried out in the following manner.

A precipitant (5% aqueous solution of Demo N, produced by Kao Atlas Co., Ltd., Japan) and an aqueous solution of magnesium sulphate (20%) were added in a ratio of 10:9 until a precipitate was produced. The precipitate was sedimented upon allowing to stand quietly, and then the supernatant liquid was removed by decantation. To the sedimented precipitate was added 800 ml. of distilled water to disperse again the precipitate. An aqueous solution of magnesium sulphate (20%) was further added to the so obtained disperse system until a precipitate was again produced. After sedimentation of the precipitate was complete, the supernatant liquid was removed by decantation. An aqueous gelatin solution containing 10 g. of ossein gelatin was added to the precipitate, and the precipitate was dispersed under stirring at 350C. for 20 min.The total volume was then adjusted to 200 ml. by adding distilled water. The so obtained emulsion is referred to hereinafter as EM-2.

EXAMPLE 3 An emulsion comprising composite silver halide crystals in which silver bromide crystals were deposited on silver iodide through epitaxial junction was prepared in the same manner as described in Example 2 (non-conversion method), except that the preparation was carried out at pAg 4, that is, under the conditions for producing silver bromide having the outer facet of the (1 0 0) plane. The silver potential was maintained at +356 mV while the solution was added. The so obtained emulsion is referred to hereinafter as EM-3.

Electromicroscopic photographs were taken on the EM-i, EM-2 and EM-3 emulsions. The photograph of EM-2 emulsion indicated that one large silver bromide crystal only was deposited on a silver iodide crystal and that most of the surface of the silver iodide crystal was free of a silver bromide layer. The photograph of EM-3 emulsion indicated, on the contrary, that a great number of silver bromide crystals were deposited on a silver iodide crystal and that the surface of the silver iodide crystal was almost perfectly covered with a silver bromide layer.

EXAMPLE 4 An emulsion comprising composite silver halide crystals in which silver bromide crystals were deposited on silver iodide crystals through epitaxial junction was prepared in the same manner as described in Example 2 except that the preparation was carried out at pAg 2. Thus, the preparation was not carried out according to the conversion method. The silver potential was maintained at +478 mV while the solution was added.

It was confirmed that a silver bromide crystal takes a ball type form with no substantial growth of (111) plane at pAg 2 and pH 6 when the formation is done in the absence of silver iodide. The so obtained emulsion is referred to hereinafter as EM-4.

EXAMPLE 5 An emulsion comprising composite silver halide crystals in which silver bromide crystals were deposited on silver iodide crystals through epitaxial junction was prepared in the same manner as described in Example 4 (non-conversion method) except that the pH value of Solution F was-replaced with 2. The silver potential was maintained at +478 mV while the solution was added.

It was confirmed that a silver bromide crystal takessn octahedral form in which a (1 1 1) plane is developed at pAg 2 and pH 2 when the formation is done in the absence of silver iodide. The so obtained emulsion is referred to hereinafter as Elm5.

EXAMPLE 6 An emulsion comprising composite silver halide crystals in which silver iodobromide crystals were deposited on silver iodide crystals through epitaxial junction was prepared at pAg 6, that is, under conditions for producing silver iodobromide having the outer facet of the (1 0 0) plane, employing the four solutions set forth in Table 3. The preparation was not carried out according to the conversion method.

TABLE 3 Solution G Same as Solution D of Example 2 Solution H Same as Solution F of Example 2 Solution I KBr 107.2 g.

Kl 10.0 9.

Add distilled water to make 1,000 ml.

Solution J 1 molar aqueous solution of KBr To Solution H were added simultaneously 70.6 ml. of Solution G and 70.6 ml. of Solution I at 350 C. over 6 min., by the double-jet method. While the addition was continued, the silver potential was checked. Solution J was added as well as Solution G and Solution I, and the flow rate of Solution J was controlled to maintain the silver potential at +233 mV, that is, to maintain pAg value of Solution H at 6.0. After the addition was complete, washing with water, desalting and redispersion were carried out in the same manner as described in Example 2. The so obtained emulsion is referred to hereinafter as EM-6.

EXAMPLE 7 An emulsion comprising composite silver halide crystals in which silver iodobromide crystals were deposited on silver iodide crystals through epitaxial junction was prepared in the same manner as described in Example 6 (non-conversion method) except that the preparation was carried out at pAg 10.0, that is, under conditions for producing silver iodobromide having the outer facet of the (1 1 1) plane. The silver potential was maintained at -1 mV while the solution was added. The so obtained emulsion is referred to hereinafter as EM-7.

EXAMPLE 8 An emulsion comprising composite silver halide crystals in which pure silver chloride crystals were deposited on silver iodide crystals through epitaxial junction was prepared employing the three solutions set forth in Table 4. This process is according to the process disclosed in the aforementioned Reference 1.

TABLE 4 Solution K 1 molar aqueous solution of AgNO3 Solution L 1 molar aqueous solution of KCI Solution M Ossein gelatin 4.4 g.

KCI lOg.

Elm1 emulsion 160 ml.

Distilled water 840 ml.

To Solution M were added simultaneously 70.6 ml. of Solution K and 70.6 ml. of Solution L at 350C. and over 6 min., by the double-jet method. After the addition was complete, washing with water, desalting and redispersion were carried out in the same manner as described in Example 2. The so obtained emulsion is referred to hereinafter as EM-8.

EXAMPLE 9 To 200 ml. of an emulsion prepared in the same manner as described in Example 8 were added, in advance of subjecting to the desalting procedure, 6.5 ml. of 1 molar aqueous solution of KBr and 4.2 ml.

of 1 molar aqueous solution of KI. The resulting mixture was heated at 500 C. for 10 min., so as to convert the silver chloride crystals combined epitaxially, to silver halide crystals comprising bromine and iodine. Then, the washing with water, desalting and redispersion were carried out in the same manner as described in Example 2. The so obtained emulsion is referred to hereinafter as EM-9.

EXAMPLE 10 The emulsions EM-2, EM-6, EM-7, Elm8 and EM-9 were treated in the following manner to prepare samples for testing.

To each of the above-mentioned silver halide emulsions were added 2.0 x 10-5 mole of sodium thiosulfate and 1.0 x 10-5 mole of sodium chloroaurate, both being based on mole of silver, and the mixture was aged at 45 C. for 60 min. Subsequently, to the resulting mixture were added 4-hydroxy-6 methyl-1 ,3,3a,7-tetraazaindene and 1 -phenyl-5-mercaptotetrazole, and bis(vinylsulfonylmethyl) ether and saponin were further added thereto just prior to being coated. The mixture was then coated on a support. The amount to be coated was so adjusted to give 0.50 g./m2 for silver and 2.0 g./m2 for gelatin, based on the surface area of the support. The coated support (sample) was preserved at 550C. and RH (relative humidity) 10%, for 3 days.The so preserved sample and the sample having been subjected to no preserving procedure were exposed to white light through an optical wedge in a KS-1 type Sensitometer (manufactured by Konishiroku Photo Industry Co., Ltd., Japan), and then developed in a developing solution having the following composition, at 200C. and for 10 min., and fixed.

Developing solution Metol 3.or.

Sodium sulfite (anhydrous) 50.0 g.

Hydroquinone 6.0g.

Sodium carbonate 29.5 g.

Potassium bromide 1.0 g.

Add water to make one liter The results obtained upon sensitometry are set forth in Table 5.

TABLE 5 With no preservation Preserved (3 days) Sample Relative Relative No. EM No. Fog sensitivity Dmax. Fog sensitivity Dmax.

1 EM-2 0.03 100 0.45 0.03 100 0.45 2 EM-6 0.02 103 0.45 0.02 100 0.45 3 EM-7 0.02 103 0.45 0.02 100 0.45 4 Elm8 0.03 100 0.44 0.11 85 0.40 5 EM-9 0.03 100 0.45 0.09 90 0.42 Note: Relative sensitivity was determined at Fog +0.10 As is clear from the results in Table 5, photographic emulsions comprising the composite crystals in which silver bromide crystals or silver iodobromide crystals are deposited on silver iodide crystals through epitaxial junction and which are prepared according to the present invention, i.e., Samples No.

1, No. 2 and No. 3 are superior in the storage stability to the photographic emulsions prepared according to the process disclosed in Reference 2, i.e., Samples No. 4 and No. 5.

EXAMPLE 11 The emulsions Elm1, EM-2, EM-3, EM-4, Elm5, EM-6 and EM-7 were all treated in the following manner to prepare samples for testing.

To each of the above-mentioned silver halide emulsions were added 2.0 x 10-5 mole of sodium thiosulfate and 1.0 x 10-5 mole of sodium chIornaurate, both being based on mole of silver, and the mixture was aged at 450C. for 60 min. Subsequently, to the resulting mixture were added 4-hydroxy-6 methyl- 1 ,3,3a,7-tetraazaindene and 1 -phenyl-5-mercapto-tetrazole, and conventional film hardening agent and spreading agent were further added thereto just prior to being coated. The mixture was then coated on a support.

The amount to be coated was so adjusted to give 0.50 g./m2 for silver and 2.0 g./m2 for gelatin, based on the surface area of the support.

The coated sample was dried and subjected to the exposure at 160 CMS by means of a tungsten lamp (color temperature: 28500 K). Subsequently, the developing process was carried out in the following manner.

Treatment Treating time Treating temperature Developing 5 min. 00 sec. 380C.

Stopping 0 min. 30 sec. 380C.

Washing with water 3 min. 00 sec. 380C..

Developing solution Sodium polyphosphate 2.0 g.

Sodium hydrogen sulfite (anhydrous) 8.0 g.

Phenidone 0.35 g.

Sodium sulfite 37.0 g.

Hydroquinone 5.5 g.

Sodium carbonate 33.0 g.

Sodium bromide 1.3 g.

Sodium thiocyanide (10% aqueous solution) 13.8 ml Potassium iodide (0.1% aqueous solution) 13.0 rnl Add water to make one liter Adjust pH value to reach 9.9 i 1 Stopping solution Sodium hydroxide 1.75 g.

Glacial acetic acid 30.0 ml.

Add water to make one liter Adjust pH value to reach 3.8 The sample which had been subjected to the exposure at 160 CMS and treated as described in the above was analyzed by X-ray fluorescence analysis to determine a ratio of the amounts of silver bromide and silver iodide remaining undeveloped in the sample against the amounts thereof having been present in the sample prior to the processing. The results and a degree of coverage on silver iodide crystals observed on an electromicroscopic photograph are set forth in Table 6.

TABL 6

Ratio of silver halide Degree of coverage undeveloped upon Conditions of preparation on Agl observed on developing process Epitaxial Equivalent eiectromicroscopic EM No. pAg pH silver hallde crystal habit photograph Agl AgBr EM-1 - - - - 0 2 % (Agl) EM-2 10 6 AgBr (1 1 1) low 75 % 0 % EM-3 4 6 AgBr (1 0 0) high 33 % 0 % EM-4 2 6 AgBr (1 0 0) high 46 % 0 % EM-5 2 2 AgBr (1 1 1) low 72 % 0 % EM-6 6 6 AgBr (1 0 0) high 29 % 0 % EM-7 10 6 AgBr (1 1 1) low 81 % 0 % Note:Degree of coverage "low" means a coverage ranging from about 20 to 30% "high" means a coverage of not less than about 80% As is clear from the results in Table 6, the composite crystals prepared according to the present invention have the coverage on the silver iodide host crystals to less extents, and this means that the crystals prepared according to the present invention are the preferred crystals.

EXAMPLE 12 The EM-2, EM-6, EM-8 and EM-9 emulsions were sensitized chemically in the same manner as in Example 10 and, after addition of 3 x 10-4 mole of the following sensitizing dye (1) and 3.5 x 10-4 mole of the following sensitizing dye (II), based on mole of silver, the emulsions were subjected to spectral sensitization.

To the so sensitized emulsion were added 4-hydroxy-6-methyl-1 ,3,3a,7-tetraazaindene and 1 phenyl-5-mercaptotetrazole. Further added was an emulsion of 50 g. of 1 -(2,4,6-trichlorophenyl)-3- [ 3 (2,4-di-t-aminophenoxyacetamido)benzamido ] -5-pyrazolone, 10 g. of 1 -(2,4,6-trichlorophenyl)-4-(1 - naphthylazo)-3-(2-chloro-5-octadecenylsuccinimidanilino)-5-pyrazolone and 1.5 g. of 2-( 1 -phenyl-5- tetrazolylthio)-4-octadecylsuccinimid-1 -indanone in 60 g. of tricresyl phosphate.The resulting emulsion was so coated on a support consisting of a transparent cellulose triacetate base with substratum layer that the amount of silver was made to 10 g1m2. The so prepared sample was then subjected to wedge exposure through yellow filter in the same manner as described in Example 10, and developed in the manner set forth below.

Developing process Process (at 37.80 C.) Processing time Color developing 3 min. 15 sec.

Bleaching 6 min. 30 sec.

Washing with water 3 min. 15 sec.

Fixing 6 min. 30 sec.

Washing with water 3 min. 15 sec.

Stabilizing 1 min. 30 sec.

Drying Processing solutions employed in each of the above-described processes had the following compositions.

Colour developing solution 4Amino3methyl-N-ethyl-l'l-(- hydroxyethyl)-aniline sulfate 4.8 g.

Sodium sulfite (anhydrous) 0.14 g.

Hydroxylamine-F sulfate 1.98 9.

Sulfuric acid O.74g.

Potassium carbonate (anhydrous) 28.85 g.

Potassium hydrogen carbonate (anhydrous) 3.46 g.

Potassium sulfite (anhydrous) 5.10 g.

Potassium bromide 1.16 g.

Trisodium nitrotriacetate (monohydrate) 1.20 g.

Potassium hydroxide 1.48 g.

Add water to make one liter Bleaching solution Ammonium iron ethylenediamine tetraacetate 100.0 g.

Diammonium ethylenediamine tetraacetate 10.0 g.

Ammonium bromide 150.0 9.

Glacial acetic acid 10.0 ml.

Add water to make one liter Add aqueous ammonium to adjust pH 6.0 Fixing solution Ammonium thiosulfate 175.0 g.

Sodium sulfite (anhydrous) 8.6 g.

Sodium methasulfite 2.3 g.

Add water to make one liter Add acetic acid to adjust pH 6.0 Stabilizing solution Formalin 1.5 ml.

Konidax (manufactured by Konishlroku Photo Industry Co., Ltd., Japan) 7.5 ml.

Add water to make one liter The results obtained upon sensitometry are set forth in Table 7.

TABLE 7 With no preservation Preserved (3 days) Sample ' Relative Relative No. EM No. Fog sensitivity Dmax. Fog sensitivity Dmax.

6 EM-2 0.06 105 1.35 0.06 104 1.34 7 EM6 0.05 106 1.35 0.06 105 1.33 8 Elm8 0.07 100 1.20 0.12 80 1.00 9 EM-9 0.06 105 1.35 0.10 95 1.25 Note: Relative sensitivity was determined at Fog +0.10 As is clear from the results in Table 7, Samples No. 6 and No. 7 according to the present invention are better in the strage stability than Samples No. 8 and No. 9.

Claims (7)

1. A photographic silver halide emulsion which comprises composite silver halide crystals consisting essentially of multi-faceted silver iodide crystals and silver bromide or silver iodobromide crystals being combined with the multi-faceted silver iodide crystals through epitaxial junction, at least half of the facets of the multi-faceted silver iodide crystals being substantially free of epitaxial silver halide, and the epitaxial silver halide being limited to not more than 75 mole % based on the total silver halide forming the composite crystals.

2. A photographic silver halide emulsion as claimed in Claim 1, in which the silver iodide crystals comprise mainly truncated bipyramid p-phase silver iodide crystals.

3. A photographic silver halide emulsion as claimed in Claim 1, in which the silver iodide crystals have the minimum average grain diameter of at least 0.1 Su.

4. A photographic silver halide emulsion as claimed in Claim 1, in which the amount of the silver bromide or silver iodide ranges from 1 to 50 mole %, based on the total silver halide forming the composite crystals.

5. A process for preparing a silver halide emulsion comprising epitaxial composite silver halide crystals consisting essentially of silver iodide crystals and silver bromide crystals or silver iodobromide crystals containing not more than 12 mole % of silver iodide which are deposited on the siliver iodide crystals through epitaxial junction, which is characterized in that both of the under-mentioned-(a) solution and (b) solution, or both of the under-mentioned (a) solution and (c) solution are added to a solution containing a protective colloid and silver iodide crystals under the conditions for producing silver bromide or silver iodobromide crystals whose equivalent crystal habits essentially are (1 1 1) planes: : (a) A solution containing a water-soluble silver salt (b) A solution containing a water-soluble bromide (c) A solution containing either of a water-soluble bromide and a water-soluble iodide.

6. An emulsion as claimed in Claim 1 and substantially as hereinbefore described.

7. A process as claimed in Claim 5 and substantially as hereinbefore described.