FR2715651A1 - Tritogenic ceramic material containing lithium and its preparation process. - Google Patents

Abstract

Lithium-containing tritigenic material and method of preparation. The material comprises a lithium-containing ceramic of formula: Li2Ti1-x-yZrxSnyO3 wherein x and y are such that 0</=x+y</=1 with (0</=x</=0,1 or 0,9</=x<1). Said material is prepared by hydrolysing a mixture comprising an alkoxide selected from Zr, Ti or Sn and lithium hydroxide and then drying and sintering the product obtained. The material of the invention is capable of releasing tritium at temperatures of 300 DEG C.

Description

TRITIGEN CERAMIC MATERIAL CONTAINING LITHIUM
AND ITS PREPARATION PROCESS
DESCRIPTION
The present invention relates to a tritigenic ceramic material containing lithium, usable in particular in thermonuclear fusion reactors controlled as an essential component of the cover.

In such reactors, the cover must be stable at operating temperatures which are generally 400 to 700 ° C., and it must allow
the production and the easy release of the tritium resulting from the bombardment of the lithium-6 atoms by the neutrons emitted by the fusion reaction DT (deuterium-tritium) of the plasma, and
- the recovery of energy, or heat, released by these reactions.

 It has already been envisaged to use ceramics based on lithium gamma aluminate in the cover of these reactors, but the temperature necessary to obtain the release of tritium by the ceramic remains high.

 The present invention specifically relates to a new tritigenic ceramic material containing lithium, which overcomes these drawbacks.

According to the invention, the tritigenic material comprises a ceramic containing lithium of formula
Li2Ti lxyZrxSnyO3 in which x and y are such that
0 <x + y <l with (0 <x <0.1 or 0.9 <x <1)
Thus, according to the invention, the tritigenic material can be either a lithium zirconate (x = l), or a lithium titanate (x = y = 0), or a lithium stannate (y = l), or a compound isostructural of lithium titanate resulting from the partial or total substitution of Ti4 + ions by Sn4 + ions (x = 0), that is an isostructural compound of lithium zirconate resulting from the substitution in lithium zirconate of at most 10 e of the ions Zr4 + by one or more ions chosen from Ti4 + and Sn4 +, an isostructrual compound of the solid solution Li2Ti1 ~ ySnyO3 resulting from the substitution of at most 10% of the Ti4 + and / or Sn4 + ions by Zr4 + ions.

These isostructural compounds are solid solutions
Li2ZrO3-Li2TiO3,
Li2TiO3-Li2SnO3 t
Li2ZrO3-Li2SnO3, or Li2ZrO3-Li2TiO3-Li2SnO3.

 The use of these lithium-containing ceramics makes it possible to obtain the relaxation of the tritium at lower temperatures than that obtained with lithium aluminate y, which constitutes an important advantage.

 Indeed, in future fusion reactors, it is expected that there will be "cold" spots within the cover, because some places of the cover will be at 7000C, while others will be only at 400-4500C. However, the release of tritium requires thermally activated processes, but for reasons of safety and supply of the deuterium-tritium fusion reactors, it is not desirable that the residence time of the tritium in the cover is too long. long. Also, a gain of 100 "C on the release temperature of tritium is interesting, because it allows release of tritium in the cold parts of the cover.

 According to an exemplary embodiment of the material of the invention, the ceramic containing lithium is a ceramic of lithium zirconate or of a solid solution Li2ZrO3-Li2TiO3, that is to say that in the formula given above, y is 0.

 Among ceramics of this type, those which also contain titanium are preferred, since the sintering of Li2ZrO3 ceramics is favored by the addition of titanium.

 Indeed, lithium zirconate has a slow establishment of monoclinic crystal symmetry between 700 "C and 1000 C. This" transition "strongly counteracts sintering. We can highlight this phenomenon by comparing the dilatometric curves recorded on a zirconate of lithium from the same powder calcined at 7500C or 8000C; the powder calcined at 8000C contains a higher proportion of monoclinic phase, the establishment of the phase is favored. Also, the sinterability is improved. stoichiometry, this transition to a more ordered state leads to a "bursting" of the ceramic during sintering, bursting all the more important as the ceramic is dense. Also, it is necessary to favor this transition a little more, either by modification of stoichiometry, either by substitution of an element. The substitution Zr4 + <---> Ti4 + promotes this transition because the ionic radius of titanium is close to that of u lithium, and its various degrees of oxidation allow deviations from the stoichiometry when the structure is put in order. Therefore, a ceramic powder of composition Li2Ti1xZrxO3 has a better sinterability than that of a Li2ZrO3 powder.

 In addition, the modification of the parameters of the crystal lattice of pure lithium zirconate induced by the substitution Ti4 + <---> Zr4 + is favorable to the mechanical properties.

 The tritigenic materials of the invention can be prepared by conventional solid phase reaction methods. However, it is preferred to prepare them by the process described below which allows better control of their microstructure.

Also, the invention also relates to a process for the preparation of a ceramic containing lithium of formula Li2Til-x-yZrxSny03 in which x and y are such that
0 <x + y <1 with (0 <x <0.1 or 0.9 <x <1) characterized in that it comprises the following steps
a) mixing in an anhydrous alcohol at least one alkoxide chosen from zirconium, titanium and tin alkoxides, with or without hydrated lithium hydroxide,
b) add water to the mixture obtained in step a) to hydrolyze it,
c) drying, at a temperature below 300 ° C, the hydrolyzed product obtained in step b) to evaporate the alcohols and the water, then calcining at a temperature between 500 and 8000C to obtain a crystallized powder,
d) shaping the powder obtained in step c), and
e) subjecting the shaped product to a thermal sintering treatment carried out at a temperature of 800 to 12000C to obtain a sintered ceramic.

 This process is advantageous because it allows good control, on the one hand, of the stoichiometric ratios and, on the other hand, the microstructure of the ceramics obtained.

 To carry out step a) of this process, anhydrous short-chain alcohols are generally used, such as methanol and ethanol and zirconium, titanium and / or tin alkoxides such as butoxides and isopropoxides of these metals. .

 Lithium hydroxide, hydrated or not, can be added to the freshly prepared solution in the form of a powder, solution or suspension in an alcohol; preferably, it is added in the form of a suspension in the same alcohol as that of the solution. Preferably, lithium hydroxide monohydrate is used.

 In step a) of this process, a partial hydrolysis is carried out with a rise in temperature, and a solution is thus obtained in which the lithium and the metal or metals (Zr, Ti, Sn) are intimately mixed.

 Starting from very pure products, one can thus obtain, by subsequent hydrolysis, a product of very high homogeneity and very high purity, which leads by drying to a fine powder which does not give rise to a significant loss of mass during a subsequent cooking.

 In step c), after drying, the powder obtained is subjected to calcination, before performing step d). This calcination can be carried out at temperatures of 500 to 800 "C, in an air atmosphere.

 The transformation of the dried or calcined powder into ceramic and its sintering are then obtained by carrying out steps d) of shaping and e) of heat treatment described above.

 By this process, ceramics can be obtained whose density varies from 70% to 100% of the theoretical density, with grains of homogeneous size whose average dimensions range from 0.1 to 10 μm depending on the temperature used.

 Generally, sintering is carried out in an oxygen atmosphere, for example in air.

 Furthermore, during sintering, the parts are preferably buried in a bed of powder of the same composition to properly control the stoichiometry.

 In the process of the invention, the characteristics of the powder (stoichiometry, crystallinity, particle size,...), And therefore its sinterability, are controlled, by appropriately regulating the amount of alcohol used in the first solution and the amount of lithium hydroxide added.

 The amount of lithium hydroxide generally corresponds to the stoichiometric amount, but the density of the final product can also be adjusted by using amounts of lithium hydroxide different from the stoichiometry to have a difference of for example up to 4% in mole.

 The characteristics of the hydrolyzed product are also checked by adding in step b) an amount of water such that the molar ratio of water to the alkoxide of Zr, Ti and / or Sn is from 2 to 26.

 In step a) of the process of the invention, the solution of alkoxide (s) is prepared by mixing, with stirring, the alkoxide (s) with alcohol, then the lithium hydroxide is immediately added and it is stirred vigorously for a sufficient time during which the temperature rises to approximately 700C, to obtain an intimate mixture of lithium with the other metal or the other metals.

 Generally, this duration ranges from 20 min to 60 min for a volume of 600 ml.

 In step a), partial hydrolysis takes place, but complete hydrolysis is then carried out in step b) by adding water with stirring.

 For this step b), preferably deionized and decarbonated water is used to obtain, by hydrolysis, a powder of high purity. Stage a) is preferably carried out under an atmosphere of inert gas, for example nitrogen, since the alkoxides are very sensitive to humidity.

 In step c), the product obtained is dried by hydrolysis at a temperature below 300 "C.

 This can be done for example at 1500C in an oven or at a temperature above the critical point of alcohol in an autoclave, for example at 250 "C under 7MPa.

Other characteristics and advantages of the invention will appear better on reading the following examples, of course given by way of illustration and not limitation, with reference to the appended drawing in which
FIG. 1 schematically represents an installation for testing the properties of the tritigenic material with regard to the release of tritium,
FIG. 2 is a diagram illustrating the flow of released tritium as a function of temperature, and
- Figure 3 is a diagram illustrating the results of dilatometric studies on different ceramic powders containing lithium according to the invention.

Example 1: Preparation of LiZrO3.

 In this example, a first solution is prepared by mixing 122.67 g of zirconium butoxide Zr (OC4Hg) from Aldrich Chemical Cy with 250 ml of ethanol under a dry nitrogen atmosphere.

 In another beaker, a suspension of lithium hydroxide is prepared by suspending 24.12 g of LiOH, H2O in 160 ml of ethanol.

 The suspension thus obtained is then added to the first solution and the whole is subjected to vigorous stirring for 30 min during which the temperature increases to 70 C. A white solution is thus obtained.

 The hydrolysis of this solution is then carried out by adding 62 ml of deionized and decarbonated water and the mixture is stirred for 15 min. about.

 The white solution obtained is then dried in an oven at 1500C for at least 2 hours, then the powder obtained is calcined at 800 ° C for 2 hours.

 This powder is then subjected to cold isostatic pressing under a pressure of 200 MPa, for 1 min to form spheres of approximately 4 mm in diameter. These spheres are then sintered in an alumina crucible at a temperature of 950 "C for 2 h with a heating rate of 3" C / min, after having buried them in a bed of powder of the same composition to limit variations in stoichiometry .

 After sintering, the relative density obtained is 87.6%.

Example 2: Preparation of Li2TiO3.

 In this example, a first solution is prepared by mixing under a dry nitrogen atmosphere 100 g of titanium isopropoxide Ti (OCH (CH3) 2) 4 (97%) from Aldrich Chemical Co. with 250 ml of ethanol.

 In another beaker, a suspension of lithium hydroxide is prepared by suspending 28.92 g of LiOH, H2O in 160 ml of ethanol.

The preceding suspension is then added to the first solution and the whole is subjected to vigorous stirring for 30 minutes during which the
o temperature increases to 70 C. A white solution is thus obtained.

 Hydrolysis of this solution is then carried out by adding deionized and decarbonated water in an amount such that the water / titanium isopropoxide molar ratio is 10, and the mixture is stirred for approximately 15 minutes.

 The white solution obtained is then dried in an oven at 1500C for at least 2 hours. The powder obtained is calcined at 7500C for 2 hours.

 This powder is then subjected to cold isostatic pressing under a pressure of 200 MPa for 1 minute to form spheres of approximately 4 mm in diameter. These spheres are then sintered in an alumina crucible at a temperature of 875 "C for 2 hours with a heating rate of 30C / min, after having buried them in a bed of powder of the same composition to limit variations in stoichiometry.

 After sintering, the relative density obtained is 86.9%.

Example 3: Preparation of Li2Zr0.9Ti0, i3.

In this example, a first solution is prepared by mixing under a dry nitrogen atmosphere 125 g of zirconium butoxide Zr (OC4Hg) 4 from
Aldrich Chemical Co. with 9.53 g of titanium isopropoxide Ti (OCH (CH3) 2) 4 (97%) from Aldrich
Chemical Co. and 250 ml of ethanol.

 In another beaker, a suspension of lithium hydroxide is prepared by suspending 27.57 g of LiOH, H2O in 160 ml of ethanol.

 The preceding suspension is then added to the first solution and the whole is subjected to vigorous stirring for 30 minutes during which the temperature increases to 700C. A white solution is thus obtained.

 The hydrolysis of this solution is then carried out by adding deionized and decarbonated water in an amount such that the water / (titanium isopropoxide + zirconium butoxide) molar ratio is 10, and the mixture is stirred for approximately 15 minutes.

 The white solution obtained is then dried in an oven at 1500C for at least 2 hours. The powder obtained is calcined at 750 "C for 2 hours.

 This powder is then subjected to cold isostatic pressing under a pressure of 200 MPa for 1 minute to form spheres of approximately 4 mm in diameter. These spheres are then sintered in an alumina crucible at a temperature of 950 "C for 2 hours with a heating rate of 3 C / min, after having buried them in a bed of powder of the same composition to limit variations in stoichiometry.

 After sintering, the relative density obtained is 86%.

Example 4
In this example, the ceramic spheres obtained in Examples 1 to 3 are subjected to a test for the release of tritium.

 For this purpose, each sphere which weighs 80 to 100 mg is subjected to a vacuum degassing at 650 "C for 3 h, then it is introduced into a quartz bulb previously filled with 27KPa (200 Torrs) d of helium, the ampoule is sealed and the assembly is subjected to neutron irradiation under a flux of approximately 10 14 neutrons / cm 2 for 2 h.

 After this irradiation which leads to the formation of tritium in the ceramic, the irradiated ceramic sphere is introduced into the test installation shown in FIG. 1.

 This installation comprises a quartz tube 1 into which the sample 3 is introduced at the level of a heating furnace 5. This quartz tube comprises downstream of the furnace 5 a filling of zinc balls 7 surrounded by a second furnace of heating 9 and it can be traversed by a carrier gas introduced at 11.

After passing through the two ovens, the tritium content of the carrier gas is measured in a proportional counter or an ionization chamber 13.

 During the test, the ceramic sphere 3 which has been irradiated, is introduced into the quartz tube 1, after having been extracted from the bulb used for its irradiation. In this quartz tube, it is heated by the oven 5 so as to undergo heating at a constant speed of 0.5 C / min, while being swept by a carrier gas consisting of helium containing 0.1% hydrogen to a flow rate of 30 ml / min.

When heated, the tritium included in the ceramic is released in the carrier gas, in the form of tritiated gas and tritiated water vapor; the tritiated water vapor is reduced to tritiated gas by the reducing agent 7 consisting of zinc balls with a large specific surface in the reduction furnace 9, at a temperature of 3700C. At the outlet of the reduction furnace, the carrier gas therefore contains the tritium in the form of tritiated gas and the tritium content of this gas is measured in the proportional counter or the ionization chamber 13.

 The results obtained (in Becquerel / second) as a function of the temperature reached in the heating furnace 5 (in OC) are shown in FIG. 2.

In this figure,
the mixed line curve refers to the ceramic of Example 1, Li2ZrO3,
the curve in solid lines refers to the ceramic of example 2, Li2TiO3, and
- the dashed curve refers to the ceramic of example 3, Li2Zro, gTio, 1 3.

 In view of these curves, we note that in all cases we obtain the maximum release of tritium at a temperature below 4000C.

 Thus, better results are obtained with the ceramics of the invention than with lithium aluminate for which the maximum release of tritium is generally at 4500C.

Example 5: Preparation of Li2ZrO3
The same procedure is followed as in Example 1 to prepare a powder of this ceramic, but the calcination is carried out at 750 ° C. for 2 h.

 This powder is then compressed into the form of a bar of 5 to 20 mm in diameter by cold isostatic compression under a pressure of 200 MPa for 1 min, then the sintering ability of the powder is studied by observing the dilatometric behavior of the bar depending on the temperature.

 The results obtained are given in FIG. 3.

 In this figure, which illustrates the withdrawal of the bar AL / Lo where Lg is the initial length of the bar and AL its withdrawal, as a function of the temperature, curve 1 which refers to Example 5, shows that the sintering does not is not completed at 11000C since we have not reached a plateau.

 By way of comparison, this figure shows the results obtained when the same study is carried out on the Li2ZrO3 powder obtained in Example 1 which was calcined at 8000C (curve 2) and on the Li2Zro powder, gTio, 1 3 of Example 3, which was calcined at 750 "C.

 Thus, it is noted that in the case of curve 2, the sintering is completed at 11000C and that it is the same in the case of Example 3 although the powder was not calcined at 7500C.

This confirms the interest of solid solutions
Li2ZrO3-Li2TiO3 to carry out sintering under better conditions.

Claims (13)

 1. Tritigenic material, characterized in that it comprises a ceramic containing lithium of formula Li2Til-x-yZrxSny03  in which x and y are such that  0 <x + y <1 with (0 <x <0.1 or 0.9 <x <1).  2. tritigenic material according to claim 1, characterized in that x is equal to 1.  3. tritigenic material according to claim 1, characterized in that x and y are equal to 0.  4. tritigenic material according to claim 1, characterized in that y is equal to 1.  5. Tritigenic material according to claim 1, characterized in that x is equal to 0.  6. tritigenic material according to claim 1, characterized in that x is equal to 0.1 and y is equal to 0.  7. Process for the preparation of a ceramic containing lithium of formula Li2Ti l-X-yZrxSnyO3 in which x and y are such that  0 <x + y <1 with (0 <x <0.1 or 0.9 <x <1), characterized in that it comprises the following steps  a) mixing in an anhydrous alcohol at least one alkoxide chosen from zirconium, titanium and tin alkoxides, with or without hydrated lithium hydroxide,  b) add water to the mixture obtained in step a) to hydrolyze it,  c) drying, at a temperature below 300 "C, the hydrolyzed product obtained in step b) to evaporate the alcohols and water, then calcining at a temperature between 500 and 800" C to obtain a crystallized powder,  d) shaping the powder obtained in step c), and  e) subjecting the shaped product to a thermal sintering treatment carried out at a temperature of 800 to 12000C to obtain a sintered ceramic.  8. Method according to claim 7, characterized in that the lithium hydroxide hydrated or not, is added to the solution in the form of a powder, a solution or a suspension of lithium hydroxide in the same anhydrous alcohol as that of the solution.  9. Method according to any one of claims 7 and 8, characterized in that the zirconium alkoxide is zirconium butoxide.  10. Method according to any one of claims 7 to 9, characterized in that the titanium alkoxide is titanium isopropoxide.  11. Method according to any one of claims 7 to 10, characterized in that one uses in step a) lithium hydroxide monohydrate.  12. Method according to any one of claims 7 to 11, characterized in that the alcohol is ethanol.  13. Method according to any one of claims 7 to 12, characterized in that step a) is carried out under a nitrogen atmosphere.