The present communication reports the results of an investigation of the properties of surface tension, density, and electrical conductance for molten Li/ sub 2/CO/sub 3/, Na/sub 2/CO/sub 3/, and K/sub 2/CO/sub 3/ and some mixtures in the temperature range of 750 deg to 1000 deg C. The surface tensions are approximately twice the values for the corresponding chlorides; the densities and electrical conductance are quite comparable to those of the chlorides. The ionic nature of the molten carbonates is examined from the physico-chemical criteria based on these properties. The mechanism of electrical transport is considered in the light of current theoretical concepts. Relative to Na/sub 2/CO/sub 3/--K/ sub 2/CO/sub 3/ mixtures, the surface tensions and partial molal volumes indicate but little deviations from the predictions for thermodynamically ideal ma-xtures. (auta)

MOLTEN CARBONATE ELECTROLYTES: PHYSICAL PROPERTIES, STRUCTURE, AND MECHANISM OF ELECTRICAL CONDUCTANCE. Technical Report No. 12

Lithiumcarbonat was treated hydrothermally in the range of temperature from 80 °C to 600 °C. The Li2CO3-crystals of the starting material were monoclinic with the unit cell a = 8.39 A, b = 5.00 A, c = 6.21 A, β = 114.5° and the space group C2/c (Zemann). During the increase of temperature the habit of the Li2CO3-crystals changed in the ranges of higher temperatures that they could be described as pseudocubic. In parallel with this development the electrical conductivity of the crystals also changed.



Lithiumkarbonat wurde hydrothermal im Temperaturbereich von 80 °C bis 600 °C behandelt. Die als Ausgangsmaterial verwendeten Li2CO3-Kristalle waren monoklin mit den Gitterkonstanten a = 8,39 A, b = 5,00 A, c = 6,21 A, β = 114,5° und der Raumgruppe C2/c (Zemann). Im Laufe der Temperatursteigerung anderte sich der Habitus der Li2CO3-Kristalle so, das sie in den hoheren Temperaturbereichen als pseudokubisch bezeichnet werden konnen. Parallel zu dieser Kristallentwicklung veranderte sich auch die elektrische Leitfahigkeit der Kristalle.

The Hydrothermal Effect on Crystal Development and Electrical Conductivity of Li2CO3

Growth and characterisation of Bi2Sr2Ca1Cu2Oy by the floating zone method

Sparingly soluble calcium sulphate dihydrate (gypsum) crystals are grown by the gel method, derived from the diffusion of calcium chloride into the set gel containing ammonium sulphate. By etching (010) matched cleavages (of gypsum grown by above method) with analar grade nitric acid as well as successively etching a thin flake, it has been established that the tracks of dislocations initially make an inclination to the cleavage face and continue almost parallel to the cleavage face, contrary to the earlier findings on natural gypsum [1] where the dislocations are shown to be oblique, parallel lines passing through the body of the crystal. The change in the tracking of dislocations in gel-grown gypsum has been attributed to the role of gel inclusion, as well as a faster growth rate. The implications are discussed.

Tracking of dislocations in gel-grown gypsum single crystals

Textured growth and orientation dependence of hardness measurement on superconducting Bi2Sr2CaCu2O(y)

Li 2 CO 3 single crystals have been grown by zone melting technique in carbondioxide atmosphere. The diameter of the grown crystal depends on the growth rate. The quality of the crystal depends on the growth rate, temperature of the molten zone, choice of the seed and the temperature of the auxiliary furnace. The crystal shows cleavage plane. The etch studies on cleavage planes show that the etch pits are always triangular in character.

Growth of Li2CO3 Single Crystals by Zone Melting Technique

IAthium carbonate (Li2CO3) single crystals were grown by the zone melting technique [1] in CO2 atmosphere, doping with 1% K2CO 3 by weight. The etch pits on undoped crystal cleavages etched in 2% citric acid were triangular in shape with well defined sides, whereas the doped crystal cleavages revealed triangular pits which were anomalous in nature. When the trapped impurities were in bulk the etch pits revealed irregular contours, while in traces, pits showed grooves extending from the bottom point of the pit. On observing the etch figures in a scanning electron microscope, as well as from the etching experiments, it was conjectured that the impurities preferred dislocation sites. That the impurities become trapped in the interior of the crystal has been ascertained from the Laue X-ray transmission diffraction patterns. Much useful information can be obtained from the studies on different types of etch pits and their density regarding the nature of defects, impurity content and the history of growth of crystals. Pate1 and co-workers [2, 3] have shown that the te.rracing nature of etch pits are found due to the precipitation of impurities along the dislocation lines. It is quite probable that the fission fragment lying along the dislocation line may deviate the geometrical shape of the etch pit due to the preferential etching along the fission track giving rise to a groove or a wing shape to the etch pits. The anomalous nature of the etch pits formed due to the dissolution of the fission fragments present in the body of the crystal has been reported by Fleischer

The role of impurities on chemical etch pits in potassium-doped Li2CO3 single crystals

Linear arrays of dislocations (straight, terminating and curved) are observed on etching with 2% citric acid (002) cleavages of lithium carbonate (Li 2 CO 3 ) single crystals, grown by zone melting technique. As the dislocations are non-uniformly spaced and disappear on successive etching, they may not be revealing low-angle grain boundary, nor slip traces as they are terminating and curved meaning crystallographically non-oriented. On the observation of experiments on successive etching and etching of matched pairs, it is concluded that the arrays of etch pits reveal dislocation walls (dislocation density 1.1 × 10 6 - 1.5 × 10 7 pits cm -2 ) probably created due to the localized thermal stresses released as a result of high temperature annealing of the crystal during growth. The implications are discussed.

Sub-boundaries on (002) cleavages of Li2CO3 single crystals

Well-defined triangular etch pits are produced on etching (002) cleavages of lithium carbonate single crystals with 2% citric acid. On etching of cleavage face and matched face these pits nucleate at the intersection sites of dislocations with the cleavage face. On successive etching of a cleavage face, shallow irregular arrays of pits do not enlarge in size as compared to other isolated pits on the same face. These irregular arrays of pits have reasonable correspondence on an etched match face, suggesting they form at dislocation sites.

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M. Palaniswamy

Papers

Growth of Li2CO3 Single Crystals by Zone Melting Technique

The role of impurities on chemical etch pits in potassium-doped Li2CO3 single crystals

Sub-boundaries on (002) cleavages of Li2CO3 single crystals

Evidence of screw dislocations on (002) cleavages of lithium carbonate single crystals