Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Patent
1993-11-12
1996-01-16
Schilling, Richard L.
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
430567, G03C 1015, G03C 1035
Patent
active
054846978
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for preparing a photographic emulsion containing gelatin and tabular silver halide grains exhibiting a narrow size distribution.
DESCRIPTION OF THE PRIOR ART
The tabular silver halide grains, their preparation methods and their use have been extensively studied these last years, and they are used in commercial products. By "tabular grain" is meant a grain defined by two parallel or substantially parallel crystalline faces, each exhibiting a notably greater surface than any other crystal face forming the grain. The aspect ratio, i.e., the diameter/thickness ratio is more than at least 2:1, and preferably, more than at least 5:1. The diameter is defined as being the diameter of an equivalent circle obtained from the grain projected area, as viewed in a photomicrograph or in an electron micrograph of an emulsion sample.
The advantages of these grains are well known: they provide a better image sharpness, a higher covering power, a better relationship between sensitivity and granularity, a better separation between the blue and the minus blue, and allow to use lower silver coverages and thinner emulsion layers.
Numerous methods for preparing the tabular silver halide emulsions have been disclosed. For example, U.S. Pat. No. 4,434,226 discloses tabular halide grains having a thickness less than 0.5 .mu.m, a diameter of at least 0.6 .mu.m, an average aspect ratio more than 8:1 and representing at least 50% of the total grain projected area. These grains are prepared by a double jet method at a pBr ranging between 0.6 and 1.6.
By this method, tabular silver halide grains exhibiting a wide size distribution are obtained.
However, it would be highly desirable to provide a method for preparing monodisperse tabular silver halide grains, i.e., exhibiting a narrow size distribution. The advantages due to the narrow size distributions are well known, the number of photographically useful grains is increased, the sensitization can be more easily controlled since the grains exhibit similar sizes, the contrast and the granularity of the resulting photographic element are improved.
In the present description, the dispersity is, unless otherwise stated, represented by the coefficient of variation of the diameter (COV), which is the ratio between the diameter standard deviation of the grains and the mean diameter of these grains. The cited values refer to measures performed on electron micrographs of the grains.
French Patent No. 2,534,036 discloses a method for preparing hexagonal and triangular monodisperse flat grains having a thickness less than 0.3 .mu.m, an aspect ratio of at least 5:1 and representing at least 97% of the total grain projected area. The coefficient of variation expressed as % of grains versus grain diameter varies between 15 and 28.4% in the examples. This method consists in precipitating fine grains having a diameter less than 0.15 .mu.m and letting them undergo a physical ripening at a pAg ranging from 8.4 to 11, without any complexing agent.
U.S. Pat. No. 4,775,617 discloses a method for preparing monodisperse flat grains having a thickness ranging between 0.5 and 6 .mu.m, an aspect ratio ranging between 5:1 and 30:1 and a coefficient of variation (COV) of at least 20%, the tabular grains forming at least 50% of the total grain projected area. The method consists in growing the grains by controlling the concentration flowrate of the halide and silver ion solutions at 50-60% of the crystal critical growth rate.
U.S. Pat. No. 4,722,886 discloses a method for preparing tabular grains having a thickness ranging between 0.05 and 0.5 .mu.m, an average grain volume ranging between 0.05 and 1 .mu.m.sup.3 and an aspect ratio more than 2:1. The emulsion predominantly contains tabular grains. The method includes several steps, the precipitation being performed in presence of ammonia, which is then neutralized before the ripening and the growth. The dispersity is calculated in volume, which is not really significative for flat grains, in the ab
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Jezequel Pierre H.
Mignot Andre G. E.
Eastman Kodak Company
Huff Mark F.
Leipold Paul A.
Schilling Richard L.
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