Thermal conductivity and diffusivity of KH 2 PO 4 and NH 4 H 2 PO 4 polycrystalline samples near their phase transitions
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Simultaneous measurements of thermal conductivity and diffusivity of ferroelectric polycrystalline samples of the KDP family type have been performed with the transient plane source (TPS) technique over the temperature range 88-300 K. The thermal conductivity behavior resembles that of the corresponding single crystals, but with values a factor of three lower. This decrease in thermal conductivity of the polycrystalline form is attributed to scattering mechanisms due to microstructural imperfections such as point defects, grain boundaries, etc. Values of porosity have been estimated using the dimensions of the unit cell deduced from the x-ray analysis and the apparent densities. The data indicate that the phase transition mechanisms (displacement type and order-disorder type) still dominate the conduction processes in spite of the presence of other scattering mechanisms due to the imperfection of the microstructure in the polycrystalline form of these materials.
I. INTRODUCTION Ferroelectric crystals are used in many important applications,1'2 and they are of prime interest due to their unique physical and thermal properties.3 Some of these ferroelectric crystals belong to the MH 2 PO 4 family, where M = K or NH4 [KH 2 PO 4 , potassium dihydrogen phosphate (KDP), or NH 4 H 2 PO 4 , ammonium dihydrogen phosphate (ADP)]. These materials are isomorphous in their paraelectric phases (i.e., above their Curie temperatures), where the structure is tetragonal. However, below their Curie temperatures both have orthorhombic structure, but with opposite ferroelectric phases, being ferroelectric for the former and antiferroelectric for the latter. The thermal conductivity for single crystals was reported many years ago,4 but as far as we know neither thermal conductivity nor thermal diffusivity data have been reported for these materials in their polycrystalline form. The idea of investigating polycrystalline samples arose due to the following two reasons: first, their preparation is easier than that for single crystals and second, larger size samples can be fabricated as compared to the size of crystals used today. The aim of this work was to compare the difference in the thermal conductivity behavior between the two forms, crystalline and polycrystalline, for both materials in the vicinity of their transition (Curie) temperatures. In addition, the applicability of the more recently developed Plane Source techniques, such as the transient plane source TPS 5 ' 6 and the dynamic plane source DPS, 7 was investigated for measuring the thermal transport J. Mater. Res., Vol. 9, No. 7, Jul 1994
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properties of solids in the vicinity of the phase transition temperatures. II. EXPERIMENTAL A. Sample preparation and characteristics Preparation of both samples is rather simple. The powders (supplied by E. Merck, Darmstadt, Germany) were mixed well by grinding in a dry agate mortar and weighed according to the required amount depending on the sample type. Then, using 28 mm pellet die
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