Thermal etching process of a ceramic under oxidizing conditions

Details Thermal etching process of a ceramic under oxidizing conditions Thermal etching process of a ceramic under oxidizing conditions

1998-10-21

2001-01-09

Gulakowski, Randy (Department: 1746)

Etching a substrate: processes

Heating or baking of substrate prior to etching to change...

C216S058000, C216S059000, C216S060000, C216S061000, C216S063000, C216S064000, C216S076000, C216S088000

Reexamination Certificate

active

06171511

ABSTRACT:

DESCRIPTION
The invention relates to a process for the thermal etching of a ceramic under oxidizing conditions with the particular aim of revealing its grain boundaries and studying its granular microstructure.
The invention applies to technical and nuclear ceramics and in particular to UO
2
and (U, Pu) O
2
mixtures.
The structural study of a material generally involves a careful metallographic preparation of said material for the morphological analysis thereof.
In order to study the structure, it is necessary to render visible the different constituent elements thereof and consequently, precisely reveal the grain boundaries in order to permit the measurement of the size of the grains of the material. In the case of polyphase materials, the size of the grains must be measured on each phase independently.
The automation of the measuring processes concerning the size of the grains involving image analysis software, requires the obtaining of high quality images. This quality is linked with that of the revealing of the grain boundaries and therefore the etching process used. Among the latter, reference can be made in the case of ceramics to acid chemical etching processes.
Chemical etching processes are e.g. described in the work by V. Tebaldi “Kernforschung und Technologie”,
Metallography and Thermal Analysis of Ceramic Nuclear Fuels
—a handbook for laboratory assistants and test engineers, commission of the European Community, 1988, EUR 11716 DE.
From the physicochemical standpoint, chemical acid etching of a polished surface of a monophase sample, as is the case with UO
2
, can be summarized by the action of two concurrent mechanisms:
an etching of the surface of the grains at different speeds as a function of their crystal orientation, said facetting of the grains inducing reflectivity differences, which will make it possible to individualize them during their observation by optical microscopy,
a specific local etching at the grain boundaries, due to the significant crystal defects and to the presence of impurities. This etching will create a groove at the location of the grain boundary and its shape will depend on the surface and intergranular tensions. It is therefore directly linked with the crystal orientation differences of two adjacent grains and will therefore be heterogeneous in the plane of the specimen.
In the case of polyphase ceramics, such as mixed uranium and plutonium fuel, the chemical potentials of the different phases are not the same. These chemical potential differences are significant and will finally lead to a preferential etching of one phase compared with the other.
Therefore the revealing of the microstructure of (U, Pu) O
2
fuels corresponds to this principle.
This specificity enables scientists to independently study the granular structure of areas having high and low plutonium contents using two specific chemical etching processes:
a first etching process for revealing the areas having a limited Pu enrichment (matrix), which is carried out using an acid solution (H
2
O, H
2
O
2
, H
2
SO
4
) at ambient temperature for 2 hours 20 minutes,
the sample is then repolished before undergoing a second acid etching (H
2
O, H
2
SO
4
, NH
4
NF
2
) at 70° C. for 1 to 3 minutes, which makes it possible to reveal all the surface including the areas with high and low plutonium contents.
However, it is particularly difficult to control the latter etching process, the areas with only a limited plutonium enrichment being liable to undergo an excessive revealing or development.
The chemical etching of a ceramic, particularly a nuclear fuel ceramic, especially of the (U, Pu) O
2
type involves a certain number of disadvantages, including:
impossibility of uniformly revealing a ceramographic section for the reasons mentioned hereinbefore,
difficulty of defining a reproducible revealing procedure, because it is necessary to start with in each case new solutions,
different colouring of the grains, generated by multiple crystal orientations of the grains, not permitting the automation of the grain size measurements,
generation of active effluents (in the case of “nuclear” ceramics) and corrosion of neighbouring equipment by the giving off of acid vapours in the glove box.
The above shows that the use of chemical etching for revealing grain boundaries of a ceramic and in particular a ceramic acting as a nuclear fuel (“nuclear” ceramic) is unsatisfactory because, due to the incomplete revealing of the grain boundaries, the images obtained on the ceramic or fuel which has been chemically etched are of mediocre quality and do not permit the automatic processing and analysis of the images.
Another method for revealing grain boundaries is thermal etching, which is currently used for studying the granular structure of technical ceramics, such as e.g. alumina, silicon carbide or cerium oxide.
Thermal etching is based on the fact that on heating a polycrystalline solid to a temperature T in the presence of a vapour or liquid phase in thermodynamic equilibrium with the solid, grooves appear at the emergence lines of the grain boundaries as a result of material transfer diffusion mechanisms (surface diffusion, volume diffusion and condensation evaporation). This phenomenon is illustrated in
FIG. 1
, which shows the groove of the thermally revealed grain boundary.
The thermal etching of ceramics in general is performed under an atmosphere identical to that used during the production thereof.
The thermal etching of UO
2
has been studied, but only under the following specific conditions, which are those of a reducing, thermal etching. The thermal etching experiments disclosed in the literature are generally performed on UO
2
pellets, which have been previously annealed for sufficiently long periods of time, e.g. two weeks at 1700° C., in comparison with the thermal etching periods. This annealing is indispensable for stabilizing the microstructure and thus overcoming a possible enlargement of the grains, which could lead to mechanisms other than that or those responsible for the thermal etching of the groove. Studies carried out on UO
2+x
of a superstoichiometric nature were carried out with a uranium oxide, whose oxygen/uranium ratio was determined and which did not evolve during the experiment.
Thus, most of the publications describe the thermal etching of UO
2x
fuels under a high temperature close to 1650 to 1700° C. and in a reducing or neutral atmosphere.
Thus, the article by G. C. Grappiolo “Thermal etching as a mean to evidence grain boundaries in high density UO
2
”, energia nucleare, vol. 11, No. 5, May 1964 describes the treatment of high density uranium oxide in a hydrogen atmosphere at 1650° C.
In the same way, the article by R. L. Colombo and I. Amato “Thermal etching figures in ceramic bodies”,
Journal of Nuclear Maerials
42, (1972) 345-347, North Holland Publishing Co., Amsterdam, describes the thermal etching of ceramics such as alumina and UO
2
in a reducing hydrogen atmosphere at a temperature close to 1700° C.
As far as is known to us, the thermal etching of mixed plutonium and uranium oxides, such as MOX, has not been described in the literature.
The inventors have found, by carrying out thermal etching operations under a reducing atmosphere on UO
2
samples, particularly at 1650° C., under dry H
2
for 15 minutes, that the revealing of grain boundaries obtained by thermal etching under a reducing atmosphere are of better quality than those obtained after chemical etching operations, although a certain number of defects and disadvantages remained, more particularly:
an optical image of a non-uniform nature of the grain boundaries, so that the automatically binarized grain boundary network has discontinuities requiring a significant manual correction during the image treatment stage,
the revealing of crystal defects, such as scratches or ridges on the surface of the material or dislocations and which generate a background noise, which becomes all the more prejudicial if the sample is porous, because confusion can arise between the porosity, the backgr

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