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The low-temperature dependence of the heat capacity of acrylonitrile-butadiene-styrene (ABS) polymer and its composite with thermally reduced graphene oxide was studied. The existence of a so-called “boson peak” characteristic of orientational and structural glasses was demonstrated. The boson peak appears in the form of a local maximum in the heat capacity curve displayed as C/T3 vs T at Tmax = 3.52 K. It was found that for both ABS polymer and its composite, as well as for a number of other substances of a crystalline and amorphous nature, the manifestation of the anomaly of the boson peak in the heat capacity has a universal character that is described by an empirical function Δ*. The value of Δ* depends on the magnitude of the anomaly in the heat capacity and the temperature of the boson peak manifestation. Thus, this study provides new physical information about the possible causes of the boson peak appearance in disordered materials and indicates the universality of boson peak anomaly for substances with short- and long-range order.

DSC and calorimetric studies of multi-walled carbon nanotubes (MWCNTs) with average diameters in the range 7, 9 and 18 nm were carried out. MWCNTs were obtained by the CVD method. The kinetic proce...

Measurements of heat capacity at constant pressure of thermally reduced graphene oxide (trGO) and graphite oxide (GtO) were performed in the temperature range from 1.8 to 275 K by thermal relaxation method. Above 15 K, the specific heat values of trGO and GtO are close to each other within 20%. Below 10 K, the ratio of the specific heat of trGO to the specific heat of GtO increases with decreasing temperature and reaches 5 at 2 K. It was found that the temperature dependence of the specific heat of trGO is of a complex character. There are temperature ranges in which of the specific heat of trGO is close to either linear or T2.5.

The thermal conductivity of crystalline methane samples with spherical palladium nanoparticles of diameter 6, 8, 10, 12, 18 and 24 nm embedded in the CH4 structure was determined in the temperature range 2–35 K. The spherical nanoparticles featured a low dispersion of the sphere diameter and the volume fraction of the nanoparticles in the nanocomposites amounted to 0.15. Time relaxation approximation analysis shows that the matrix resistive phonon scattering processes are dominated by diffuse scattering of phonons by matrix-nanoparticle boundaries. Other considered intensities of resistive scattering mechanisms of phonons, i.e., scattering by point defects, by dislocation strain fields and by phonons in U-processes remain almost the same in nanocomposites and pure methane crystal. In transfer of the heat through the investigated nanocomposites non-resistive scattering of phonons by the nanoparticles, such as phonon specular scattering and multiple scattering of phonons show very high intensity. The nanocomposites formed of 18 and 24 nm nanospheres show anomalous power increase of the thermal conductivity coefficient dependence on temperature, exceeding the values known for a typical dielectric crystal.The thermal conductivity of crystalline methane samples with spherical palladium nanoparticles of diameter 6, 8, 10, 12, 18 and 24 nm embedded in the CH4 structure was determined in the temperature range 2–35 K. The spherical nanoparticles featured a low dispersion of the sphere diameter and the volume fraction of the nanoparticles in the nanocomposites amounted to 0.15. Time relaxation approximation analysis shows that the matrix resistive phonon scattering processes are dominated by diffuse scattering of phonons by matrix-nanoparticle boundaries. Other considered intensities of resistive scattering mechanisms of phonons, i.e., scattering by point defects, by dislocation strain fields and by phonons in U-processes remain almost the same in nanocomposites and pure methane crystal. In transfer of the heat through the investigated nanocomposites non-resistive scattering of phonons by the nanoparticles, such as phonon specular scattering and multiple scattering of phonons show very high intensity. The nanoco...

The results of experimental investigations into the thermoelectric properties (electrical conductivity, thermoelectric power, and thermal conductivity) of microtextured foils and single-crystal wires based on semimetal and semiconductor Bi1 –xSbx alloys are presented in the temperature range of 4.2–300 K. It is found that the band gap ΔE in Bi–17 at % Sb wires increases with decreasing wire diameter d, which is a manifestation of the quantum-size effect. At low temperatures (T < 50 K), in the wires with d < 400 nm, the electrical conductivity increases due to the significant contribution of highly conductive surface states characteristic of topological insulators. It is found for the first time that the thermal conductivity of semimetal Bi–3 at % Sb foils at low temperatures is two orders of magnitude lower, and that of semiconductor Bi–16 at % Sb foils one order of magnitude lower, than that in bulk samples of the corresponding composition due to significant phonon scattering at grain boundaries and surfaces. This effect leads to considerable enhancement of the thermoelectric figure-of-merit ZT and can be used in miniature low-temperature thermoelectric energy converters.

Low-temperature thermal diffusivity of molecular cryocrystals (O₂, N₂, CO, N₂O and CO₂) has been investigated and some anomalies were found. The thermal diffusivity of these crystals vary by more than 4–6 orders of the magnitude in the temperature range from 1 K up to their triple points. The thermal diffusivity displays jumps at the phase transition points. It has been found that the thermal diffusivity of solid oxygen is nonmonotonic. Below 4 K, the temperature dependence shows a plateau which is due to the fact that both the heat capacity and the thermal conductivity have the cubic temperature dependence in this temperature range. For the cryocrystals of the nitrogen group, a similar plateau is shifted to lower temperatures. The temperature dependence of the equalization time for these crystals is studied. Досліджено низькотемпературну температуропровідність молекулярних кріокристалів (O₂, N₂, CO, N₂O і CO₂). Температуропровідність цих кристалів змінюється більш ніж на 4–6 порядків в області температур від 1 К до їх потрійних точок. В області фазових перетворень спостерігаються стрибки температуропровідності. Виявлено, що температуропровідність твердого кисню є немонотонною. Нижче 4 К на температурній залежності спостерігається плато, яке обумовлено тим, що як теплоємність, так і теплопровідність в цьому температурному діапазоні мають кубічну температурну залежність. Для кріокристалів азотної групи подібне плато зміщується в бік більш низьких температур. Вивчено температурну залежність часів вирівнювання температури для цих кристалів. Исследована низкотемпературная температуропроводность молекулярных криокристаллов (O₂, N₂, CO, N₂O и CO₂). Температуропроводность этих кристаллов изменяется более чем на 4–6 порядков в области температур от 1 К до их тройных точек. В области фазовых переходов наблюдаются скачки температуропроводности. Обнаружено, что температуропроводность твердого кислорода является немонотонной. Ниже 4 К на температурной зависимости наблюдается плато, обусловленное тем, что как теплоемкость, так и теплопроводность в этом температурном диапазоне имеют кубическую температурную зависимость. Для криокристаллов азотной группы подобное плато смещается в сторону более низких температур. Изучена температурная зависимость времен выравнивания температуры для этих кристаллов.

The specific heat of multi-walled carbon nanotubes (MWCNTs) with a low defectiveness and with a low content of inorganic impurities has been measured in the temperature range from 1.8 to 275 K by the thermal relaxation method. The elemental composition and morphology of the MWCNTs were determined using scanning electron microscopy analysis and energy dispersion x-ray spectroscopy. The MWCNTs were prepared by chemical catalytic vapor deposition and have mean diameters from 7 nm up to 18 nm and lengths in some tens of microns. MWCNTs purity is over 99.4 at.%. The mass of the samples ranged from 2–4 mg. It was found that the temperature dependence of the specific heat of the MWCNTs differs significantly from other carbon materials (graphene, bundles of SWCNTs, graphite, diamond) at low temperatures. The specific heat of MWCNTs systematically decreases with increasing diameter of the tubes at low temperatures. The character of the temperature dependence of the specific heat of the MWCNTs with different diameters demonstrates the manifestation of different dimensions from 1D to 3D, depending on the temperature regions. The crossover temperatures are about 6 and 40 K. In the vicinity of these temperatures, a hysteresis is observed.

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

Graphene, a two-dimensional structure of carbon, due to its structure has unique physico-chemical properties that can be used in numerous research and industry areas. Although this structure is already well known, there are still technological (and cost) barriers which do not allow to produce this material in large quantities and hence prevent its use in various applications. For this reason, many technologies are currently being developed to obtain graphene in forms that would enable its widespread use. The graphene-like ceramics were fabricated by the high isostatic pressure method at different temperatures. This technique allows to obtain dense ceramics with various shapes. The structure and morphology of sintered graphene were investigated by XRD, SEM and the Raman spectroscopy. The hardness, thermal conductivity and electric transport measurements recorded in a wide range of temperatures were used to analyze the physical properties of the obtained ceramics.

The temperature dependencies of thermal conductivity χ(Т), electrical conductivity σ(Т), and thermopower α(Т) in foils of Bi1–xSbx alloys in the semimetal and semiconductor states, in a temperature range of 4.2–300 K, were experimentally studied. Foils of Bi1–xSbx alloys were prepared by high-speed crystallization of a thin layer of the melt on the polished inner surface of a rotating copper cylinder. High crystallization rates (v = 5 × 105 m/s) enabled a uniform distribution of the components throughout the volume. The thickness of the foils was 10–30 μm with the texture 101¯2 parallel to the foil plane and the С3 axis coinciding with the normal to the foil surface. It was shown that, in the low-temperature range (T

Experimental setup for the thermal conductivity measurement of amorphous solids with the glass transition temperature below 250 K is presented. This setup allows to heat the specimen to the melting point and then cool it fast to the glass transition temperature. Thermal conductivity can be investigated in the temperature range from 5 K to 300 K. Thermal conductivity of ethanol in amorphous and polycrystalline states was measured for the verification of the setup function. The results of the measurements agree well with those presented in literature.

The preliminary results of experimental investigations of the thermal conductivity of nanocomposites built of amorphous silica nanoparticles embedded in the structure of crystalline methane are presented. The investigations were carried out on the samples containing the particles of linear dimensions of 5, 10–20 and 42 nm. The measurements were performed with steady-state heat flow method in the temperature interval 2–35 K. A very simple thermal conductivity model shows that the dependence of the thermal conductivity coefficient of the investigated nanocomposites on temperature can be successfully described by taking into account only two mechanisms of the heat carrier dissipation: scattering of phonons by methane crystal grain boundaries and phonon-phonon interaction in U-processes. It was found that the low-temperature phonon mean free path is inversely proportional to the linear dimension of the nanopowder particles embedded in the methane crystal structure.

Influence of temperature on spectroscopic and thermal properties of Tm 3+ and Er 3+ -doped GdVO 4 and LuVO 4 crystals was examined. Polarized absorption and emission spectra as well as excited state luminescence lifetimes have been measured and analyzed especially at cryogenic temperatures in the 1.5–1.6 μm and 1.8–1.9 μm spectral ranges to assess laser qualities of these materials. The emission cross sections for 4 I 132 → 4 I 15/2 (Er) and 3 F 4 → 3 H 6 (Tm) transitions at T = 77 K were determined and confronted to the results obtained at room temperature. Effective emission cross sections estimated for aforementioned erbium and thulium transitions indicate that effective laser operation at cryogenic temperatures can be achieved in the gadolinium and lutetium vanadate crystals. Efficiency of laser material depends on its ability to heat distribution upon high-power energy pumping process. Accordingly, it has been found that the anisotropy of thermal conductivity for the undoped crystals is weakly influenced by the temperature albeit conductivity values for the GdVO 4 are higher than those for LuVO 4 at all temperatures. Incorporation of erbium or thulium ions results in a decrease of thermal conductivity in the two vanadate crystals and an anomalous temperature dependence of thermal conductivity in the LuVO 4 was observed. Anisotropy of temperature dependence of thermal conductivity was found for the LuVO 4 :3.2 at%Tm 3+ crystal. In this case the thermal conductivity along c-axis is inferior to that along a-axis of the crystal.

Mixed garnet crystals with nominal stoichiometry Gd3Al2.5Ga2.5O12 undoped, single doped with Sm3+, Ho3+, Er3+ and Yb3+ and co-doped with Ho3+ + Yb3+ and Er3+ + Yb3+ were grown by the Czochralski method. Optical spectra and thermal conductivity of obtained single crystal samples were measured and analyzed. Recorded optical absorption spectra revealed that the intensity and spectral characteristics of color centre absorption in the UV region, close to the UV absorption edge of the host, are affected by the type of incorporated rare earth ions. On the other hand, spectral characteristics of stable color centers induced by an intense UV irradiation of crystals depend weakly on the rare earth admixture. Low temperature optical spectra provided the evidence that Sm3+ ions and Er3+ ions occupy at least two different types of sites in this host but Yb3+ ions seem to be located preferentially in identical sites. Thermal conductivity of the crystals at 300 K, weakly affected by incorporated rare earth ions, is comparable to that reported for ordered gallium gadolinium garnet crystals. At cryogenic temperatures, i.e. below ca 80 K, thermal conductivities of the crystals containing rare earth ions are markedly smaller than that of undoped one, however.

The effective thermal conductivity of samples of cryocrystal nanocomposite obtained from argon and SiO2 nanopowder was determined in the temperature interval 2–35 K using the steady-state method. The thermal conductivity of crystalline argon with nanoparticles of amorphous silica oxide embedded in its structure shows a weak dependence on particle linear dimension in the interval 5–42 nm. The temperature dependence of the thermal conductivity of the nanocomposites can be well approximated by taking into account only the two mechanisms of heat carrier scattering: phonon-phonon interaction in U-processes and scattering of phonons by dislocations.

Thermal conductivity κ of a single crystal triclinic 4-bromobenzophenone polymorph was measured along axis b from 4.5 to 303 K. The κ values versus temperature were represented as a sum of κ 1 , which generally describes the thermal conductivity of ordered crystals, and κ TA responsible for activation processes, which fits nicely experimental data from 120 up. DFT analysis led to the conclusion that the activation agent responsible for the high-temperature behavior is the molecular mode that involves C–Br oscillations. It is for the first time that the exponential thermal conduction was ascribed to a specific intramolecular excitation.

High thermal conductivity is an important parameter for nitride-based power electronic and deep-UV light emitters. Especially in the latter case short period superlattices and multicomponent alloys are used and the knowledge of the thermal properties of the binary compounds is sufficient. In-plane and cross-plane thermal conductivity of AlGaN/GaN superlattices were measured by differential two-wire 3ω method in the temperature range from 147 to 325 K. Samples were grown by metalorganic vapor phase epitaxy; the structure quality and accuracy of superlattice structures preparation were verified by means of HRXRD and transmission electron microscopy. It was observed, that value of thermal conductivities decrease with decreasing period thickness, while temperature dependencies differ from each other—in-plane thermal conductivity decreases, and cross-plane—increases with increasing temperature. Callaway method was used for thermal conductivity calculation; dependence of boundary scattering rate on the phonon wavelength was taken into account. Minimum thermal conductivity was added to calculated values to include the influence of high frequency acoustic phonons and optical phonons on the heat transport. Calculations are in good agreement with experimental results.

The preliminary results of investigations of heat transfer in nanocomposites consisting of nanoparticles randomly distributed in solid carbon monoxide matrix are presented. In the experiment the thermal conductivity coefficient dependence on temperature for CO crystal with silica and palladium nanoparticles of different size embedded in the crystal structure was determined over the temperature range 2.2–35 K by steady-state heat flow method. The results of the measurements were analyzed within the frame of relaxation time approximation. The analysis shows that lowering of the thermal conductivity of the nanocomposites relative to pure carbon monoxide crystal observed for both types of the investigated nanoparticles, palladium and silica, is caused mostly by scattering of phonons by boundaries of the nanoparticles. Additionally, the presence of the nanoinclusions promotes higher density of dislocations and influences the matrix lattice dynamics. Наведено попередні результати досліджень теплопереносу в нанокомпозитах, які містять наночастинки, що випадково розподілені у твердій матриці монооксиду вуглецю. Методом стаціонарного теплового потоку в температурному інтервалі 2,2–35 К визначено експериментальні залежності коефіцієнта теплопровідності від температури для кристалевого СО, що містить наночастинки оксиду кремнію та паладію різних розмірів. Результати вимірювань проаналізовано в рамках апроксимації часу релаксації. Аналіз показує, що зниження теплопровідності нанокомпозитів щодо чистого кристала монооксиду вуглецю, яке спостерігається для досліджених наночастинок паладію та оксиду кремнію, обумовлено головним чином розсіюванням фононів на границях наночастинок. Крім того, наявність нановключень сприяє більш високій щільності дислокацій і впливає на динаміку гратки матриці. Приведены предварительные результаты исследований теплопереноса в нанокомпозитах, содержащих наночастицы, случайным образом распределенные в твердой матрице монооксида углерода. Методом стационарного теплового потока в температурном интервале 2,2–35 К определены экспериментальные зависимости коэффициента теплопроводности от температуры для кристаллического СО, содержащего внедренные наночастицы оксида кремния и палладия различных размеров. Результаты измерений проанализированы в рамках аппроксимации времени релаксации. Анализ показывает, что понижение теплопроводности нанокомпозитов относительно чистого кристалла монооксида углерода, наблюдаемое для исследуемых наночастиц палладия и оксида кремния, обусловлено главным образом рассеянием фононов на границах наночастиц. Кроме того, наличие нановключений способствует более высокой плотности дислокаций и влияет на динамику решетки матрицы.

Study on half-Heusler compounds RENiSb, RE = Sc, Dy, Ho, Er, Tm, Lu, was performed in perspective of high-temperature thermoelectric application. Relatively small electrical resistivity (4–60 μ Ω m in 300–1000 K) combined with fairly high positive Seebeck coefficient (40–160 μV/K) yielded for the RENiSb ternaries rather large thermoelectric power factors at 750 K, namely 14 μW K−2cm−1 for ScNiSb and 11 μW K−2cm−1 for TmNiSb. At room temperature the thermal conductivity measured for the two materials was 8.1 W K−1m−1 and 4.4 W K−1m−1, respectively. The value obtained for the latter compound is smaller than those reported for unoptimized half-Heusler phases based on d-electron elements. This finding may be explained by strong point defect scattering occurring due to intrinsic disorder in crystal structure of TmNiSb, or to smaller extent by enhanced scattering on incoherent grain boundaries with the impurity phases. Nevertheless, achieving good thermoelectric performance of the RENiSb phases at high temperatures requires proper tuning of carrier concentration, as well as further reduction in their thermal conductance by means of alloying and/or nanostructurization.

The heat capacity $C(T)$ of a 2-bromobenzophenone (2-BrBP) single crystal measured at temperatures from 0.4 to 30 K demonstrates anomalies inherent in disordered solids: Instead of the Debye law ${C}_{\mathrm{D}}\ensuremath{\propto}{T}^{3}, C(T)$ shows a linear temperature dependence and a boson peak, i.e., peculiarities typical of solids with disorder. Computations for a pair of interacting 2-BrBP molecules revealed a few low-frequency states which are expected to actively couple to long-wave phonons. Raman scattering spectra demonstrate two strong spikes at energies close to the boson peak center at $26\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We relate the Raman spikes to low-energy intramolecular modes, which are the rotational oscillations of the substituted phenyl ring around the C-C link to the ketone. Thus, we have found that a completely ordered crystal made up of molecules with low-energy intramolecular modes can show low-$T$ properties, which are inherent in irregular solids such as various glasses, etc.

The measurements of thermal conductivity coefficient of a solid carbon monoxide crystal containing argon and nitrogen admixtures at different concentrations (1.5, 3, 6, 12.5, 25% for N2 and 0.5, 1, 1.25, 2, 4% for Ar) were performed in the temperature range from 1.5 to 40 K by steady-state heat flow method. For analysis of the experimental data the Callaway method in the framework of the Debye model was used. The contribution of various mechanisms of phonon scattering, including scattering by disordered dipoles of the CO matrix, to the thermal conductivity of CO–N2 and CO–Ar solid solutions were taken into account.

The low-temperature dependence of thermal conductivity coefficient of dielectric crystals on temperature spreads over a vast region, both in terms of the absolute value of the coefficient as well as its functional dependence on the temperature. However, we were able to notice a group of simple molecular polycrystals which show their thermal conductivity very close to each other. What is more, the similarity of the thermal conductivity of the polycrystals for many reasons resembles known and studied for over 30 years the universality observed for low-temperature thermal conductivity of amorphous solids. Here, utilizing already developed thermal conductivity models, we try to understand what phenomena could result in the similarity of the thermal conductivity of the polycrystals and therefore explain the finding. Provided that some other conditions are met at least each of three different phonon scattering mechanisms could lead to the similarity.

8 pags.; 4 figs.; 3 tabs.; 7th IDMRCS: Relaxation in Complex Systems, © 2014 Elsevier B.V. All rights reserved. The thermal conductivity of CBrnCl4 -n,(n = 0,1,2) and 2-adamantanone (C10H14O) crystals which exhibit substitutional disorder has been measured for temperatures between 2 K and 150 K. Such temperatures comprise signatures of dynamical processes involving the site occupancy of the chlorine and bromine atoms for CBrnCl4 -n,(n = 0,1,2) and the oxygen atom for (C10H14O), as it has been revealed by both, calorimetry and spectroscopic techniques. The data analysis is carried out in terms of several phonon scattering channels contributing to a resistive relaxation rate which can be represented by two contributions due to propagating phonons whose mean-free path exceeds half the phonon wavelength as well as a term attributed to localized short wavelength modes. The implications of the present findings for studies pertaining to glassy dynamics are briefly discussed., This work is supported by the Spanish Ministry of Science and Innovation (grant no. FIS2011-24439), the Catalonia Government (grant no. 2009SGR-1251). The authors acknowledge the support from the National Academy of Science of Ukraine in the frame of the joint Ukrainian– Russian Project (agreement no. 10-2012) on the subject “Metastable states of simple condensed systems”. One of us A.I. K. thanks the Spanish Ministry of Education (grant SAB2011-0070) for an invited position at the Universitat Politècnica de Catalunya.

6 pags., 4 figs., The low-temperature thermal and transport properties of an unusual kind of crystal exhibiting minimal molecular positional and tilting disorder have been measured. The material, namely, low-dimensional, highly anisotropic pentachloronitrobenzene has a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that low-temperature glassy anomalies can be found in a system with minimal disorder due to the freezing of (mostly in-plane) reorientational jumps of molecules between equivalent crystallographic positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of the above-mentioned universal glassy properties at low temperature., This work has been supported by the Spanish MINECO through projects FIS2014-54734-P, FIS2014-54498-R, and MAT2014-57866-REDT and “María de Maeztu” Program for Units of Excellence in R&D (MDM-2014-0377 and the National Science Centre (Poland) Grant No. UMO-2013/08/M/ST3/00934. We also acknowledge the Generalitat de Catalunya under Project 2014SGR-581 and the Autonomous Community of Madrid through program NANOFRONTMAG-CM (S2013/MIT-2850).

Order or disorder in subsystem of dipolar constituents of a solid often significantly influences its physical properties. The results of investigation of the effect of glassy state in subsystem of carbon monoxide molecules on thermal conductivity of CO crystal in its equilibrium vapor pressure are reported. The thermal conductivity of a high quality carbon monoxide crystal was specified over the temperature range 1.2–45 K. The results of these measurements were analyzed within the frame of relaxation time approximation. It was shown that at low temperatures phonon scattering by subsystem of disordered CO molecules is the most significant dissipative mechanism of heat transfer in the investigated crystal.

Micron-grained (2–11 μm), erbium-doped, polycrystalline lithium titanium germinate ceramics were fabricated out of the heat-treated of the presintered samples at a temperature of 970 °C. X-ray diffraction showed a high crystallinity of the synthesized ceramics and confirmed formation of the natisite type structure with the P4/nmm symmetry. FE-SEM analysis revealed that the microstructure of obtained ceramics depends on dopant concentration. The number of pores increases with increasing the dopant dose. The specific heat measurements showed that the temperature of the ferroelastic phase transition was significantly reduced for Li2TiGeO5 ceramics (222 K) compared to the bulk (233.5 K). The investigations of the thermal conductivity showed its untypical behavior, proportional to the square of temperature, for a dielectric at low temperatures which could be explained by the microstructure of the samples.

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.

A “porous glass + indium” nanocomposite has been prepared. The thermal conductivity κ(T) and electrical resistivity ρ(T) of the nanocomposite have been measured in the temperature range 5–300 K, and their fractions accounted for by nanoindium embedded in 7-nm channels of the porous glass have been determined. For comparison, κ and ρ of the bulk polycrystalline indium sample have been measured in the same temperature range. The electronic and phonon components of the thermal conductivity have been calculated for the nanoindium and bulk indium. It has been demonstrated that, as the result of the emergence of boundary electron and phonon scattering in the nanoindium, the electrical resistivity of this material becomes larger, and the phonon thermal conductivity, smaller than those of the bulk indium.

By applying the Klemens–Callaway expression for the thermal conductivity in the framework of the Debye's model of phonons we present an analysis of the thermal transport by phonons in ceramic composites Si3N4/BN of various fibre structure (including plaited fibre configurations denoted [0/45] and [0/90]) and biomorphic composites (or ecocomposites) SiC/Si based on beech matrices filled with Si of two different volume percentage (2% and 26%). We obtained a good agreement with the Debye–Klemens–Callaway theory of the thermal conductivity at low temperatures for the ceramic composites Si3N4/BN with the plaited fibre configurations. A good fitting of the Callaway expression for the thermal conductivity coefficient in the whole temperature range can be made to our experimental results for the biomorphic composites SiC/Si. The latter fitting allowed us to assess the magnitude of the mean free path of phonons scattered by the boundaries of grains. We also estimated that the contribution of the conduction electrons to the heat transfer at low temperatures is 5–7% of the effective thermal conductivity in case of SiC/Si.

The results of measurements of the thermal conductivity dependence on temperature of pure ammonia crystal obtained in the temperature range 1.3–44 K have been shown. Quantitatively, the sample displays the thermal conductivity typical for a dielectric crystal. Below 2 K a singularity of the dependence of thermal conductivity coefficient on temperature was observed. The singularity was linked to the existence of TLS in the crystal. The analysis of the temperature dependence of the phonon mean free path in the NH3 crystal was also done.

We show, through thermal conductivity coefficient measurements of Li2TiGeO5 ceramics, that the anomaly associated with the ferroelastic phase transition can be distinguished. We correlate this anomaly with the measurement of the domain structure observed in the vicinity of phase transition temperature Tc. Near the Tc, the appearance of ferroelastic domains which become new scattering centers for thermal waves, strongly affect the thermal conductivity in Li2TiGeO5 ceramics.

Newly discovered phase transition at 281 K in Na2TiGeO5 ceramics was studied with thermal analysis techniques. The results of thermal conductivity and dilatometric studies unequivocally demonstrated a ferroelastic character of the second-order phase transition.

This paper reports on measurement of the heat capacity at constant pressure C p of silicon bio-carbide prepared within the 5–300 K temperature interval from beech tree wood (bio-SiC(BE)), and within 80–300 K, from tree wood of sapele (bio-SiC(SA)), as well as SiC/Si ecoceramics of beech, sapele, and white eucalyptus wood. It has been shown that in bio-SiC(BE) the measured heat capacity contains a significant contribution of surface heat capacity, whose magnitude decreases with increasing temperature. Of the ecoceramics, only SiC/Si(SA) characterized by a high enough porosity has revealed a small contribution to the heat capacity coming from its surface component. The experimental results obtained are discussed.

{Ca□}[NbGa□](GaNb□)O-type cubic Ia3d garnets (CNGG) have been grown by the Czochralski method, either unmodified or incorporating different crystalline modifiers (Li or Mg), and Yb as a laser dopant. From nuclear magnetic resonance, single crystal X-ray and powder neutron diffraction studies it is shown that Li incorporates exclusively into the 24d tetrahedral site of the host, removing tetrahedral-sited Nb and filling cationic vacancies in this site. A comparison of low temperature (6 K) Yb spectroscopy in crystals with different compositions at the tetrahedral sites and a modeling of energy positions of the F(0) and F(0′) Yb levels show that the strongest contribution to the Yb optical absorption/emission bandwidth is associated with the electric charge of cations/vacancies occupying the two tetrahedra at the shortest distance (3.12 Å) from the central 24c dodecahedral Yb. Cationic disorder over the remaining four tetrahedral, octahedral and dodecahedral sites also contributes to the Yb bandwidth but to a lesser extent. To obtain the largest Yb bandwidth, the two nearest tetrahedra must contain cations/vacancies with electric charges as different as possible. Although the decrease in the concentration of vacancies at tetrahedral sites associated with Li incorporation induces some reduction of the Yb optical bandwidth with regard to the unmodified Yb:CNGG crystal, Li incorporation along with the use of high purity precursors yields crystals with less coloration, longer Yb lifetime, and slightly larger thermal conductivity, which favors laser operation performance., This work was supported by the Spanish Ministry of Economy and Competitiveness and by the European Regional Development Fund through Projects MAT2014-56607-R and and RTC-2015-3631-5.

The thermopower coefficients of bio-SiC and SiC/Si ecoceramics prepared from sapele tree wood have been measured in the temperature interval 5–300 K. The measurements have been performed both along and perpendicular to empty (bio-SiC), as well as empty and partially silicon-filled (SiC/Si) channels in the samples. In bio-SiC, a contribution to thermopower associated with electron drag by phonons has been shown to exist within the temperature interval 5–200 (250) K. No such effect is realized in SiC/Si. This is assumed to derive from the presence in this material of heavily doped silicon embedded in SiC channels and the dominant part it plays in the behavior of the thermopower of this ceramics. The results obtained for the thermopower are compared with the available data for bio-SiC prepared from white eucalyptus tree wood and heavily doped bismuth.

Samples of β-SiC/Si ecoceramics with a silicon concentration of ∼21 vol % have been prepared using a series of consecutive procedures (carbonization of sapele wood biocarbon, synthesis of high-porosity biocarbon with channel-type pores, infiltration of molten silicon into empty channels of the biocarbon, formation of β-SiC, and retention of residual silicon in channels of β-SiC). The electrical resistivity ρ and thermal conductivity κ of the β-SiC/Si ecoceramic samples have been measured in the temperature range 5–300 K. The values of ρ Si chan (T) and κ Si chan (T) have been determined for silicon Sichan located in β-SiC channels of the synthesized β-SiC/Si ecoceramics. Based on the performed analysis of the obtained results, the concentration of charge carriers (holes) in Sichan has been estimated as p ∼ 1019 cm−3. The factors that can be responsible for such a high value of p have been discussed. The prospects for practical application of β-SiC/Si ecoceramics have been considered.

The electrical resistivity and thermal conductivity of high-porosity (∼52 vol %, channel-type pores) bio-SiC samples prepared from sapele wood biocarbon templates have been measured in the temperature range 5–300 K. An analysis has been made of the obtained results in comparison with the data for bio-SiC samples based on beech and eucalyptus, as well as for polycrystalline β-SiC. The conclusion has been drawn that the electrical resistivity and thermal conductivity of bio-SiC samples based on natural wood are typical of heavily doped polycrystalline β-SiC.

Undoped Li3Lu3Ba2(MoO4)8 monoclinc (C2/c) single crystals, as well as crystals doped with Tm or Ho and codoped with Tm and Ho, have been grown by the top seeded solution growth method using Li2MoO4 as the solvent. A convenient growth flux was found for the solute/solvent molar composition of 1:5; for this, the growth temperature interval was ≈1073–1053 K, with a cooling rate of 0.05 K/h. The crystal shape is controlled by the seed orientation, and a- and b-oriented seeds give rise to near rectangular cross sections while c-oriented seeds produce rhombohedral cross sections. The grown crystals are deficient in Li with regard to the nominal formula. Li and Lu (Tm, Ho) ions share the same 8f crystal site with occupancy factors near 0.2 and 0.8, respectively. The relationships between the crystallographic and physical frames of some crystal properties (thermal expansion, refractive indices, and optical absorption) are determined as well as the specific heat and thermal conductivity (along a- and b-crystal axe...

The Neel temperature T N(n) of the Co3O4 antiferromagnet nanostructured in channels of porous borosilicate glass with channel cross sections of ∼7 nm has been determined from thermal conductivity measurements. It has been shown that the Neel temperature T N(n) of this nanomaterial is approximately equal to 20 K, which is considerably lower than T N = (30–40) K for the bulk Co3O4 sample.

This paper reports on measurements of the thermal conductivity κ and the electrical resistivity ρ in the temperature range 5–300 K, and, at 300 K, on X-ray diffraction studies of high-porosity (with a channel pore volume fraction of ∼47 vol %) of the beech wood biocarbon prepared by pyrolysis (carbonization) of tree wood in an argon flow at the carbonization temperature T carb = 800°C. It has been shown that the biocarbon template of the samples studied represents essentially a nanocomposite made up of amorphous carbon and nanocrystallites—“graphite fragments” and graphene layers. The sizes of the nanocrystallites forming these nanocomposites have been determined. The dependences ρ(T) and κ(T) have been measured for the samples cut along and perpendicular to the tree growth direction, thus permitting determination of the magnitude of the anisotropy of these parameters. The dependences ρ(T) and κ(T), which have been obtained for beech biocarbon samples prepared at T carb = 800°C, are compared with the data amassed by us earlier for samples fabricated at T carb = 1000 and 2400°C. The magnitude and temperature dependence of the phonon thermal conductivity of the nanocomposite making up the beech biocarbon template at T carb = 800°C have been found.

This paper reports on measurements of the thermopower S of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures (T carb). The thermopower S has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves S(T carb) have revealed a noticeable increase of S for T carb < 1000°C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for T carb ∼ 1000°C they undergo a phase transition of the insulator-(at T carb < 1000°C)-metal-(at T carb > 1000°C) type. The existence of this transition is attested also by the character of the temperature dependences S(T) of beech wood biocarbon samples prepared at T carb above and below 1000°C.

The heat capacity at constant pressure C p of the biocarbon matrix prepared at a beech wood carbonization temperature of 1000°C has been measured in the temperature range 80–300 K. It has been shown that, in the temperature range 90–180 K, the heat capacity is C ∼ T 0.8 and, at T = 190–300 K, it is C p ∼ T 1.2. The phonon mean free path l(T) in the biocarbon matrix has been calculated using the obtained dependences C p (T), our previous results on the phonon thermal conductivity of the carbon framework of this biocarbon matrix, and data available in the literature on the sound velocity in the matrix. It has been demonstrated that, in the temperature range 200–300 K, the mean value of l is ∼ 15 A, which is close to the sizes of nanocrystallites (“carbon fragments”) of ∼ 12A, obtained earlier from X-ray diffraction data for the carbon matrix under consideration. These nanocrystallites participate in the formation of the carbon framework of the beech wood biocarbon matrix.

A nanocomposite chrysotile-KDP (KH2PO4) was prepared. KDP was introduced into empty nanochannels of chrysotile asbestos with diameters of ∼5 nm. Thermal conductivity κ and heat capacity at a constant pressure C p of the samples of chrysotile asbestos and nanocomposite chrysotile asbestos-KDP were measured in a temperature range of 80–300 K. Based on the analysis of the behavior of temperature dependences κ(T) and C p (T) of the composite, temperatures of the ferroelectric transition T F for KDP in nanochannels of chrysotile asbestos were determined. It turned out to be equal to ∼250 K at T F ∼ 122 K for massive KDP samples.

Thermal conductivities of two benzophenone single crystals have been measured at temperatures from 4.7 to 270 K. The experimental data for both are consistent for temperatures above 15 K. The thermal conductivity of benzophenone can be represented as a sum of two contributions: κ1 + κTA where κ1 is due to the standard phonon mechanisms accepted for ordered crystals and κTA takes into account the heat flow due to intermolecular hopping of thermally activated intramolecular vibrational modes. The thermal activation contribution in unsubstituted benzophenone is substantially smaller when compared to that in any of the two para-bromobenzophenone polymorphs studied previously. Unlike in the 4-bromobenzophenone crystals, the microscopic agent responsible for intramolecular excitation(s) was not determined. The characteristic intramolecular excitation energy was evaluated to be 220 K, about three times less compared to 4-bromobenzophenone.

The thermopower S(T) of the composites prepared by filling empty sap channels in high-porosity biocarbon preforms of white pine wood by melted copper in vacuum and the thermopower S(T) of these preforms have been measured in the temperature range 5–300 K. The biocarbon preforms have been obtained by pyrolysis of pine wood in an argon flow at two carbonization temperatures, 1000 and 2400°C. An analysis of the experimental data has demonstrated that the thermopower of the composites is determined by the contribution related to the copper filling the channels of the biocarbon preform and exhibits a characteristic temperature dependence with a deep minimum close to 20 K. This suggests that copper in the preform channels is essentially a Kondo alloy with the iron and manganese impurities entering into it from the carbon preform in the course of infiltration of melted copper.

The electrical resistivity ρ(T) of the novel type of composites prepared by infiltrating melted copper in vacuum in empty sap channels of white pine high-porosity biocarbon preforms has been measured in the temperature range 5–300 K. Biocarbon preforms have been prepared by pyrolysis of tree wood in an argon flow at two carbonization temperatures, 1000 and 2400°C. The electrical resistivity of the composites has been found to vary relatively weakly with temperature and to pass through a characteristic minimum near 40–50 K, which can be ascribed to iron and manganese impurities penetrating into copper from the carbon preform when liquid copper is infiltrated into it. It has been shown that the electrical resistivity ρ(T) of the composites is governed primarily by the specific microstructure of the preform, which is made up of parallel channels with an average diameter of about 50 μm interrupted by systems of thin capillaries. The small cross section of the copper-filled capillaries accounts for these regions providing the major contribution to the electrical resistivity of the composites. An increase in the wood carbonization temperature brings about a noticeable increase in the effective capillary cross section and a decrease in the electrical resistivity ρ(T) of the composite.