Photothermographic element with core-shell-type silver halide grains

A negative-acting photothermographic element comprising a support bearing at least one heat-developable, photosensitive, image-forming photothermographic emulsion layer comprising: PA1 (a) core-shell-type, photosensitive silver halide grains containing a total silver iodide content of less than 4 mole %, the core having a tint silver iodide content of from about 4-14 mole %, the shell having a second silver iodide content lower than the silver iodide content of the core; PA1 (b) a non-photosensitive, reducible source of silver; PA1 (c) a reducing agent for the non-photosensitive, reducible source of silver; PA1 (d) a binder; and PA1 (e) at least one compound selected from the group consisting of: a halogen molecule; an organic haloamide; and hydrobromide acid salts of nitrogen-containing heterocyclic compounds which are further associated with a pair of bromine atoms.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photothermographic element and in particular, 
it relates to a photothermographic element containing core-shell-type 
silver halide grains. 
2. Background to the Art 
Silver halide containing photothermographic imaging materials (i.e., 
heat-developable photographic elements) processed with heat, and without 
liquid development, have been known in the art for many years. These 
materials, also known as "dry silver" compositions or emulsions, generally 
comprise a support having coated thereon: (1) a photosensitive material 
that generates elemental silver when irradiated; (2) a non-photosensitive, 
reducible silver source; 3) a reducing agent for the non-photosensitive 
reducible silver source; and (4) a binder. The photosensitive material is 
generally photographic silver halide which must be in catalytic proximity 
to the non-photosensitive, reducible silver source. Catalytic proximity 
requires an intimate physical association of these two materials so that 
when silver specks or nuclei are generated by the irradiation or light 
exposure of the photographic silver halide, those nuclei are able to 
catalyze the reduction of the reducible silver source. It has long been 
understood that elemental silver (Ag.degree.) is a catalyst for the 
reduction of silver ions, and the photosensitive photographic silver 
halide may be placed into catalytic proximity with the non-photosensitive, 
reducible silver source in a number of different fashions, such as by 
partial metathasis of the reducible silver source with a 
halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); 
coprecipitation of silver halide and reducible silver source material 
(see, for example, U.S. Pat. No. 3,839,049); and other methods that 
intimately associate the photosensitive photographic silver halide and the 
non-photosensitive, reducible silver source. 
In both photographic and photothermographic emulsions, exposure of the 
photographic silver halide to light produces small clusters of silver 
atoms (Ag.degree.). The imagewise distribution of these clusters is known 
in the art as a latent image. This latent image generally is not visible 
by ordinary means and the photosensitive emulsion must be further 
processed in order to produce a visible image. The visible image is 
produced by the reduction of silver ions, which are in catalytic proximity 
to silver halide grains beating the clusters of silver atoms, i.e. the 
latent image. This produces a black-and-white image. 
The non-photosensitive, reducible silver source is a material that contains 
silver ions. The preferred non-photosensitive, reducible silver source 
comprises silver salts of long chain aliphatic carboxylic acids, typically 
having from 10 to 30 carbon atoms. The silver salt of behenic acid or 
mixtures of acids of similar molecular weight are generally used. Salts of 
other organic acids or other organic materials, such as silver 
imidazolates, have been proposed, and U.S. Pat. No. 4,260,677 discloses 
the use of complexes of inorganic or organic silver salts as 
non-photosensitive, reducible silver sources. 
As the visible image is produced entirely by elemental silver (Ag.degree.), 
one cannot readily decrease the amount of silver in the emulsion without 
reducing the maximum image density. However, reduction of the amount of 
silver is often desirable in order to reduce the cost of raw materials 
used in the emulsion. 
One method of attempting to increase the maximum image density in 
black-and-white photographic and photothermographic emulsions without 
increasing the amount of silver in the emulsion layer is by incorporating 
toning agents into the emulsion. Toning agents improve the color of the 
silver image of the photothermographic emulsions, as described in U.S. 
Pat. Nos. 3,846,136; 3,994,732; and 4,021,249. 
Another method of increasing the maximum image density of photographic and 
photothermographic emulsions without increasing the amount of silver in 
the emulsion layer is by incorporating dye-forming materials in the 
emulsion. For example, color images can be formed by incorporation of 
leuco dyes into the emulsion. Leuco dyes are the reduced form of a 
color-bearing dye. Upon imaging, the leuco dye is oxidized, and the 
color-bearing dye and a reduced silver image are simultaneously formed in 
the exposed region. In this way a dye enhanced silver image can be 
produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 
4,426,441; 4,374,921; and 4,460,681. 
Multicolor photothermographic imaging articles typically comprise two or 
more monocolor-forming emulsion layers (often each emulsion layer 
comprises a set of bilayers containing the color-forming reactants) 
maintained distinct from each other by barrier layers. The barrier layer 
overlaying one photosensitive, photothermographic emulsion layer typically 
is insoluble in the solvent of the next photosensitive, photothermographic 
emulsion layer. Photothermographic articles having at least 2 or 3 
distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 
4,021,240 and 4,460,681. Various methods to produce dye images and 
multicolor images with photographic color couplers and leuco dyes are well 
known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 
3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 
1989, item 29963. 
With the increased availability of low-irradiance light sources such as 
light emitting diodes (LED), cathode ray tubes (CRT), and semi-conductor 
laser diodes, have come efforts to produce high-speed, photothermographic 
elements which require shorter exposure times. Such photothermographic 
systems would find use in, for example, conventional black-and-white or 
color photothermography, in electronically-generated black-and-white or 
color hardcopy recording, in graphic arts laser recording, for medical 
diagnostic laser imaging, in digital color proofing, and in other 
applications. 
Various techniques are typically employed to try and gain higher 
sensitivity in a photothermographic material. These techniques center 
around making the silver halide crystals' latent image centers more 
efficient such as by introducing imperfections into the crystal lattice or 
by chemical sensitization of the silver halide grains and by improving the 
sensitivity to particular wavelengths of light by formulating new improved 
sensitizing dyes or by the use of supersensitizers. 
In efforts to make more sensitive photothermographic materials, one of the 
most difficult parameters to maintain at a very low level is the various 
types of fog or D.sub.min. Fog is spurious image density which appears in 
non-developmentally sensitized areas of the element and is often reported 
in sensitometric results as D.sub.min. Photothermographic emulsions, in a 
manner similar to photographic emulsions and other light-sensitive 
systems, tend to suffer from fog. 
Traditionally, photothermographic materials have suffered from fog upon 
coating. The fog level of freshly prepared photothermographic elements 
will be referred to herein as initial fog or initial D.sub.min. 
In addition, the fog level of photothermographic elements often rises as 
the material is stored, or "ages." This type of fog will be referred to 
herein as shelf-aging fog. Adding to the difficulty of fog control on 
shelf-aging is the fact that the developer is incorporated in the 
photothermographic element. This is not the case in most silver halide 
photographic systems. A great amount of work has been done to improve the 
shelf-life characteristics of photothermographic materials. 
A third type of fog in photothermographic systems results from the 
instability of the image after processing. The photoactive silver halide 
still present in the developed image may continue to catalyze formation of 
metallic silver (known as "silver print-out") during room light handling 
or post-processing exposure such as in graphic arts contact frames. Thus, 
there is a need for post-processing stabilization of photothermographic 
materials. 
Without having acceptable resistance to fog, a commercially useful material 
is difficult to prepare. Various techniques have been employed to improve 
sensitivity and maintain resistance to fog. 
U.S. Pat. No. 4,212,937 describes the use of a nitrogen-containing organic 
base in combination with a halogen molecule or an organic haloamide to 
improve storage stability and sensitivity. 
Japanese Patent Kokai 61-129642, published Jun. 17, 1986, describes the use 
of halogenated compounds to reduce fog in color-forming photothermographic 
emulsions. These compounds include acetophenones such as 
phenyl(.alpha.,.alpha.-dibromobenzyl)ketone. 
U.S. Pat. No. 4,152,160 describes the use of carboxylic acids, such as 
benzoic acids and phthalic acids, in photothermographic elements. These 
acids are used as antifoggants. 
U.S. Pat. No. 3,589,903 describes the use of small amounts of mercuric ion 
in photothermographic silver halide emulsions to improve speed and aging 
stability. 
U.S. Pat. No. 4,784,939 describes the use of benzoic acid compounds of a 
defined formula to reduce fog and to improve the storage stability of 
silver halide photothermographic emulsions. The addition of halogen 
molecules to the emulsions are also described as improving fog and 
stability. 
U.S. Pat. No. 5,064,753 discloses a thermally-developable, photographic 
material containing core-shell-type silver halide grains that contain a 
total of 4-40 mole % of silver iodide and which have a lower silver iodide 
content in the shell than in the core. Incorporating silver iodide into 
the silver halide crystal in amounts greater than 4 mole % is reported to 
result in increased photosensitivity and reduced D.sub.min. The silver 
halide itself is the primary component reduced to silver metal during 
development. The shelf stability properties of the preferred formulations 
is not addressed. This material is primarily used for color applications. 
Japan Patent Kokai 63-300,234, published Dec. 7, 1988, discloses a 
heat-developable, photosensitive material containing a photosensitive 
silver halide, a reducing agent, and a binder. The photosensitive silver 
halide has a silver iodide content of 0.1-40 tool % and a core/shell grain 
structure. The photosensitive silver halide grains are further sensitized 
with gold. The material is reported to afford constructions with good 
sensitivity and low fog. 
Japan Kokai 62-103,634, published May, 14, 1987; Japan Kokai 62-150,240, 
published Jul. 4, 1987; and Japan Kokai 62-229,241, published Oct. 8, 
1987, describe heat-developable photosensitive materials incorporating 
core-shell grains with an overall iodide content greater that 4 mol %. 
U.S. Pat. No. 5,028,523 discloses radiation-sensitive, 
thermally-developable imaging elements comprising; a photosensitive silver 
halide; a light-insensitive silver salt oxidizing agent; a reducing agent 
for silver ion; and an antifoggant or speed enhancing compound comprising 
hydrobromic acid salts of nitrogen-containing heterocyclic compounds which 
are further associated with a pair of bromine atoms. These antifoggants 
are reported to be effective in reducing spurious background image 
density. 
SUMMARY OF THE INVENTION 
The present invention provides heat-developable, photothermographic 
elements capable of providing high photographic speed; stable, high 
density images of high resolution and good sharpness; and good shelf 
stability. 
It has now been discovered that core-shell-type silver halide grains with 
certain concentrations of silver iodide in the core and in the shell, when 
used in conjunction with either a halogen molecule; an organic haloamide 
compound; or compounds comprising hydrobromic acid salts of 
nitrogen-containing heterocyclic compounds which are further associated 
with a pair of bromine atoms, .give enhanced photothermographic properties 
when used as part of a preformed dry silver soap formulation. By 
controlling the amounts and ratio of silver iodide in both the core and 
the shell, significant improvement over non-core-shell type emulsions in 
sensitometric properties such as speed D.sub.min (i.e., lower initial 
fog), and shelf-life stability (i.e., shelf-aging fog) have been obtained 
while retaining the desired high sensitivity and D.sub.max. 
These negative-acting, heat-developable, photothermographic elements 
comprise a support bearing at least one photosensitive, image-forming, 
photothermographic emulsion layer comprising: 
(a) core-shell-type, photosensitive silver halide grains containing a total 
silver iodide content of less than 4 mole %, the core having a first 
silver iodide content of from about 4-14 mole %, the shell having a second 
silver iodide content lower than the silver iodide content of the core; 
(b) a non-photosensitive, reducible source of silver; 
(c) a reducing agent for the non-photosensitive, reducible source of 
silver; 
(d) a binder; and 
(e) at least one compound selected from the group consisting of: a halogen 
molecule; an organic haloamide compound; and hydrobromic acid salts of 
nitrogen-containing heterocyclic compounds which are further associated 
with a pair of bromine atoms. 
The reducing agent for the non-photosensitive, reducible source of silver 
may optionally comprise a compound capable of being oxidized to form or 
release a dye. Preferably, the dye-forming material is a leuco dye. 
The core-shell-type photosensitive type silver halide grains used in the 
present invention should have an overall silver iodide content of less 
than 4 mole %. The silver iodide content in the core grains is within the 
range of 4-14 mole %, and preferably, within the range of 6-10 mole %. For 
the silver halide composition of the shell, the silver iodide content is 
preferably within the range of 0-2 mole %. 
In contrast to the above-mentioned U.S. Pat. No. 5,064,753, the present 
invention provides a system based on core-shell-type silver halide acting 
only as a photosensitive catalyst for a non-photosensitive, reducible 
source of silver (such as silver behenate) which is reduced to become the 
primary source of metallic silver in the system. 
Other aspects, advantages, and benefits of the present invention are 
apparent from the detailed description, the examples, and the claims. 
DETAILED DESCRIPTION OF THE INVENTION 
The negative-acting photosensitive element of the present invention 
comprises a support having at least one photosensitive, image-forming, 
photothermographic emulsion layer comprising: 
(a) core-shell-type, photosensitive silver halide grains containing a total 
silver iodide content of less than 4 mole %, the core having a first 
silver iodide content of from about 4-14 mole %, the shell having a second 
silver iodide content lower than the silver iodide content of the core; 
(b) a non-photosensitive, reducible source of silver; 
(c) a reducing agent for the non-photosensitive, reducible source of 
silver; 
(d) a binder; and 
(e) at least one compound selected from the group consisting of a halogen 
molecule; an organic haloamide compound; or hydrobromic acid salts of 
nitrogen-containing heterocyclic compounds which are further associated 
with a pair of bromine atoms. 
The reducing agent for the non-photosensitive, reducible silver source may 
optionally comprise a compound capable of being oxidized to form or 
release a dye. Preferably, the dye forming material is a leuco dye. 
Improvements in photothermographic properties can be attained by utilizing 
core-shell-type (sometimes referred to as "layered") silver halide grains 
where the core contains 4-14 mole % silver iodide and the shell contains a 
lesser amount of silver iodide with the requirement that the total silver 
iodide contained in the silver halide grains is less than 4 mole %. 
Preferably, the core comprises up to 50 mole % of the total silver halide 
content in the silver halide grains. The grains may be grown by any 
variety of known procedures and to any grain size, however, it is 
preferable to grow grains that are less than 0.1 .mu.m (0.1 micron or 0.1 
micrometer). Grains of reduced size result in reduced haze and lower 
D.sub.min. When used with compounds comprising hydrobromic acid salts of 
nitrogen-containing heterocyclic compounds; a halogen molecule; or an 
organic haloamide compound which are further associated with a pair of 
bromine atoms, the present invention provides heat-developable, 
photothermographic elements capable of providing high photographic speed, 
stable, high density images of high resolution, good sharpness, and good 
shelf stability. 
The photothermographic elements of this invention may be used to prepare 
black-and-whim, monochrome, or full-color images. The photothermographic 
element of this invention can be used, for example, in conventional 
black-and-white or color photothermography, in electronically-generated 
black-and-white or color hardcopy recording, in the graphic arts laser 
recording, for medical diagnostic laser imaging, in digital color 
proofing, and in other applications. The element of this invention 
provides high photographic speed, provides strongly absorbing 
black-and-white or color images, and provides a dry and rapid process 
while possessing low D.sub.min. 
The Photosensive Core-Shell-Type Silver Halide 
The photosensitive silver halide grains used in the present invention are 
characterized by-their core-shell-type structure wherein the surface layer 
(such as in the form of a shell) has a lower silver iodide content than 
the internal phase or bulk (such as in the form of a core). If the silver 
content in the surface layer of the core-shell-type silver halide grains 
is higher than or equal to that in the internal phase, disadvantages such 
as increased D.sub.min and increased fog upon storage or shelf aging, (as 
often simulated by accelerated aging at elevated temperature) will occur. 
There is no particular limitation on the types of silver halides other than 
silver iodide in the core of the photosensitive silver halide grains, but 
preferable examples are silver iodobromide and silver chloroiodobromide. 
The difference in silver iodide content between the surface layer (shell) 
and internal phase (core) of a silver halide grain may be abrupt, so as to 
provide a distinct boundary, or diffuse so as to create a gradual 
transition from one phase to the other. 
The silver iodide-containing core of the photosensitive silver halide 
grains may be prepared by the methods described in various .references 
such as: P. Giafkides, Chimie et Physique Photographique, Paul Montel, 
1967; G.E Duffin. Photographic Emulsion Chemistry, The Focal Press, 1966; 
and V.L. Zelikman et al., Making and Coating Photographic Emulsions, The 
Focal Press, 1964. 
An emulsion of the core-shell-type silver halide grains used in the present 
invention may be prepared by first making cores from monodispersed 
photosensitive silver halide grains, then coating a shell over each of the 
cores. The term "monodispersed silver halide emulsion" as used in the 
present invention means an emulsion wherein the silver halide grains 
present have such a size distribution that the size variance with respect 
to the average particle size is not greater than the level specified 
below. An emulsion made of a photosensitive silver halide that consists of 
silver halide grains that are uniform in shape and which have small 
variance in grain size (this type of emulsion is hereinafter referred to 
as a "monodispersed emulsion") has a virtually normal size distribution 
and allows its standard deviation to be readily calculated. If the spread 
of size distribution (%) is defined by (standard deviation/ average grain 
size) .times.100, then the monodispersed photosensitive silver halide 
grains used in the present invention preferably have a spread of 
distribution of less than 15% and, more preferably, less than 10%. 
Monodispersed silver halide grains with desired sizes that serve as cores 
can be formed by using a "double-jet" method with the pAg being held at a 
constant level. In the double-jet method, the silver halide is formed by 
simultaneous addition of a silver source (such as silver nitrate) and a 
halide source (such as potassium chloride, bromide, or iodide) such that 
the concentration of silver (i.e., the pAg) is held at a constant level. 
Preparation of monodispersed silver halide grains using a double-jet 
method is described in Example 1 of this application. 
A silver halide emulsion comprising highly monodispersed photosensitive 
silver halide grains to serve as cores for the core-shell-type emulsion 
may be prepared by employing the method described in Japanese Patent 
Application No. 48521/1979. A shell is then allowed to grow continuously 
on each of the thus prepared monodispersed core grains in accordance with 
the method employed in making the monodispersed emulsion. As a result, a 
silver halide emulsion comprising the monodispersed core-shell-type silver 
halide grains suitable for use in the present invention is attained. 
While it suffices for the core-shell-type photosensitive silver halide 
grains used in the present invention to have a lower silver iodide content 
in the surface layer (shell) than in the internal phase (Core), the silver 
iodide content of the shell is preferably at least about 2-12 mole % lower 
than the silver iodide content of the core. The shell may be comprised of 
silver chloride, silver bromide, silver chlorobromide, or silver iodide. 
The average size of the core-shell-type photosensitive silver halide grains 
used in the present invention is not limited to any particular value, but 
is preferably less than 0.1 .mu.m in average diameter with the range of 
0.02 to 0.08 .mu.m being more preferable. 
The average size of the photosensitive core-shell-type silver halide grains 
is expressed by the average diameter if the grains are spherical and by 
the average of the diameters of equivalent circles for the projected 
images if the grains are cubic or in other non-spherical shapes. 
Grain size may be determined by any of the methods commonly employed in the 
art for particle size measurement. Representative methods are described by 
in "Particle Size Analysis," ASTM Symposium on Light Microscopy, R.P. 
Loveland, 1955, pp. 94-122; and in The Theory of the Photographic Process, 
C.E. Kenneth Mees and T.H. James, Third Edition, Chapter 2, Macmillan 
Company, 1966. Particle size measurements may be expressed in terms of the 
projected areas of grains or approximations of their diameters. These will 
provide reasonably accurate results if the grains of interest are 
substantially uniform in shape. 
Pre-formed core-shell-type silver halide emulsions in the element of this 
invention can be unwashed or washed to remove soluble salts. In the latter 
case the soluble salts can be removed by chill-setting and leaching or the 
emulsion can be coagulation washed, e.g., by the procedures described in 
Hewitson, et al., U.S. Pat. No. 2,618,556; Yutzy et al., U.S. Pat. No. 
2,614,928; Yackel, U.S. Pat. No. 2,565,418; Hart et at., U.S. Pat. No. 
3,241,969; and Waller et at., U.S. Pat. No. 2,489,341. The silver halide 
grains may have any crystalline habit including, but not limited to, 
cubic, tetrahedral, orthorhombic, tabular, laminar, platelet, etc. 
The shape of the photosensitive core-shell-type silver halide grains of the 
present invention is in no way limited; they may be normal crystals (such 
as cubes, tetradecahedrons and octahedrons), twinned, or tabular. If 
desired, a mixture of these crystals may be employed. 
The light sensitive core-shell-type silver halide used in the present 
invention can be employed in a range of 0.005 mol to 0.5 mol and, 
preferably, from 0.01 mol to 0.15 mol, per mole of non-photosensitive 
reducible source of silver. The silver halide may be added to the emulsion 
layer in any fashion which places it in catalytic proximity to the 
non-photosensitive reducible source of silver. 
Addition of sensitizing dyes to the core-shell-type silver halides of this 
invention serves to provide them with high sensitivity to visible and 
infra-red light by spectral sensitization. The photosensitive silver 
halides may be spectrally sensitized with various known dyes that 
spectrally sensitize silver halide. Sensitization may be in the visible or 
infra-red. Non-limiting examples of sensitizing dyes that can be employed 
include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex 
merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, 
and hemioxanol dyes. Of these dyes, cyanine dyes, merocyanine dyes, and 
complex merocyanine dyes are particularly useful. 
An appropriate amount of sensitizing dye added is generally in the range of 
from about 10.sup.-10 to 10.sup.-1 mole, and preferably from about 
10.sup.-8 to 10.sup.-3 moles, per mole of silver halide. 
The Non-Photosensitive Reducible Silver Source Material 
As noted above, the non-photosensitive silver salt which can be used in the 
present invention is a silver salt which is comparatively stable to light, 
but forms a silver image when heated to 80.degree. C. or higher in the 
presence of an exposed photocatalyst (such as silver atoms) and a reducing 
agent. 
Silver salts of organic acids, particularly silver salts of long chain 
fatty carboxylic acids, are preferred. The chains typically contain 10 to 
30, preferably 15 to 28, carbon atoms. Suitable organic silver salts 
include silver salts of organic compounds having a carboxyl group. 
Preferred examples thereof include a silver salt of an aliphatic 
carboxylic acid and a silver salt of an aromatic carboxylic acid. 
Preferred examples of the silver salts of aliphatic carboxylic acids 
include silver behenate, silver stearate, silver oleate, silver laurate, 
silver caprate, silver myristate, silver palmitate, silver maleate, silver 
fumarate, silver tartarate, silver furoate, silver linoleate, silver 
butyrate, silver camphorate, and mixtures thereof, etc. Silver salts which 
are substitutable with a halogen atom or a hydroxyl group can also be 
effectively used. Preferred examples of the silver salts of aromatic 
carboxylic acids and other carboxyl group-containing compounds include 
silver benzoate, a silver-substituted benzoate such as silver 
3,5-dihydroxybenzoate, silver o-methylbenzoate, silver m-methylbenzoate, 
silver p-methylbenzoate, silver 2,4-dichlorobenzoate, silver 
acetamidobenzoate, silver p-phenylbenzoate, etc., silver gallate, silver 
tannate, silver phthalate, silver terephthalate, silver salicylate, silver 
phenylacetate, silver pyromellilate, a silver salt of 
3-carboxymethyl-4-methyl-4-thiazoline-2-thione or the like as described in 
U.S. Pat. No. 3,785,830, and silver salt of an aliphatic carboxylic acid 
containing a thioether group as described in U.S. Pat. No. 3,330,663. 
Silver salts of compounds containing mercapto or thione groups and 
derivatives thereof can be used. Preferred examples of these compounds 
include a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, a silver salt 
of 2-mercaptobenzimidazole, a silver salt of 
2-mercapto-5-aminothiadiazole, a silver salt of 
2-(2-ethylglycolamido)benzothiazole, a silver salt of thioglycolic acid 
such as a silver salt of a S-alkylthioglycolic acid (wherein the alkyl 
group has from 12 to 22 carbon atoms) as described in Japanese patent 
application No. 28221/73, a silver salt of a dithiocarboxylic acid such as 
a silver salt of dithioacetic acid, a silver salt of thioamide, a silver 
salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silver salt of 
mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, a silver salt as 
described in U.S. Pat. No. 4,123,274, for example, a silver salt of 
1,2,4-mercaptothiazole derivative such as a silver salt of 
3-amino-5-benzylthio-1,2,4-thiazole, or a silver salt of a thione compound 
such as a silver salt of 3-(2-carboxyethyl)-4-methyl-4 
-thiazoline-2-thione as disclosed in U.S. Pat. No. 3,201,678. 
Furthermore, a silver salt of a compound containing an imino group can be 
used. Preferred examples of these compounds include a silver salt of 
benzothiazole and a derivative thereof as described in Japanese patent 
publications Nos. 30270/69 and 18146/70, for example, a silver salt of 
benzothiazole such as silver salt of methylbenzotriazole, etc., a silver 
salt of a halogen-substituted benzotriazole, such as a silver salt of 
5-chlorobenzotriazole, etc., a silver salt of 1,2,4-triazole, of 
1H-tetrazole as described in U.S. Pat. No. 4,220,709, a silver salt of 
imidazole and an imidazole derivative, and the like. 
It is also convenient to use silver half soaps, of which an equimolar blend 
of silver behenate and behenic acid, prepared by precipitation from 
aqueous solution of the sodium salt of commercial behenic acid and 
containing about 14.5% silver, represents a preferred example. Transparent 
sheet materials made on transparent film backing require a transparent 
coating and for this purpose the silver behenate full soap, containing not 
more than about 4 or 5 wt % of free behenic acid and containing about 25.2 
wt % silver may be used. 
The method used for making silver soap dispersions is known in the art and 
is disclosed in Research Disclosure, April 1983, item no 22812; Research 
Disclosure, October 1983, item no. 23419; and U.S. Pat. No. 3,985,565. 
The core-shell-type silver halide and the organic silver salt which are 
separately formed in a binder can be mixed prior to use to prepare a 
coating solution, but it is also effective to blend or homogenize them in 
an homogenizer for a long period of time. Further, it is also effective to 
use a process which comprises adding a halogen-containing compound to the 
core-shell-type silver halide and the organic silver salt prepared to 
partially convert the silver of the organic silver salt to silver halide. 
Methods of preparing these silver halide and organic silver salts and 
manners of blending them are described in Research Disclosure, No. 17029, 
Japanese Patent Applications No. 32928/75 and 42529/76, U.S. Pat. No. 
3,700,458, and Japanese Patent Applications Nos. 13224/74 and 17216/75. 
The silver halide and the non-photosensitive reducible silver source 
material that form a starting point of development should be in "reactive 
association." By "reactive association" is meant that they should be in 
the same layer, in adjacent layers, or in layers separated from each other 
by an intermediate layer having a thickness of less than 1 micrometer (1 
.mu.m). It is preferred that the silver halide and the non-photosensitive 
reducible silver source material be present in the same layer. 
Photothermographic emulsions containing preformed silver halide in 
accordance with this invention can be sensitized with spectral sensitizers 
as described above. 
The source of reducible silver material generally constitutes from 15 to 
70% by weight of the emulsion layer. It is preferably present at a level 
of 30 to 55% by weight of the emulsion layer. 
The Reducing Agent for the Non-Photosensitive Reducible Silver Source 
The reducing agent for the organic silver salt may be any material, 
preferably organic material, that can reduce silver ion to metallic 
silver. Conventional photographic developers such as phenidone, 
hydroquinones, and catechol are useful, but hindered phenol reducing 
agents are preferred. 
A wide range of reducing agents has been disclosed in dry silver systems 
including amidoximes such as phenylamidoxime, 2-thienylamidoxime and 
p-phenoxyphenylamidoxime, azines (e.g., 
4-hydroxy-3,5-dimethoxybenzaldehydeazine); a combination of aliphatic 
carboxylic acid aryl hydrazides and ascorbic acid, such as 
2,2'-bis(hydroxymethyl)propionylbetaphenyl hydrazide in combination with 
ascotbit acid; a combination of polyhydroxybenzene and hydroxylamine, a 
reductone and/or a hydrazine, e.g., a combination of hydroquinone and 
bis(ethoxyethyl)hydroxylamine, piperidinohexose reductone or 
formyl-4-methylphenylhydrazine, hydroxamic acids such as phenylhydroxamic 
acid, p-hydroxyphenylhydroxamic acid, and o-alaninehydroxamic acid; a 
combination of azines and sulfonamidophenols, e.g., phenothiazine and 
2,6-dichloro-4-benzenesulfonamidophenol; .alpha.-cyanophenylacetic acid 
derivatives such as ethyl .alpha.-cyano-2-methylphenylacetate, ethyl 
.alpha.-cyano-phenylacetate; bis-o-naphthols as illustrated by 
2,2'-dihydroxyl-1-binaphthyl, 6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, 
and bis(2-hydroxy-1-naphthyl)methane; a combination of bis-o-naphthol and 
a 1,3-dihydroxybenzene derivative, (e.g., 2,4-dihydroxybenzophenone or 
2,4-dihydroxyacetophenone); 5-pyrazolones such as 
3-methyl-1-phenyl-5-pyrazolone; reductones as illustrated by 
dimethylaminohexose reductone, anhydrodihydroaminohexose reductone, and 
anhydrodihydro-piperidone-hexose reductone; sulfamidophenol reducing 
agents such as 2,6-dichloro-4-benzenesulfonamidophenol, and 
p-benzenesulfonamidophenol; 2-phenylindane-1,3 -dione and the like; 
chromans such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman; 
1,4-dihydropyridines such as 
2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine; bisphenols, e.g., 
bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane; 
2,2-bis(4-hydroxy-3-methylphenyl)propane; 
4,4-ethylidene-bis(2-t-butyl-6-methylphenol); and 
2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane;ascorbic acid derivatives, 
e.g., 1-ascorbylpalmitate, ascorbylstearate and unsaturated aldehydes and 
ketones; 3-pyrazolidones; and certain indane-1,3-diones. 
The reducing agent should be present as 1 to 10% by weight of the imaging 
layer. In multilayer constructions, if the reducing agent is added to a 
layer other than an emulsion layer, slightly higher proportions, of from 
about 2 to 15%, tend to be more desirable. 
The Optional Dye-Forming or Dye-Releasing Material 
As noted above, the reducing agent for the reducible source of silver may 
be a compound that can be oxidized directly or indirectly to form or 
release a dye. 
The dye-forming or releasing material may be any colorless or lightly 
colored compound that can be oxidized to a colored form, when heated, 
preferably to a temperature of from about 80.degree. C. to about 
250.degree. C. (176.degree. F. to 482.degree. F.) for a duration of from 
about 0.5 to about 300 seconds. When used with a dye- or image-receiving 
layer, the dye can diffuse through emulsion layers and interlayers into 
the image-receiving layer of the dement of the invention. 
Leuco dyes are one class of dye-releasing material that form a dye upon 
oxidation. Any leuco dye capable of being oxidized by silver ion to form a 
visible image can be used in the present invention. Leuco dyes that are 
both pH sensitive and oxidizable can be used, but are not preferred. Leuco 
dyes that are sensitive only to changes in pH are not included within 
scope of dyes useful in this invention because they are not oxidizable to 
a colored form. 
As used herein, the term "change in color" includes: (1) a change from an 
uncolored or lightly colored state (optical density less than 0.2) to a 
colored state (an increase-in optical density of at least 0.2 units); and 
(2) a substantial change in hue. 
Representative classes of leuco dyes that are suitable for use in the 
present invention include, but are not limited to, bisphenol and 
bisnaphthol leuco dyes, phenolic leuco dyes, indoaniline leuco dyes, 
imidazole leuco dyes, azine leuco dyes, oxazine leuco dyes, diazine leuco 
dyes, and thiazine leuco dyes. These classes of dyes are described in U.S. 
Pat. Nos. 4,460,681 and 4,594,307. 
One class of leuco dyes useful in this invention are those derived from 
imidazole dyes. Imidazole leuco dyes are described in U.S. Pat. No. 
3,985,565. 
Another class of leuco dyes useful in this invention are those derived from 
so-called "chromogenic dyes." These dyes are prepared by oxidative 
coupling of a p-phenylenediamine with a phenolic or anilinic compound. 
Leuco dyes of this class are described in U.S. Pat. No. 4,594,307. Leuco 
chromogenic dyes having short chain carbamoyl protecting groups are 
described in copending application U.S. Ser. No. 07/939,093, incorporated 
herein by reference. 
A third class of dyes useful in this invention are "aldazine" and 
"ketazine" dyes. Dyes of this type are described in U.S. Pat. Nos. 
4,587,211 and 4,795,697. 
Another class of leuco dyes are reduced forms of dyes having a diazine, 
oxazine, or thiazine nucleus. Leuco dyes of this type can be prepared by 
reduction and acylation of the color-bearing dye form. Methods of 
preparing leuco dyes of this type are described in Japanese Patent No. 
52-89131 and U.S. Pat. Nos. 2,784,186; 4,439,280; 4,563,415; 4,570,171; 
4,622,395; and 4,647,525. 
Another class of dye-releasing materials that form a dye upon oxidation are 
known as preformed-dye-release (PDR) or redox-dye-release (RDR) materials. 
In these materials, the reducing agent for the organic silver compound 
releases a preformed dye upon oxidation. Examples of these materials are 
disclosed in Swain, U.S. Pat. No. 4,981,775. 
Also Useful are neutral, phenolic leuco dyes such as 
2-(3,5-di-t-butyl-4-hydroxyphenyl)-4,5-diphenylimidazole, or 
bis(3,5-di-t-butyl-4-hydroxy-phenyl)phenylmethane. Other phenolic leuco 
dyes useful in practice of the present invention are disclosed in U.S. 
Pat. Nos. 4,374,921; 4,460,681; 4,594,307; and 4,782,010. 
Other leuco dyes may be used in imaging layers as well, for example, 
benzylidene leuco compounds cited in U.S. Pat. No. 4,923,792. The reduced 
form of the dyes should absorb less strongly in the visible region of the 
electromagnetic spectrum and be oxidized by silver ions back to the 
original colored form of the dye. Benzylidene dyes have extremely sharp 
spectral characteristics giving high color purity of low gray level. The 
dyes have large extinction coefficients, typically on the order of 
10.sup.4 to 10.sup.5 mole-era liter.sup.-1, and possess good compatibility 
and heat stability. The dyes are readily synthesized and the reduced leuco 
forms of the compounds are very stable. 
Leuco dyes such as those disclosed in U.S. Pat. Nos. 3,442,224; 4,021,250; 
4,022,617; and 4,368,247 are also useful in the present invention. 
The dyes formed from the leuco dye in the various color-forming layers 
should, of course, be different. A difference of at least 60 nm in 
reflective maximum absorbance is preferred. More preferably, the 
absorbance maximum of dyes formed will differ by at least 80-100 nm. When 
three dyes are to be formed, two should preferably differ by at least 
these minimums, and the third should preferably differ from at least one 
of the other dyes by at least 150 nm, and more preferably, by at least 200 
nm. Any leuco dye capable of being oxidized by silver ion to form a 
visible dye is useful in the present invention as previously noted. 
The dyes generated by the leuco compounds employed in the elements of the 
present invention are known and are disclosed, for example, in The Colour 
Index; The Society of Dyes and Colourists: Yorkshire, England, 1971; Vol. 
4, p. 4437; and Venkataraman, K. The Chemistry of Synthetic Dyes; Academic 
Press: New York, 1952; Vol. 2, p. 1206; and U.S. Pat. No. 4,478,927. 
Leuco dye compounds may readily be synthesized by techniques known in the 
art. Suitable methods are disclosed, for example, in: F.X. Smith et at. 
Tetrahedron Lett. 1983, 24(45), 4951-4954; X. Huang., L. Xe, Synth. 
Commun. 1986, 16(13) 1701-1707; H. Zimmer et al. J. Org. Chem. 1960, 25, 
1234-5; M. Sekiya et al. Chem. Pharm. Bull. 1972, 20(2),343; and T. Sohda 
et al. Chem. Pharm. Bull. 1983, 31(2) 560-5; H.A. Lubs The Chemistry of 
Synthetic Dyes and Pigments; Hafner; New York, NY; 1955 Chapter 5; in H. 
Zollinger Color Chemistry: Synthesis, Properties and Applications of 
Organic Dyes and Pigments; VCH; New York, NY; pp. 67-73, 1987, and in U.S. 
Pat. No. 5,149,807; and EPO Laid Open Application No. 0,244,399. 
Further, as other image-forming materials, materials where the mobility of 
the compound having a dye part changes as a result of an 
oxidation-reduction reaction with silver halide, or an organic silver salt 
at high temperature can be used, as described in Japanese Patent 
Application No. 165054 (1984). Many of the above-described materials are 
materials wherein an image-wise distribution of mobile dyes corresponding 
to exposure is formed in the photosensitive material by heat development. 
Processes of obtaining visible images by transferring the dyes of the 
image to a dye-fixing material (diffusion transfer) have been described in 
the above-described cited patents and Japanese Patent Application Nos. 
168,439 (1984) and 182,447 (1984). 
Still further the reducing agent may be a compound that releases a 
conventional photographic dye coupler or developer on oxidation as is 
known in the art. When the heat developable, photosensitive element used 
in this invention is heat developed in a substantially water-free 
condition after or simultaneously with imagewise exposure, a mobile dye 
image is obtained simultaneously with the formation of a silver image 
either in exposed areas or in unexposed areas with exposed photosensitive 
silver halide. 
The total amount of optional leuco dye used as a reducing agent utilized in 
the present invention should preferably be in the range of 0.5-25 weight 
percent, and more preferably, in the range of 1-10 weight percent, based 
upon the total weight of each individual layer in which the reducing agent 
is employed. 
The Binder 
The photosensitive core-shell-type silver halide and the organic silver 
salt oxidizing agent used in the present invention are generally added to 
at least one binder as described herein below. 
The binder(s) that can be used in the present invention can be employed 
individually or in combination with one another. It is preferred that the 
binder be selected from polymeric materials, such as, for example, natural 
and synthetic resins and that the binder be sufficiently polar to hold the 
other ingredients of the emulsion in solution or suspension. The binder 
may be hydrophilic or hydrophobic. 
A typical hydrophilic binder is a transparent or translucent hydrophilic 
colloid, examples of which include a natural substance, for example, a 
protein such as gelatin, a gelatin derivative, a cellulose derivative, 
etc.; a polysaccharide such as starch, gum arabic, pullulan, dextrin, 
etc.; and a synthetic polymer, for example, a water-soluble polyvinyl 
compound such as polyvinyl alcohol, polyvinyl pyrrolidone, acrylamide 
polymer, etc. Another example of a hydrophilic binder is a dispersed vinyl 
compound in latex form which is used for the purpose of increasing 
dimensional stability of a photographic element. 
Examples of typical hydrophobic binders are polyvinyl acetals, polyvinyl 
chloride, polyvinyl acetate, cellulose acetate, polyolefins, polyesters, 
polystyrene, polyacrylonitrile, polycarbonates, methacrylate copolymers, 
maleic anhydride ester copolymers, butadiene-styrene copolymers, and the 
like. Copolymers, e.g. terpolymers, are also included in the definition of 
polymers. The polyvinyl acetals, such as polyvinyl butyral and polyvinyl 
formal, and vinyl copolymers such as polyvinyl acetate and polyvinyl 
chloride are particularly preferred. The binders can be used individually 
or in combination with one another. Although the binder may be hydrophilic 
or hydrophobic, it is preferrably hydrophobic. 
The binders are generally used at a level of from about 20 to about 80% by 
weight of the emulsion layer, and preferably, from about 30 to about 55% 
by weight. Where the proportions and activities of leuco dyes require a 
particular developing time and temperature, the binder should be able to 
withstand those conditions. Generally, it is preferred that the binder not 
decompose or lose its structural integrity at 200.degree. F. (90.degree. 
C.) for 30 seconds, and more preferred that it not decompose or lose its 
structural integrity at 300.degree. F. (149.degree. C.) for 30 seconds. 
Optionally, these polymers may be used in combination of two or more 
thereof. Such a polymer is used in an amount sufficient to carry the 
components dispersed therein, that is, within the effective range of the 
action as the binder. The effective range can be appropriately determined 
by one skilled in the art. 
The generation of fog in photothermographic elements comprising a 
core-shell-type, photosensitive silver halide; a non-photosensitive, 
reducible source of silver; a reducing agent for the non-photosensitive, 
reducible source of silver; and a binder, can be further reduced by the 
addition of a fog-reducing amount of hydrobromic acid salts of 
nitrogen-containing heterocyclic ring compounds which are further 
associated with a pair of bromine atoms; a halogen molecule; or an organic 
haloamide. 
The Hydrobromic Acid Salt of Nitrogen-Containing Heterocyclic Compounds 
which are Further Associated with a Pair of Bromine Atoms. 
The central nucleus of the nitrogen-containing heterocyclic compounds of 
the present invention may be represented by any of the following formulae: 
##STR1## 
in which Q represents the atoms (preferably selected from C, S, N, Se, and 
O, more preferably C, N, and O) necessary to complete a 5-, 6-, or 
7-membered heterocyclic ring group. This ring group may be monocyclic or 
polycyclic (especially bicyclic, with a fused-on benzene ring). The 
heterocyclic ring group may be unsubstituted or further substituted with 
such moieties as alkyl, alkoxy, and aryl groups, halogen atoms, hydroxy 
groups, cyano groups, nitro groups, and the like. Exemplary and preferred 
nitrogen-containing heterocyclic ring compounds include pyridine, 
pyrolidone, and pyrrolidinone. Other useful heterocyclic ring groups 
include, but are not limited to, pyrrolidines, phthalazinone, phthalazine, 
etc. 
Preferred structures for use in the practice of the present invention may 
be defined by the formulae: 
##STR2## 
and the like, wherein each possible R group is independently selected form 
substituents such as alkyl groups, alkoxy groups, hydrogen, halogen, aryl 
groups (e.g., phenyl, naphthyl, thienyl, etc.) nitro, cyano, and the like. 
R substituents on adjacent positions may form fused ring groups so that 
formula (1) above would in fact be inclusive of formulae (2) and (4). n is 
zero or a whole positive integer such as 1, 2, 3, or4. 
These compounds are used in general amounts of at least 0.005 mol per mole 
of silver halide in the emulsion layer. Usually the range is between 0.005 
and 1.0 mol of the compound per mol of silver halide and preferably 
between 0.01 and 0.3 mol of antifoggant per mol of silver. 
The Halogen Molecule 
The halogen molecules which can be employed in this invention include 
iodine molecule, bromine molecule, iodine monochloride and iodine 
trichloride, iodine bromide and bromine chloride. The bromine chloride is 
preferably used in the form of a hydrate which is solid. 
The term "halogen molecule" as used herein includes not only the 
above-described halogen molecules, but also complexes of a halogen 
molecule, for example, complexes of a halogen molecule with p-dioxane 
which are generally void. Of the halogen molecules that can be used in 
this invention, iodine molecule which is solid under normal conditions is 
especially preferred. 
The Organic Haloamide Compounds 
The organic haloamide compounds which can be employed in this invention 
include, for example, N-chlorosuccinimide, N-bromosuccinimide, 
N-iodosucinimide, N-chlorophthalimide, N-bromophthalimide, 
N-iodophthalimide, N-chlorophthalazinone, N-bromophthalazinone, 
N-iodophthalazinone, N-chloroacetamide, N-bromoacetamide, N-iodoacetamide, 
N-chloroacetanilide, N-bromoacetanilide, N-iodoacetanilide, 
1-chloro-3,5,5,-trimethyl-2,4-imidazolidinedione, 
1-bromo-3,5,5,-trimethyl-2,4-imidazonidinedionel-iodo-3,5,5,-trimethyl-2,4 
-imidazolidinedione, 1,3-di-chloro-5,5-dimethyl-2,4-imidazolidinedione, 
1,3-dibromo-5,5-dimethyl-2,4-imidazolidinedione, 
1,3-dibromo-5,5-dimethylimidazolidinedione, 
N,N-dichlorobenzenesulfonamide, N,N-dibromobenzenesulfonamide, 
N-bromo-N-methylbenzenesulfonamide, N-chloro-N-methylbenzenesulfonamide, 
N,N-diiodobenzenesulfonamide, N-iodo-N-methylbenzenesulfonamide, 
1,3-dichloro-4,4-dimethylhydantoin, 1,3-dibromo-4,4-dimethylhydantoin, and 
1,3-diiodo-4,4-dimethylhydantoin. 
In general, the halogen molecules are more effective for improving both the 
sensitivity and the storage stability of the photosensitive materials than 
the organic haloamide compounds. The amount of the halogen molecules or 
the organic haloamide compounds typically ranges from about 0.001 mole to 
about 0.5 mole, and preferably from about 0.01 mole to about 0.2 mole, 
based on the mole of the organic silver salt oxidizing agent. 
Dry Silver Formulations 
The formulation for the photothermographic emulsion layer can be prepared 
by dissolving and dispersing the binder, the photosensitive 
core-shell-type silver halide, the non-photosensitive source of reducible 
silver, the reducing agent for the non-photosensitive reducible silver 
source (such as, for example, the optional leuco dye), and optional 
additives, in an inert organic solvent, such as, for example, toluene, 
2-butanone, or tetrahydrofuran. 
The use of "toners" or derivatives thereof which improve the image, is 
highly desirable, but is not essential to the element. Toners may be 
present in amounts of from 0.01 to 10 percent by weight of the emulsion 
layer, preferably from 0.1 to 10 percent by weight. Toners are well known 
materials in the photothermographic art as shown in U.S. Pat. Nos. 
3,080,254; 3,847,612; and 4,123,282. 
Examples of toners include phthalimide and N-hydroxyphthalimide; cyclic 
imides such as succinimide, pyrazoline-5-ones, and a quinazolinone, 
1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, quinazoline and 
2,4-thiazolidinedione; naphthalimides such as N-hydroxy-1,8-naphthalimide; 
cobalt complexes such as cobaltic hexamine trifluoroacetate; mercaptans as 
illustrated by 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 
3-mercapto-4,5-diphenyl-1,2,4-triazole and 
2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboximides, e.g. 
(N,N-dimethylaminomethyl)-phthalimide, and 
N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide; and a combination of 
blocked pyrazoles, isothiuronium derivatives and certain photobleach 
agents, e.g., a combination of 
N,N'-hexamethylenebis(1-carbamoyl-3,5-dimethylpyrazole), 
1,8-(3,6-diaza-octane)bis(isothiuronium)trifluoroacetate and 
2-(tribromomethylsulfonyl benzothiazole); and merocyanine dyes such as 
3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methyl-ethylidenel-2-thio-2 
,4-o-azolidinedione; phthalazinone, phthalazinone derivatives or metal 
salts or these derivatives such as 4-(1-naphthyl)phthalazinone, 
6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 
2,3-dihydro-1,4-phthalazinedione; a combination of phthalazine plus one or 
more phthalic acid derivatives, e.g., phthalic acid, 4-methylphthalic 
acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride; 
quinazolinediones, benzoxazine or naphthoxazine derivatives; rhodium 
complexes functioning not only as tone modifiers but also as sources of 
halide ion for silver halide formation in situ, such as ammonium 
hexachlorodate (IlI), rhodium bromide, rhodium nitrate and potassium 
hexachlorodate (III); inorganic peroxides and persulfates, e.g., ammonium 
peroxydisulfate and hydrogen peroxide; benzoxazine-2,4-diones such as 
1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and 
6-nitro-1,3-benzoxazine-2,4-dione; pyrimidines and asym-triazines, e.g., 
2,4-dihydroxypyrimidine, 2-hydroxy-4-aminopyrimidine, and azauracil, and 
tetrazapentalene derivatives, e.g., 3,6-dimercapto-1,4-diphenyl-1H, 
4H-2,3a,5,6a-tetrazapentalene, and 
1,4-di(o-chlorophenyl)-3,6-dimercapto-1H, 4H-2,3a,5,6 a-tetrazapentalene. 
Photothermographic emulsions used in this invention may be further 
protected against the additional production of fog and can be stabilized 
against loss of sensitivity during keeping. While not necessary for the 
practice of the invention, it may be advantageous to add mercury (II) 
salts to the emulsion layer(s) as an antifoggant. Preferred mercury (II) 
salts for this purpose are mercuric acetate and mercuric bromide. 
Suitable antifoggants and stabilizers, which can be used alone or in 
combination, include the thiazolium salts described in U.S. Pat. Nos. 
2,131,038 and U.S. Pat. No. 2,694,716; the azaindenes described in U.S. 
Pat. Nos. 2,886,437 and 2,444,605; the mercury salts described in U.S. 
Pat. No. 2,728,663; the urazoles described in U.S. Pat. No. 3,287,135; the 
sulfocatechols described in U.S. Pat. No. 3,235,652; the oximes described 
in British Patent No. 623,448; the polyvalent metal salts described in 
U.S. Pat. No. 2,839,405; the thiuronium salts described in U.S. Pat. No. 
3,220,839; and palladium, platinum and gold salts described in U.S. Pat. 
Nos. 2,566,263 and 2,597,915. 
Emulsions used in the invention may contain plasticizers and lubricants 
such as polyalcohols, e.g., glycerin and diols of the type described in 
U.S. Pat. No. 2,960,404; fatty acids or esters such as those described in 
U.S. Pat. Nos. 2,588,765 and 3,121,060; and silicone resins such as those 
described in British Patent No. 955,061. 
The photothermographic elements of the present invention may include image 
dye stabilizers. Such image dye stabilizers are illustrated by U.K. Patent 
No. 1,326,889; and U.S. Pat. Nos. 3,432,300; 3,698,909; 3,574,627; 
3,573,050; 3,764,337; and 4,042,394. 
Photothermographic elements according to the present invention can be used 
in photographic elements which contain light-absorbing materials and 
filter dyes such as those described in U.S. Pat. Nos. 3,253,921; 
2,274,782; 2,527,583; and 2,956,879. If desired, the dyes can be 
mordanted, for example, as described in U.S. Pat. No. 3,282,699. 
Photothermographic elements described herein may contain matting agents 
such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads 
including beads of the type described in U.S. Pat. Nos. 2,992,101 and 
2,701,245. 
Emulsions in accordance with this invention may be used in 
photothermographic elements which contain antistatic or conducting layers, 
such as layers that comprise soluble salts, e.g., chlorides, nitrates, 
etc., evaporated metal layers, ionic polymers such as those described in 
Minsk, U.S. Pat. Nos. 2,861,056, and 3,206,312 or insoluble inorganic 
salts such as those described in Trevoy, U.S. Pat. No. 3,428,451. 
Photothermographic Constructions 
The photothermographic dry silver elements of this invention may be 
constructed of one or more layers on a substrate. Single layer 
constructions should contain the silver source material, the 
core-shell-type silver halide, the developer, and at least one compound 
selected from the group consisting of: hydrobromic acid salts of 
nitrogen-containing heterocyclic compounds which are further associated 
with a pair of bromine atoms; a halogen molecule; or an organic haloamide; 
and binder as well as optional materials such as toners, dye-forming 
materials, coating aids, and other adjuvants. Two-layer constructions 
should contain the silver source and silver halide in one emulsion layer 
(usually the layer adjacent to the substrate) and some of the other 
ingredients in the second layer or both layers, although two layer 
constructions comprising a single emulsion layer coating containing all 
the ingredients and a protective topcoat are envisioned. Multicolor 
photothermographic dry silver constructions may contain sets of these 
bilayers for each color or they may contain all ingredients within a 
single layer as described in U.S. Pat. No. 4,708,928. In the case of 
multilayer, multicolor photothermographic elements, the various emulsion 
layers are generally maintained distinct from each other by the use of 
functional or non-functional barrier layers between the various 
photosensitive layers as described in U.S. Pat. No. 4,460,681. 
Photothermographic emulsions used in this invention can be coated by 
various coating procedures including wire wound rod coating, dip coating, 
air knife coating, curtain coating, or extrusion coating using hoppers of 
the type described in U.S. Pat. No. 2,681,294. If desired, two or more 
layers may be coated simultaneously by the procedures described in U.S. 
Pat. No. 2,761,791 and British Patent No. 837,095. Typical wet thickness 
of the emulsion layer can range from about 10 to about 100 micrometers 
(.mu.m), and the layer can be dried in forced air at temperatures ranging 
from 20.degree. C. to 100.degree. C. It is preferred that the thickness of 
the layer be selected to provide maximum image densities greater than 0.2, 
and, more preferably, in the range 0.5 to 2.5, as measured by a MacBeth 
Color Densitometer Model TD 504. When used in color elements, a color 
filter complementary to the dye color should be used. 
Additionally, it may be desirable in some instances to coat different 
emulsion layers on both sides of a transparent substrate, especially when 
it is desirable to isolate the imaging chemistries of the different 
emulsion layers. 
Barrier layers, preferably comprising a polymeric material, may also be 
present in the photothermographic element of the present invention. 
Polymers for the material of the barrier layer can be selected from 
natural and synthetic polymers such as gelatin, polyvinyl alcohols, 
polyacrylic acids, sulfonated polystyrene, and the like. The polymers can 
optionally be blended with barrier aids such as silica. 
Alternatively, the formulation may be spray-dried or encapsulated to 
produce solid particles, which can then be redispersed in a second, 
possibly different, binder and then coated onto the support. 
The formulation for the emulsion layer can also include coating aids such 
as fluoroaliphatic polyesters. 
The substrate with backside resistive heating layer may also be used in 
color photothermographic imaging systems such as shown in U.S. Pat. Nos. 
4,460,681 and 4,374,921. 
Development conditions will vary, depending on the construction used, but 
will typically involve heating the imagewise exposed material at a 
suitably elevated temperature, e.g. from about 80.degree. C. to about 
250.degree. C., preferably from about 120.degree. C. to about 200.degree. 
C., for a sufficient period of time, generally from 1 second to 2 minutes. 
In some methods, the development is carried out in two steps. Thermal 
development takes place at a higher temperature, e.g. about 150.degree. C. 
for about 10 seconds, followed by thermal diffusion at a lower 
temperature, e.g. 80.degree. C., in the presence of a transfer solvent. 
The second heating step at the lower temperature prevents further 
development and allows the dyes that are already formed to diffuse out of 
the emulsion layer to the receptor layer. 
The Support 
Photothermographic emulsions used in the invention can be coated on a wide 
variety of supports. The support or substrate can be selected from a wide 
range of materials depending on the imaging requirement. Substrates may be 
transparent or opaque. Typical supports include polyester film, subbed 
polyester film, polyethylene terephthalate film, cellulose nitrate film, 
cellulose ester film, polyvinyl acetal film, polycarbonate film and 
related or resinous materials, as well as glass, paper, metal and the 
like. Typically, a flexible support is employed, especially a paper 
support, which can be partially acetylated or coated with baryta and/or an 
.alpha.-olefin polymer, particularly a polymer of an alpha-olefin 
containing 2 to 10 carbon atoms such as polyethylene, polypropylene, 
ethylene-butene copolymers, and the like. Preferred polymeric materials 
for the support include polymers having good heat stability, such as 
polyesters. A particularly preferred polyester is polyethylene 
terephthalate. 
The Image-Receiving Layer 
The photothermographic element may further comprise an image-receiving 
layer. Images derived from the photothermographic elements employing 
compounds capable of being oxidized to form or release a dye, such as, for 
example, leuco dyes, are typically transferred to an image-receiving 
layer. 
When the reactants and reaction products of photothermographic systems that 
contain compounds capable of being oxidized to form or release a dye 
remain in contact after imaging, several problems can result. For example, 
thermal development often forms turbid and hazy color images because of 
dye contamination by the reduced metallic silver image on the exposed area 
of the emulsion. In addition, the resulting prints tend to develop color 
in unimaged background areas. This "background stain" is caused by slow 
reaction between the dye-forming or dye-releasing compound and reducing 
agent during storage. It is therefore desirable to transfer the dye formed 
upon imaging to a receptor, or image-receiving layer. 
The image-receiving layer of this invention can be any flexible or rigid, 
transparent layer made of thermoplastic polymer. The image-receiving layer 
preferably has a thickness of at least 0.1 .mu.m, more preferably from 
about 1 to about 10 .mu.m, and a glass transition temperature (T.sub.g) of 
from about 20.degree. C. to about 200.degree. C. In the present invention, 
any thermoplastic polymer or combination of polymers can be used, provided 
the polymer is capable of absorbing and fixing the dye. Because the 
polymer acts as a dye mordant, no additional fixing agents are required. 
Thermoplastic polymers that can be used to prepare the image-receiving 
layer include polyesters, such as polyethylene terephthalates; 
polyolefins, such as polyethylene; cellulosics, such as cellulose acetate, 
cellulose butyrate, cellulose propionate; polystyrene; polyvinyl chloride; 
polyvinylidine chloride; polyvinyl acetate; copolymer of 
vinylchloride-vinylacetate; copolymer of vinylidene 
chloride-acrylonitrile; copolymer of styrene-acrylonitrile; and the like. 
The optical density of the dye image and even the actual color of the dye 
image in the image-receiving layer is very much dependent on the 
characteristics of the polymer of the image-receiving layer, which acts as 
a dye mordant, and, as such, is capable of absorbing and fixing the dyes. 
A dye image having a reflection optical density in the range of from 0.3 
to 3.5 (preferably, from 1.5 to 3.5) or a transmission optical density in 
the range of from 0.2 to 2.5 (preferably, from 1.0 to 2.5) can be obtained 
with the present invention. 
The image-receiving layer can be formed by dissolving at least one 
thermoplastic polymer in an organic solvent (e.g., 2-butanone, acetone, 
tetrahydrofuran) and applying the resulting solution to a support base or 
substrate by various coating methods known in the art, such as curtain 
coating, extrusion coating, dip coating, air-knife coating, hopper 
coating, and any other coating method used for coating solutions. After 
the solution is coated, the image-receiving layer is dried (e.g., in an 
oven) to drive off the solvent. The image-receiving layer may be 
strippably adhered to the photothermographic element. Strippable 
image-receiving layers are described in U.S. Pat. No. 4,594,307, 
incorporated herein by reference. 
Selection of the binder and solvent to be used in preparing the emulsion 
layer significantly affects the strippability of the image-receiving layer 
from the photosensitive element. Preferably, the binder for the 
image-receiving layer is impermeable to the solvent used for coating the 
emulsion layer and is incompatible with the binder used for the emulsion 
layer. The selection of the preferred binders and solvents results in weak 
adhesion between the emulsion layer and the image-receiving layer and 
promotes good strippability of the emulsion layer. 
The photothermographic element can also include coating additives to 
improve the strippability of the emulsion layer. For example, 
fluoroaliphatic polyesters dissolved in ethyl acetate can be added in an 
amount of from about 0.02 to about 0.5 weight percent of the emulsion 
layer, preferably from about 0.1 to about 0.3 weight percent. A 
representative example of such a fluoroaliphatic polyester is "Fluorad FC 
431", (a fluorinated surfactant, available from 3M Company, St. Paul, MN). 
Alternatively, a coating additive can be added to the image-receiving 
layer in the same weight range to enhance strippability. No solvents need 
to be used in the stripping process. The strippable layer preferably has a 
delaminating resistance of 1 to 50 g/cm and a tensile strength at break 
greater than, preferably at least two times greater than, its delaminating 
resistance. 
Preferably, the image-receiving layer is adjacent to the emulsion layer to 
facilitate transfer of the dye that forms after the imagewise exposed 
emulsion layer is subjected to thermal development, for example, in a 
heated shoe-and-roller type heat processor. 
Multi-layer constructions containing blue-sensitive emulsions containing a 
yellow leuco dye of this invention may be overcoated with green-sensitive 
emulsions containing a magenta leuco dye of this invention. These layers 
may in turn be overcoated with a red-sensitive emulsion layer containing a 
cyan leuco dye. Imaging and heating form the yellow, magenta, and cyan 
images in an imagewise fashion. The dyes so formed may migrate to an 
image-receiving layer. The image-receiving layer may be a permanent part 
of the construction or may be removable "i.e., strippably adhered" and 
subsequently peeled from the construction. Color-forming layers may be 
maintained distinct from each other by the use of functional or 
non-functional barrier layers between the various photosensitive layers as 
described in U.S. Pat. No. 4,460,681. False color address, such as that 
shown in U.S. Pat. No. 4,619,892, may also be used rather than 
blue-yellow, green-magenta, or red-cyan relationships between sensitivity 
and dye formation. 
In another embodiment, the colored dye released in the emulsion layer can 
be transferred onto a separately coated image-receiving sheet by placing 
the exposed emulsion layer in intimate face-to-face contact with the 
image-receiving sheet and heating the resulting composite construction. 
Good results can be achieved in this second embodiment when the layers are 
in uniform contact for a period of time of from 0.5 to 300 seconds at a 
temperature of from about 80.degree. C. to about 220.degree. C. 
Alternatively, a multi-colored image may be prepared by superimposing in 
register a single image-receiving sheet successively with two or more 
imagewise exposed photothermographic or thermographic elements, each of 
which release a dye of a different color, and heating to transfer the 
released dyes as described above. This method is particularly suitable for 
the production of color proofs especially when the dyes released have hues 
which match the internationally-agreed standards for color reproduction 
(SWOP colors). Dyes with this property are disclosed in U.S. Pat. No. 
5,023,229. In this embodiment, the photothermographic or thermographic 
element preferably comprise compounds capable of being oxidized to release 
a preformed dye as this enables the image dye absorptions to be tailored 
more easily to particular requirements of the imaging system. When used in 
a photothermographic dement, the elements are preferably all sensitized to 
the same wavelength range regardless of the color of the dye released. For 
example, the elements may be sensitized to ultra-violet radiation with a 
view toward contact exposure on conventional printing frames, or they may 
be sensitized to longer wavelengths, especially red or near infra-red to 
enable digital address by lasers. 
Objects and advantages of this invention will now be illustrated by the 
following examples, but the particular materials and amounts thereof 
recited in these examples, as well as other conditions and details, should 
not be construed to unduly limit this invention. All percentages are by 
weight unless otherwise indicated.

EXAMPLES 
All materials used in the following examples were readily available from 
standard commercial sources such as Aldrich Chemical Co. (Milwaukee, WI) 
unless otherwise specified. The following additional terms and materials 
were used. 
Acryloid.TM. B-66 is a poly(methyl methacrylate) available from Rohm and 
Haas, Philadelphia, PA.. 
Airvol.TM. 523 is a poly(vinyl alcohol) available from Air Products, 
Allentown, PA. 
Butvar.TM. B-76 is a poly(vinyl butyral) available from Monsanto Company, 
St. Louis, MO. 
Desmodur.TM. N3300 is an isocyanate resin available from Mobay Chemicals, 
Pittsburgh, PA. 
MEK is methyl ethyl ketone (2-butanone). 
PAZ is 1-(2H)-phthalazinone. 
PET is poly(ethylene terephthalate) 
PVP K-90 is a poly(vinyl pyrrolidone) available from International 
Specialty Products. 
Styron 685D is a polystyrene resin available from Dow Chemical Company, 
Midland, MI. 
VAGH is a vinyl chloride/vinyl acetate copolymer available from Union 
Carbide Corp., Danbury, CT. 
Dye-1 has the following structure (disclosed in G.B. Patent Appln. No. 
305324.7, filed March 16, 1993): 
##STR3## 
"Ethyl ketazine" is described in U.S. Pat. Nos. 4,587,211 and 4,795,697 and 
has the following formula: 
##STR4## 
EXAMPLE 1 
Preparation of Non-Core-Shell-Type Silver Iodobromide Emulsions: 
Comparative non-core-shall-type silver iodobromide emulsions 1-A, 1-B, and 
1-C were prepared by double-jet addition in aqueous phthalated gelatin 
solution at controlled pAg and temperature conditions by the following 
procedure. These examples demonstrate that higher silver iodide content 
non-core-shell-type emulsions results in thermal fogging of the emulsion. 
To a first solution (Solution A) having 50-100 g of phthalated gelatin 
dissolved in 1500 ml of deionized water, held at a temperature between 
30.degree.-38.degree. C., were simultaneously added; a second solution 
(Solution B) containing predetermined amounts of potassium iodide and 
potassium bromide; and a third solution (Solution C) which was an aqueous 
solution containing 1.4 to 1.8 moles of silver nitrate (AgNO.sub.3) per 
liter. pAg was held at a constant value by means of a pAg feedback control 
loop as described in Research Disclosure No. 17643; U.S. Pat. Nos. 
3,415,650; 3,782,954; and 3,821,002. The size of the emulsion grains being 
formed were adjusted by controlling the addition rates, silver nitrate 
concentration, gelatin concentration in the kettle, and the kettle 
temperature. 
As a result, three silver iodobromide emulsions were obtained that were 
cubic, monodispersed silver halide having different silver iodide (AgI) 
contents, but of the same grain size. These emulsions were washed with 
water and desalted. 
Table 1-1 shows the comparative emulsions 1-A, 1-B, and 1-C as to pAg at 
make, the average grain size, and the silver iodide content. 
TABLE 1-1 
______________________________________ 
Example pAg Average grain size 
AgI Content: mole % 
______________________________________ 
1-A 2 0.04.mu. m 2 
1-B 8.2 0.04.mu. m 2 
1-C 2 0.04.mu. m 3.5 
______________________________________ 
Preparation of Core-Shell-Type Silver Iodobromide Emulsion: Nine core- 
shell-type emulsions, 1-D to 1-L having different silver iodide content 
were prepared by the following procedure. 
To a first solution (Solution A) having 50-100 g of phthalated gelatin 
dissolved in 1500 ml of deionized water, held at a temperature between 
30.degree.-38.degree. C., were simultaneously added; a second solution 
(Solution B) containing predetermined amounts of potassium bromide and 
potassium iodide, and a third solution (Solution C) which was an aqueous 
solution containing 1.4 to 1.8 moles silver nitrate per liter. pAg was 
held at a constant value by means of a pAg feedback control loop as 
described in Research Disclosure No. 17643, U.S. Pat. Nos. 3,415,650; 
3,782,954; and 3,821,002. After a certain percentage of the total 
delivered silver nitrate was added, the second halide solution (Solution 
B), was replaced with Solution D which contained different predetermined 
amounts of potassium iodide and potassium bromide; and Solution C was 
replaced with Solution E. In this manner a core of particular silver 
iodide percentage with a shell of different silver iodide percentage could 
be obtained. 
Since pAg was an area of interest as to its effect on the subsequent use of 
the silver halide emulsions in photothermographic constructions, the 
emulsions investigated were made at two different pAg's. As previously 
described, the sizes of the emulsion grains were adjusted by controlling 
the addition rates, silver nitrate concentration, gelatin concentration in 
the kettle, and the kettle temperature. As a result, nine monodispersed 
core-shell-type emulsions were obtained. They differed only in pAg during 
make, total amount of silver iodide distributed in between the core and 
shell, and the portion of the grain designated as the core vs. shell. 
For illustration, the procedure for the preparation of 2 moles of emulsion 
1-E is shown below. 
Solution A was prepared at 32.degree. C. as follows: 
gelatin 50 g 
aleionized Water 1500 ml 
0.1 M KBr 6 ml 
adjust to pH=5.0 with 3N HNO.sub.3 
Solution B was prepared at 25.degree. C. as follows: 
KBr 27.4 g 
KI 3.3 g 
deionized Water 275.0 g 
Solution C was prepared at 25.degree. C. as follows: 
AgNO.sub.3 42.5 g 
deionized Water 364.0 g 
Solutions B and C were jetted into Solution A over 9.5 minutes. 
Solution D was prepared at 25.degree. C. as follows: 
KBr 179. g 
deionized Water 812. g 
Solution E was prepared at 25.degree. C. as follows: 
AgNO.sub.3 127. g 
deionized Water 1090. g 
Solutions D and E were jetted into Solution A over 28.5 minutes. 
The emulsions were washed with water and then desalted. The average grain 
size, pAg at make, and silver iodide content of each of the 
core-shell-type silver halide emulsions, 1-D to 1-L, are shown in Table 
1-2. Silver halide grain size was determined by Scanning Electron 
Microscopy (SEM). 
TABLE 1-2 
__________________________________________________________________________ 
Core Iodide 
Core Silver 
Shell Halide 
Silver in Shell 
Total Iodide 
Average 
Example 
mole % mole % 
mole % mole % mole % pAg 
Grain Size, .mu.m 
__________________________________________________________________________ 
1-D 6 25 100% Br 
75 1.5 8.2 
0.05 
1-E 8 25 100% Br 
75 2 8.2 
0.04 
1-F 16 12.5 100% Br 
87.5 2 8.2 
0.04 
1-G 8 25 100% Br 
75 2 2 0.04 
1-H 6 25 2% I/98% Br 
75 3 8.2 
0.04 
1-I 6 50 100% Br 
50 3 8.2 
0.04 
1-J 6 50 100% Br 
50 3 2 0.04 
1-K 14 25 100% Br 
75 3.5 8.2 
0.04 
1-L 14 25 100% Br 
75 3.5 2 0.04 
__________________________________________________________________________ 
Preparation of Preformed Silver Halide/Silver Organic Salt Dispersion 
A silver halide/silver organic salt dispersion was prepared as described 
below. This material is also referred to as a silver soap dispersion or 
emulsion. 
I. Ingredients 
1. Preformed silver halide emulsion (non-core-shell Examples 1A-1C; 
Core-Shell Examples 1D-1L) 0.22 mole at 700 g/mole in 1.25 liter H.sub.2 O 
at 42.degree. C. 
2. NaOH 89.18 g in 1.50 liter H.sub.2 O 
3. AgNO.sub.3 364.8 g in 2.5 liter H.sub.2 O 
4. Fatty acid 131 g (Humko Type 9718) [available from Witco. Co., Memphis, 
TN] 
5. Fatty acid 634.5 g (Humko Type 9022) [available from Witco. Co., 
Memphis, TN] 
6. HNO.sub.3 19 ml in 50 ml H.sub.2 O 
II. Reaction 
1. Dissolve ingredients #4 and #5 at 80.degree. C. in 13 liter of H.sub.2 O 
and mix for 15 minutes. 
2. Add ingredient #2 to Step 1 at 80.degree. C. and mix for 5 minutes to 
form a dispersion. 
3. Add ingredient #6 to the dispersion at 80.degree. C., cooling the 
dispersion to 55.degree. C. and stirring for 25 minutes. 
4. Add ingredient #1 to the dispersion at 55.degree. C. and mix for 5 
minutes. 
5. Add ingredient #3 to the dispersion at 55.degree. C. and mix for 10 
minutes. 
6. Wash until wash water has a resistivity of 20,000 ohm/cm.sup.-2. 
7. Dry at 45.degree. C. for 72 hours. 
Homogenization of Preformed Soaps (Homogenate): A preformed silver fatty 
acid salt homogenate was prepared by homogenizing 200 g of pre-formed 
soaps, prepared above, in solvent and Butvar.TM. B-76 poly(vinyl butyral) 
according to the following procedure. 
1. Add 200 g of preformed soap to 350 g of toluene, 1116 g of 2-butanone, 
and 33 g of Butvar.TM. B-76. 
2. Mix the dispersion for 10 minutes and hold for 24 hours. 
3. Homogenize at 4000 psi. 
4. Homogenize again at 8000 psi. 
Preparation of Photothermographic Light Sensitive Material: The homogenized 
photothermographic emulsion (200g) and 50 ml 2-butanone were cooled to 
55.degree. F. with stirring. Butvar.TM. B-76 (30.2g) was added and the 
mixture was stirred for 20 minutes. Pyridinium hydrobromide perbromide 
(PHP, 0.18g) was added and stirred for 2 hours. The addition of 1.30 ml of 
a calcium bromide solution (1 g of CaBr.sub.2 and 10 ml of methanol) was 
followed by 16 hours of stirring at 55.degree. F. The following were then 
added in 15 minute increments with stirring: 
1.0 g of 2-(4-chlorobenzoyl)benzoic acid 
0.0168 g IR Dye-1 
0.084 g 2-mercapto-(5-methylbenzimidazole) in 5 g methanol 
6.56 g 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane 
0.70 g 5-tribromomethylsulphonyl-2-methyl- 1,3,4-thiadiazole 
0.272 g Desmodur.TM. N3300 isocyanate 
A protective topcoat solution was prepared with the following ingredients: 
256.0 g acetone 
123.0 g 2-butanone 
50.0 g methanol 
20.2 g cellulose acetate 
2.89 g phthalazine 
1.52 g 4-methylphthalic acid 
1.01 g tetrachlorophthalic acid 
1.50 g tetrachlorophthalic anhydride 
Coating of Photothermographic Light Sensitive Material: The 
photothermographic emulsions were coated on 3 mil (76.2 .mu.m) polyester 
base by means of a knife coater and dried at 175.degree. F. for four 
minutes. The dry coating weight was 23 g/m.sup.2 and the silver coating 
weight was 2.0 g/m.sup.2. 
The topcoat solution was then coated over the silver-containing layer. The 
dry weight was 3.0 g/m.sup.2. The layer was dried at 175.degree. F. for 
four minutes. 
Sensitometric and Thermal Stability Measurements: The sensitometry of 
samples of freshly coated materials was determined by exposure with a 
laser sensitometer incorporating a 780 nm laser diode. After exposure, the 
samples were developed by heating at 250.degree. F. (121 .degree. C.) for 
15 seconds to give an image. 
Densitometry measurements were made on a custom-built, computer-scanned 
densitometer and are believed to be comparable to measurements obtainable 
from commercially available densitometers. Sensitometric results include 
D.sub.min, D.sub.max, speed, contrast, and .DELTA.D.sub.min. 
D.sub.min is the density corresponding to an exposure at 1.40 log E beyond 
a density of 0.25 above D.sub.min. 
Speed is the relative speed at a density of 1.0 above D.sub.min versus 
coating 1-A setat 100. 
Contrast is measured by the slope of the line joining the density points of 
0.50 and 1.70 above D.sub.min. 
.DELTA.D.sub.min is the change in D.sub.min of samples aged in an oven for 
14 days at 120.degree. F./50% RH minus D.sub.min of the non-aged samples. 
Accelerated aging studies are a very good method of determining the degree 
of thermal fog that might result from natural storage and aging. Unexposed 
strips, prepared above, were aged in ovens maintained at 120.degree. 
F./50% relative humidity (%RH). After 14 days, the samples were removed, 
exposed, and processed in a manner similar to the freshly coated samples. 
Of considerable interest was the D.sub.min that results due to accelerated 
oven testing. 
The results, shown in table 1-3, indicate that uniformly distributed silver 
iodide emulsions are generally not as good in regards to sensitometry or 
D.sub.min stability upon oven aging (known as .DELTA.D.sub.min). However, 
it can be seen that much better results are obtained when one distributes 
the silver iodide in a core-shell-type manner. In particular, emulsions 
made at pAg 2 or pAg 8.2 seem equally good regarding D.sub.min or 
.DELTA.D.sub.min unless the silver iodide in the core is higher than 14% 
as shown by 1-F or as one starts to approach 4 mole % overall silver 
iodide content as shown in Example 1-L. 
TABLE 1-3 
______________________________________ 
Experiment Relative Delta 
No. D.sub.min 
D.sub.max 
Speed Speed Contrast 
D.sub.min 
______________________________________ 
1-A 0.153 3.05 2.46 100 3.35 &gt;0.2 
1-B 0.111 3.60 2.36 79 3.91 0.04 
1-C 0.120 3.90 2.51 112 4.60 &gt;0.2 
1-D 0.106 3.63 2.58 132 4.34 0. 
1-E 0.097 3.62 2.42 91 4.97 0.04 
1-F 0.101 3.68 2.61 141 3.68 0.1 
1-G 0.116 3.86 2.60 138 4.36 0. 
1-H 0.097 3.51 2.38 83 4.00 0.01 
1-I 0.092 3.52 2.50 110 2.89 0.01 
1-J 0.091 3.56 2.41 89 3.86 0.01 
1-K 0.108 3.69 2.48 105 4.21 0. 
1-L 0.104 3.67 2.63 148 3.65 0.08 
______________________________________ 
EXAMPLE 2 
Preparation of Core-Shell-Type Iodobromide Emulsion: Four core-shell-type 
emulsions, labeled 2-A to 2-D, having different silver iodide contents 
were prepared according to the procedures described in Example 1. A higher 
kettle temperature was used to obtain a larger average grain size. 
As a result, 4 monodispersed core-shell-type emulsions were obtained. Table 
2-1 shows the core and shell silver iodide content, pAg, total mole % 
silver iodide, and average grain size. 
Sensitometric and Thermal Stability Measurements: Emulsions 2-A to 2-D were 
prepared as pre-formed soaps and formulated into photothermographic light 
sensitive elements as described in Example 1. The samples were coated, 
exposed, and processed as described in Example 1. The results, shown in 
Table 2-2, indicate that initial D.sub.min is very much affected by grain 
size and also higher silver iodide percentage. 
TABLE 2-1 
__________________________________________________________________________ 
Core Iodide 
Core Silver 
Shell Halide 
Silver in Shell 
Total Iodide 
Average 
Example 
mole % mole % 
mole % mole % mole % pAg 
Grain Size, .mu.m 
__________________________________________________________________________ 
2-A 8 25 100% Br 
75 2 2 0.08 
2-B 8 25 2% I/98% Br 
75 3 2 0.08 
2-C 12 25 100% Br 
75 3 2 0.08 
2-D 12 25 2% I/98% Br 
75 4.5 2 0.08 
__________________________________________________________________________ 
TABLE 2-2 
______________________________________ 
Experiment Relative .DELTA. 
No. D.sub.min 
D.sub.max 
Speed Speed Contrast 
D.sub.min 
______________________________________ 
2-A 0.115 3.32 2.96 316 3.15 0.02 
2-B 0.167 3.37 3.04 380 3.24 
2-C 0.161 3.27 2.97 325 2.95 
2-D 0.182 3.20 2.80 174 2.77 
______________________________________ 
EXAMPLE 3 
Core-Shell-Type Silver Halide Emulsion with Silver Iodide Core and Silver 
Chlorobromide Shell: A core-shell-type emulsion, 3-A, was prepared 
according to the procedures described in Example 1. The main difference 
was that solution D contained a mixture of potassium bromide and potassium 
chloride. As shown in Table 3,1, the ratio of KCl to KBr was 35 mol % Cl 
and 65 mol % Br. As a result, a monodispersed core-shell-type emulsion was 
obtained. Table 3-1 shows the core and shell silver iodide Content, pAg, 
total % silver iodide, % silver chloride, and % silver bromide in the 
shell, and average grain size. 
Emulsion 3-A was prepared as a pre-formed soap and formulated into a 
photothermographic, light-sensitive film as described in Example 1. The 
samples were coated, exposed, and processed as described in Example 1. The 
results, shown in Table 3-2, indicate that the shell can also be silver 
chloride/silver bromide. 
TABLE 3-2 
______________________________________ 
Experiment Relative Delta 
No. D.sub.min 
D.sub.max 
Speed Speed Contrast 
D.sub.min 
______________________________________ 
3-A 0.110 3.55 2.71 178 3.14 0.01 
______________________________________ 
TABLE 3-1 
__________________________________________________________________________ 
Core Iodide 
Core Silver 
Shell Halide 
Silver in Shell 
Total Iodide 
Average 
Example 
mole % mole % 
mole % mole % mole % pAg 
Grain Size, .mu.m 
__________________________________________________________________________ 
3-A 6 25 35% Cl/65% 
75 1.5 8.2 
0.06 
Br 
__________________________________________________________________________ 
EXAMPLE 4 
Color Photothermographic Material: A silver formulation was prepared by 
using 200 g of pre-formed core-shell-type emulsions 1-G and 1-F with the 
following ingredients, each added in its listed order with mixing for the 
time given at 55.degree. F. (12.8.degree. C.). 
40 g 2-butanone (methyl ethyl ketone) and 25 g Butvar.TM. B-76 were mixed 
for minutes. 
0.055 g pyridinium hydrobromide perbromide was added and the mixture was 
held for 1 hour. 
An additional 0.055 g pyridinium hydrobromide perbromide was added and the 
mixture held for 1 hour. 
An additional 0,055 g pyridinium hydrobromide perbromide was added and the 
mixture held for 4 hours. 
0.13 g calcium bromide was added and the mixture was held for 1 hour. 
0.4 g 2-bromobutyl-2'-tribromomethylsulfone was added and the mixture held 
overnight. 
0.7 g 2-quinoline-tribromomethylsulfone was added and the mixture held for 
minutes. 
0.6 g 2-(4-chlorobenzoyl)benzoic acid, 0.1 g of 2-mercaptobenzoimidazole, 
and 0.029 g Dye-1 in 5 g methanol was added and the mixture held for 15 
minutes. 
A second solution was prepared separately by mixing of the following 
reagents: 
0.9 g ethyl ketazine 
1.8 g phthalazinone 
80.0 g tetrahydrofuran 
6.3 g VAGH 
4.0 g Butvar.TM. poly(vinyl butyral) 
The silver formulation, 6 g, was then mixed with 13.5 g of the second 
solution and coated at 2 rail (50.8 .mu.m) wet thickness, and dried at 
170.degree. F. for 4 minutes. 
A protective topcoat solution was prepared with the following ingredients: 
53.56% acetone 
26.44% 2-butanone 
10.68% toluene 
8.65% Styron.TM. 685D Polystyrene 
The topcoat was then coated at 2 mil (50.8 .mu.m) wet thickness over the 
silver layer and dried at 170.degree. F. (76.7.degree. C.) for 4 minutes. 
The resulting film was exposed on an EG&G Sensitometer at 780 nm through a 
narrow bandpass filter for 10.sup.-3 seconds and processed at 136.degree. 
C. for 16 seconds to give a magenta image. The sensitometric response is 
shown in Table 4-1. 
______________________________________ 
Emulsion D.sub.min 
D.sub.max Speed Contrast 
______________________________________ 
1-G 0.09 2.28 1.43 4.97 
1-F 0.17 2.37 1.27 5.89 
______________________________________ 
The results indicate that core-shell-type silver halide emulsions can be 
used to form colored images in photothermographic elements. Sample 1-F 
demonstrates that cores having an silver iodide concentration greater than 
14% result in undesirably high D.sub.min. 
Reasonable modifications and variations are possible from the foregoing 
disclosure without departing from either the spirit or scope of the 
present invention as defined by the claims.