ISSN 1063-7834, Physics of the Solid State, 2009, Vol. 51, No. 11, pp. 2274–2281. © Pleiades Publishing, Ltd., 2009. Original Russian Text © L.S. Parfen’eva, N.F. Kartenko, B.I. Smirnov, I.A. Smirnov, D. Singh, K.C. Goretta, H. Misiorek, J. Mucha, D. Wlosewicz, A. Jezowski, A.I. Krivchikov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 11, pp. 2144–2150. 2274 1. INTRODUCTION Studies of the physico-chemical properties of ceramics-based composites are of considerable inter- est both for basic science and for the field of applica- tions. This stems primarily from their displaying con- siderable strength combined with a high resistance to high-temperature creep. These composites have, however, a shortcoming consisting in the possibility of catastrophic brittle fracture driven by fast propagation of incipient cracks. To suppress this adverse effect, fiber monoliths (FM), ceramic composite materials produced by pressing ceramic fibers coated by an appropriate binder were developed. The Si 3 N 4 /BN fiber monolith was shown [1–4] to offer the best promise among the various fiber monoliths. The present publication reports on a continuation of studies of the physical properties of Si 3 N 4 /BN fiber monoliths being conducted by the present authors [5– 10], in which attention was primarily focused on prob- ing their mechanical characteristics. This study pre- sents the first low-temperature (5–300 K) measure- ments of the thermal conductivity κ and heat capacity at constant pressure C p of the Si 3 N 4 /BN FM. 2. SAMPLE PREPARATION AND MEASUREMENT TECHNIQUE The technique adopted in FM preparation is dis- cussed in considerable detail in [6, 8]. Polycrystalline samples of the Si 3 N 4 /BN fiber monolith were fabri- cated Advanced Research (Tucson, AZ, USA). The fibers consisted of a Si 3 N 4 core (85 vol %) coated by a BN layer (15 vol %). Besides the commercial Si 3 N 4 (92 wt %), the core contained also the Y 2 O 3 (6 wt %) and Al 2 O 3 (2 wt %) oxides added for densification of the material. One first prepared monolayer sheets of parallel-oriented fibers. Next, these sheets were stacked successively one upon another and held under Thermal Conductivity and Heat Capacity of Si 3 N 4 /BN Fiber Monoliths L. S. Parfen’eva a , N. F. Kartenko a , B. I. Smirnov a, *, I. A. Smirnov a, **, D. Singh b , K. C. Goretta b , H. Misiorek c , J. Mucha c , D. Wlosewicz c , A. Jezowski c , and A. I. Krivchikov d a Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * e-mail: smir.bi@mail.ioffe.ru ** e-mail: igor.smirnov@mail.ioffe.ru b Argonne National Laboratory, Argonne, Cass Avenue Argonne, IL, 60439 United States c Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okolna 2, Wroclaw, 50-950 Poland d Verkin Institute for Low-Temperature Physics and Engineering, National Academy of Sciences of Ukraine, pr. Lenina 47, Kharkov, 61103 Ukraine Received February 24, 2009 Abstract—This paper reports on measurements within the 5–300-K temperature interval of the thermal con- ductivity of Si 3 N 4 and BN polycrystalline ceramic samples and Si 3 N 4 /BN fiber monoliths (FM) with differ- ent fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5–300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si 3 N 4 and BN sam- ples and in Si 3 N 4 /BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions—with minor assumptions, it can be maintained that, in the case of the Si 3 N 4 /BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si 3 N 4 and BN components with due account of their contributions to formation of the Si 3 N 4 /BN FM. It has been established that in the low-temperature domain (5–25 K), phonons in Si 3 N 4 /BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislo- cations. This effect is not observed in ceramic Si 3 N 4 and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si 3 N 4 and BN samples and the effective mean free path length in the Si 3 N 4 /BN [0] FM. PACS numbers: 65.60.+a, 65.90.+i DOI: 10.1134/S1063783409110134 SEMICONDUCTORS AND DIELECTRICS