DE1936689C3 - Process for producing an oxide with high conductivity - Google Patents

Description

The invention relates to an oxide with high electron conductivity and a method for its production.

_{Lanthanum chromite LaCrO 3 is} known as a substance which has high electron conductivity. Since then, lanthanum chromite with a melting point of 2500 ° C and a crystal structure of the Perovoy kit type and higher electron conductivity has been used as an electrode material for magnetohydrodynamic generators (MHD generators)

The invention aims to provide an oxide with a much higher Electron conductivity as lanthanum chromite as well as a Oxide whose change in conductivity with temperature is small and which has high electronic conductivity having at low temperature

Furthermore, a method for easily producing an oxide having such properties is an object of US Pat Invention.

description

Lanthanum chromite LaCrO ₃ is known for its high electron conductivity. This material with a melting point of 2500 ° C and a crystal structure of the perovskite type is used, among other things, as an electrode material for magnetohydrodynamic generators (MHD generators)

However, there is a need for a material with an even higher electron conductivity in particular for the aforementioned purpose.

The invention has therefore set itself the task of an oxide with an even higher electron conductivity than Lantnanchromit to create, in addition, the Conductivity fluctuates only slightly with temperature and especially a high one Electron conductivity is present at low temperatures

This object is achieved according to the invention in an eo process for the production of an oxide with high conductivity, consisting of a solid solution of calcium ions in lanthanum chromite, in that a mixture of lanthanum oxide, calcium carbonate and chromium oxide at approximately 1600 to replace 1 to 15% of the lanthanum ions with calcium ions ° C over a period of approximately 1 hour to produce a solid solution of (LaCa) CrO ₃ 69-81: 0.5-7.5: 38

to mix and then prepare a solid solution of (LaCa) CrO ₃ by baking the mixture for about 1 hour at about 1600 ° C. If the ratio of lanthanum oxide in the mixing ratio of lanthanum oxide and chromium oxide is larger than the above value, the solid solution obtained is easily weathered, and if the ratio is lower, it is easily evaporated at high temperature. For the solid solution, lanthanum hydroxide can be used in place of lanthanum oxide.

After the obtained solid solution of (LaCa) CrO _{3 has been} pulverized, an organic substance such as paraffin, wax emulsion or CMC (carboxymethyl cellulose) etc. is added to the pulverized (LaCa) CrO ₃ as a binder or plasticizer, and the organic substance containing ( LaCa) CrO ₃ is molded and dried using isostatic pressing (rubber pressing), uniaxial dry pressing or extrusion, etc. Thereafter, the solid solution is fired in an oxidizing atmosphere at 1750 to 1850 ° C for about 5 hours

If hot press sintering is used, after pulverizing the solid solution, the powder is placed in a graphite mold and sintered under a pressure of about 200 kg / cm ² at a temperature of 1600-1700 ° C for 20-40 minutes a reducing atmosphere is carried out, defects occur in the oxygen ion lattice of the sintered substance, and the electron conductivity of the substance is lowered in order to eliminate the defects sintered in the reducing atmosphere substance is h again in air at about 1500 ⁰ C for 1 to its oxidation burned.

The chemical formula of the solid solution of (LaCa) CrO ₃ can be represented as follows:

(La, - JCa _x ) CtO ₃

wherein χ represents the amount of lanthanum ions in the lanthanum chromite (LaCrO ₃ ) that have been replaced by calcium ions in the solid solution. The value of χ is preferably in the range of 0.01-0.15, since the properties of the solid solution χ at lower than 0.01 are virtually the same as those of lanthanum chromite, and at greater than 0.15 χ

the mechanical properties of the solid solution are inadequate, the mixing ratios of the components of the solid solution, in which Af the values 0.01; 0.02; 0.05; O ₁ JO or 0.15 are listed in the following table

Table I.

X LbjOj CaCOi CnOj 0.01 80.6 03 38.0 0.02 79.8 1.0 38.0 0.05 773 2.5 38.0 0.10 733 5.0 38.0 0.15 69.2 7.5 38.0 the following table II shows the lattice constants

and the conductivity at room temperature, 500 ^° C., 1000 ^° C. and 1500 ^° C. for lanthanum chromite and for the oxides which were obtained by sintering raw material containing the constituents in the respective ratios of Table I according to the present process.

Table II

at conductivity (ohm-'cm " ¹ ) a

Room 500 ° C 1000 ⁰ C 1500 ° C (A)

temperature

0.124 0.214 0.292 3.880

0.82 UO 1.85 3.879

1.39 233 3.13 3.878

338 6.46 6.67 3.874

83 11 14 3.863

113 14 17 3.858

As is clear from Table Π, the Oxides according to the invention have significantly higher conductivities than lanthanum chromite, the change in conductivity with the temperature is small and in particular the oxides can, since they have high conductivities have a lower temperature than electrode materials for the semi-hot MHD generator or as Resistance heating elements are used.

The following examples illustrate the invention without restricting it.

example 1

77.5 g of lanthanum oxide, 23 g of calcium carbonate and 38.0 g of chromium oxide were mixed and the mixture was ^{heated to 1600 °} C. and kept at this temperature for 1 hour

O 0.00168 0.01 0.0085 0.02 0.0113 0.05 0375 0.10 1.27 0.15 1.62

held to make a solid solution of (LaCa) CrOj. The solid solution was pulverized and placed in a graphite mold inserted solid solution in the graphite mold was kg / heated cm ² from room temperature to 1650 ⁰ C under a pressure of 200 and 20 min at 1650 ° C under pressure sintering, the volume density of the sintered substance, the diameter of which was 10 mm and the length of which was about 20 mm was 5.4 and its porosity was 20%.

ίο On both end surfaces of the sintered substance platinum powder was applied and then fired in air in an electric furnace at 1500 "C for 1 hour. Als As a result, a large number of electron holes were formed in the fired substance, which serve as a carrier, and the platinum on the end surface-in was glazed In the resulting oxide, the amount of calcium ions in the solid solution was replaced lanthanum ions 0.05 and the conductivity of the oxide had the same value as in Table II for λγ = 0.05 given.

Example 2

The solid solution obtained in Example 1 was pulverized in a proportion of 1% by weight of the In the solid solution, boric acid as a binder was mixed with the powdered solid solution

With the mixture, a graphite mold was filled, and the mixture wihde from room temperature to 1380 ⁰ C under a pressure of 200 kg / cm ² and heated for 20 minutes at 1380 ⁰ C under pressure sintered The volume density of the sintered substance was 6.72 and its porosity 1%.

On both end surfaces of the sintered substance platinum powder was applied and h in air at 1 J5 1500 ⁰ C baked

The conductivity of the fired substance was the same as in Example 1.

Example 3

The solid solution obtained in Example 1 was pulverized into particles less than 2 µm in diameter. 3% by weight of paraffin, based on the weight of the solid solution, was mixed therewith, and the mixture was molded by isostatic pressing into a rod 15 mm in diameter and 250 mm in length. The solid solution molded rod was left in an oxidizing atmosphere for about 5 hours fired at 1750-1850 ⁰ C. The volume density of the oxide obtained was 6.28, its porosity was 7.6%, and its conductivity was as high as that in Example 1.

Claims (2)

Patent claims: 1. A process for the production of an oxide with high conductivity, consisting of a solid solution of calcium ions in lanthanum chromite, characterized in that a mixture of lanthanum oxide, calcium carbonate and chromium dioxide at about 1600 ° C during the replacement of 1 to 15% of the lanthanum ions by calcium ions a period of about 1 hour is fired to produce a solid solution of (LaCa) CrO ₃ , the solid solution is pulverized, the powdered solid solution is shaped, optionally with the addition of a binder, and then the shaped solid solution in an oxidizing atmosphere at 1750 to Fired at 1850 "C. 2. Use of an oxide according to claim 1 as Electrode material for MHD generators or as resistance heating elements. is burned, the solid solution that is pulverized pulverized solid solution, optionally with the addition of a binder, is shaped and then the shaped solid Solution in an oxidizing atmosphere at 1750 to 1850 ° C is fired. The oxide produced according to the invention has high heat resistance together with high electron conductivity
In BE-PS 6 81 120 is the production of a ίο conductive oxide from a solid solution of Calcium ions are described in lanthanum chromite, although the introduction of calcium ions is in the form of Calcium zirconate takes place while calcium carbonate is used according to the invention. By using of calcium carbonate as well as the specific process measures mentioned above is a Obtained oxide, which surprisingly has a significantly higher conductivity than that from the said BE-PS has known oxide In detail, the method according to the invention consists in lanthanum oxide, calcium carbonate and chromium oxide in the ratio of