Phonon softening on the specific heat of nanocrystalline metals

Specific heat enhancement in several common nanocrystalline metals is established by comparison with their bulk counterparts. Measurements were carried out in Fe, Cu, Ni and binary alloy LaAl(2). The excess specific heat is evidenced as a low temperature peak below 65 K and a high temperature slope above 150 K. The experimental data are in good agreement with a model which considers contributions from the grain boundary and core atoms in the nanoparticles. This model is supported by Raman spectroscopy measurements, showing a softening of the frequency phonon modes associated with a size reduction and increase of the atomic disorder.     

Thermal Conductivity of Small Nickel Particles

The thermal conductivity of nanoscale nickel particles due to phonon heat transfer is extrapolated from thin film results calculated using nonequilibrium molecular dynamics (NEMD). The electronic contribution to the thermal conductivity is deduced from the electrical conductivity using the Wiedemann–Franz law. Based on the relaxation time approximation, the electrical conductivity is calculated with the Kubo linear-response formalism. At the average temperature of T=300 K, which is lower than the Debye temperature Θ_D=450 K, the results show that in a particle size range of 1.408–10.56 nm, the calculated thermal conductivity decreases almost linearly with decreasing particle size, exhibiting a remarkable reduction compared with the bulk value. The phonon mean free path is estimated, and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.     

Study of optical phonon modes of CdS nanoparticles using Raman spectroscopy

The reduction in the grain size to nanometer range can bring about radical changes in almost all of the properties of semiconductors. CdS nanoparticles have attracted considerable scientific interest because they exhibit strongly size-dependent optical and electrical properties. In the case of nanostructured materials, confinement of optical phonons can produce noticeable changes in their vibrational spectra compared to those of bulk crystals. In this paper we report the study of optical phonon modes of nanoparticles of CdS using Raman spectroscopy. Nanoparticle sample for the present study was synthesized through chemical precipitation technique. The CdS nanoparticles were then subjected to heat treatment at low temperature (150°C) for extended time intervals. The crystal structure and grain size of the samples were determined using X-ray diffraction and HRTEM. The Raman spectra of the as-prepared and heat treated samples were recorded using conventional Raman and micro-Raman techniques. The spectrum of as prepared sample exhibited an intense, broad peak at 301 cm⁻¹ corresponding to the LO phonon mode. Higher order phonon modes were also observed in the spectra. A noticeable asymmetry in the Raman line shape indicated the effect of phonon confinement. Other features in the spectra are discussed in detail.     

Low-temperature thermal properties of polypropylene

We have studied the low temperature thermal properties of a polypropylene copolymer (PP): thermal conductivity (between 0.1 and 4 K), specific heat (between 0.06 and 1 K) and thermal expansion (from 4.2 K to room temperature).Both the thermal conductivity and the specific heat temperature data were interpreted using the tunnelling model for phonon scattering.The measured thermal properties show that PP is suitable for use as thermal insulating support material in cryogenic devices.     

The influence of nanoparticle size on the magnetostrictive properties of cluster-assembled Tb-Fe nanofilms

The giant magnetostrictive Tb-Fe films assembled by nanoparticles have been prepared by the low energy cluster beam deposition. The dependence of the magnetostriction on the size of the nanoparticles is examined for the films. It is shown that the nanofilms have obtained higher saturation magnetostriction at the cluster size of 30 nm in average. The dependence of magnetostriction on particle size is ascribed to the degree of magnetic anisotropy which is related to the effective distance of exchange coupling between the adjacent Tb-Fe nanoparticles. This work demonstrates that the magnetostriction can be varied by tuning the particle size, which is important for control over the magnetostrictive properties of the films at nanoscale.     

Thermal Properties of Ln0.7Ca0.3CoO3 (Ln = La, Pr, and Nd) Perovskites

In this article, a detailed investigation of the thermal properties of doped perovskite cobaltates Ln_0.7Ca_0.3CoO₃ (Ln = La, Pr, and Nd) at temperatures 0 ${{\rm K} \leq T \leq}$ 350 K using the modified rigid ion model (MRIM) is presented. Theoretically, MRIM provides arguably the most realistic interaction potential to treat these properties. The variation of the specific heat and volumetric thermal expansion coefficient for these cobaltates in the temperature range 0 ${{\rm K} \leq T \leq}$ 350 K is computed. The computed specific heat is in reasonably good agreement with available experimental data. Present investigations reaffirm the presence of strong electron?Cphonon interactions in these compounds. The dominant contribution to the specific heat is the phonon term that follows here the Debye-type solid. In addition, the results on the temperature dependence of the molecular force constant (f), the reststrahlen frequency (??), the Debye temperature (?? _D), and the Gruneisen parameter (?? ₀) are also discussed.     

NiFe Nanoparticles: A Soft Magnetic Material?

Polytetrahedral NiFe nanoparticles with diameters of (2.8+/-0.3) nm have been obtained by hydrogenation of Ni[(COD)(2)] (COD=1,5-cyclooctadiene) and Fe[{N(SiMe(3))(2)}(2)] at 150 degrees C using stearic acid and hexadecylamine as stabilizing ligands. The nanoparticles are superparamagnetic at room temperature and display a blocking temperature of 17.6 K. Their anisotropy (2.7x10(5)J m(-3)) is determined to be more than two orders of magnitude higher than that of the bulk NiFe alloy (10(3)J m(-3)) and is close to that determined for Fe nanoparticles of the same size. Still, they display a magnetization of (1.69+/-0.05) mu(B) per metallic atom, identical to that of the bulk NiFe alloy. Combining the results from X-ray absorption and M?ssbauer studies, we evidence a progressive enrichment in iron atoms from the core to the surface of the nanoparticles. These results are discussed in relation to both size and chemical effects. They show the main role played by the enriched Fe surface on the magnetic properties and address the feasibility of soft magnetic materials at the nanoscale.     

Influence of Spin Fluctuations Upon Heat Capacity of 2D Fermi Liquid Formed on Hectorite

We measured low-temperature heat capacities of two-dimensional Fermi liquid (2D FL) formed on Hectorite down to 15mK. At a coverage where mean atomic distance is comparable to that of bulk ³ He liquid, we found that the temperature dependence deviates considerably from the expected linearity. To account for the deviation we carried through integrals including RPA spin susceptibility for the Stoner-Hubbard(SH) or the Landau's Fermi liquid (LFL) models over the effectively entire, energy-momentum space without imposing the paramagnon approximation or the like. We found numerically that the spin fluctuation develops a T ² correction at T<T* which, at T>T*, is taken over by a Tlog T term where T* is some hundredths of T _F . With these corrections we are able to interprete the data and have determined the parameters for each model consistently at the densities observed.     

Nontoxic and abundant copper zinc tin sulfide nanocrystals for potential high-temperature thermoelectric energy harvesting

Improving energy/fuel efficiency by converting waste heat into electricity using thermoelectric materials is of great interest due to its simplicity and reliability. However, many thermoelectric materials are composed of either toxic or scarce elements. Here, we report the experimental realization of using nontoxic and abundant copper zinc tin sulfide (CZTS) nanocrystals for potential thermoelectric applications. The CZTS nanocrystals can be synthesized in large quantities from solution phase reaction and compressed into robust bulk pellets through spark plasma sintering and hot press while still maintaining nanoscale grain size inside. Electrical and thermal measurements have been performed from 300 to 700 K to understand the electron and phonon transports. Extra copper doping during the nanocrystal synthesis introduces a significant improvement in the performance.     

The glass transition and thermodynamics of liquid and amorphous TiO(2) nanoparticles

The glass transition and thermodynamics of spherical liquid TiO(2) nanoparticles, with different sizes ranging from 2 to 5?nm, have been studied in a model under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. The structural properties of liquid nanoparticles at 3500?K have been analyzed in detail through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Moreover, we also show the radial density profile in nanoparticles. Calculations show that size effects on the structure of a model are significant and that liquid TiO(2) nanoparticles have a distorted pentahedral network structure with the mean coordination numbers Z(Ti-O)≈5.0 and Z(O-Ti)≈2.5, while amorphous TiO(2) nanoparticles have an octahedral network structure. The temperature dependence of the surface structure and surface energy of the nanoparticles has been obtained and is presented. In addition, the size dependence of the glass transition temperature and the temperature dependence of the diffusion constant of atomic species have been found and are discussed.     

Spin Density Wave Excitations in the Specific Heat of La2CuO4.11 Single Crystals

We measured the specific heat vs. temperature in single crystal samples of superconducting La₂CuO_4.11, in magnetic fields up to 15 T. After subtraction of the phonon contribution to the specific heat, obtained measuring a nonsuperconducting crystal, we found a broad anomaly centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu²⁺ spins possibly enhanced by an interplay with the charge degrees of freedom.     

Lattice dynamics of solid Ne

Previously derived interatomic potentials for Ne are used to examined selected solid state properties. The self-consistent phonon harmonic approximation with suitable corrections for cubic anharmonic terms is used to calculate phonon dispersion curves at 5 K anda=4.454 Å. The isochoric temperature shifts between 5 and 22 K of phonons propagating in the [100] direction are evaluated using two approximations for the phonon self-energy. ThepV isotherm at 4.2 K and the volume dependence of the elastic constants and the zero-temperature Debye theta from the heat capacity are also calculated. There is reasonable agreement with experiment at low temperatures but near the melting point there is a real discrepancy with recent Brillouin scattering data and reasons for this are discussed.     

Far field thermal radiation through nanoholes and apertures

Thermal radiation through a nanohole or a nanoaperture is calculated in this paper. Results show that for aperture size < dominant wavelength of thermally excited photons, heat flux deviates significantly from the well-known Planck blackbody expression of sigmaT4, where sigma is the Stefan-Boltzmann constant and T is the temperature. The heat flux through a nanohole is proportional to T8 and a4, where a is the radius of the hole. Finally it is shown that for aT < 10(6), where a is expressed in nm and T is expressed in K, radiative heat flux through the hole is proportional to T8 and for aT > 10(6) radiative heat flux is equal to the Planck blackbody heat flux; i.e., it is proportional to T4.     

Dielectric and thermal properties of a machinable glass—ceramic at low temperatures

Measurements of the dielectric properties (2–300 K), specific heat (2–20 K), and thermal conductivity (2–22 K) are reported for a mica-containing glass-ceramic which has a machinability in the range from brass to low-carbon steel. The dielectric constant increases with increasing temperature and is field independent for field strengths up to at least 70 kV cm^?1 at low temperatures. Power-supply-limited attempts to measure the dielectric breakdown strength at low temperatures are consistent with the reported strength at room temperature (1.4 MV cm^?1). The thermal properties are similar to fused SiO₂ with two exceptions: the thermal conductivity does not show the ‘knee’ at ～ 10 K typical of amorphous materials, and the specific heat deviates strongly from a T³ law below 3.5 K     

Effect of aggregation kinetics on the thermal conductivity of nanoscale colloidal solutions (nanofluid)

The thermal conductivity, k, of nanoscale colloidal suspensions (also known as nanofluid), consisting of nanoparticles suspended in a base liquid, is much higher than the thermal conductivity of the base liquid at very small volume fractions of the nanoparticles. However, experimental results from various groups all across the world have shown various anomalies such as a peak in the enhancement of k with respect to nanoparticle size, an increase as well as a decrease in the ratio of k of these colloidal solutions with the k of the base fluid with increasing temperature, and a dependence of k on pH and time. In this paper, the aggregation kinetics of nanoscale colloidal solutions are combined with the physics of thermal transport to capture the effects of aggregation on k. Results show that the observed anomalies reported in experimental work can be well described by taking aggregation kinetics into account. Finally, we show that colloidal chemistry plays a significant role in deciding the k of colloidal nanosuspensions.     

Thermal conductivity of ge and ge-si core-shell nanowires in the phonon confinement regime

Heterostructure core-shell semiconductor nanowires (NWs) have attracted tremendous interest recently due to their remarkable properties and potential applications as building blocks for nanodevices. Among their unique traits, thermal properties would play a significant role in thermal management of future heterostructure NW-based nanoelectronics, nanophotonics, and energy conversion devices, yet have been explored much less than others. Similar to their electronic counterparts, phonon spectrum and thermal transport properties could be modified by confinement effects and the acoustic mismatch at the core-shell interface in small diameter NWs (<20 nm). However, fundamental thermal measurement on thin core shell NWs has been challenging due to their small size and their expected low thermal conductivity (κ). Herein, we have developed an experimental technique with drastically improved sensitivity capable of measuring thermal conductance values down to ～10 pW/K. Thermal conductivities of Ge and Ge-Si core-shell NWs with diameters less than 20 nm have been measured. Comparing the experimental data with Boltzmann transport models reveals that thermal conductivities of the sub-20 nm diameter NWs are further suppressed by the phonon confinement effect beyond the diffusive boundary scattering limit. Interestingly, core-shell NWs exhibit different temperature dependence in κ and show a lower κ from 300 to 388 K compared to Ge NWs, indicating the important effect of the core-shell interface on phonon transport, consistent with recent molecular dynamics studies. Our results could open up applications of Ge-Si core shell NWs for nanostructured thermoelectrics, as well as a new realm of tuning thermal conductivity by "phononic engineering".     

Low temperature heat capacity and sound velocity in fullerite C60 orientational glasses

The nature of the linear term in the heat capacity of fullerite C₆₀ has been investigated. The low-temperature dependence of the sound velocity has been determined from the data of the heat capacity at temperatures below 4 K. A model of the dynamic configuration excitations (DCE) is proposed to describe the contribution of the linear term in heat capacity and calculate the dependence of sound velocity. It is shown that this model, apparently, adequately describes the dynamics of cluster formations of the short-range order in fullerite C₆₀ by taking into account excitations of both the atomic and electronic subsystems. In the framework of this model, it is shown that low-energy tunnel states that are located at the boundaries of C₆₀ domains make a dominant contribution to the low-temperature effects in the heat capacity and sound velocity of C₆₀.     

Low-temperature transient thermal anomalies in vitreous quartz

Fast-time (10–100 microseconds) heat-pulse methods have been employed to obtain simultaneously the specific heats and thermal diffusivities of vitreous quartz at low temperatures. The experiments were designed to look for a time dependence in the specific heat as predicted by the tunneling model used to describe the low-temperature thermal properties of amorphous materials. No time dependence was observed over the range 0.6 to 3.4 kelvins when the pulses were analyzed using the response predicted by standard heat-diffusion theory. However, a large, reproducible, and systematic dependence on the input energy of the heat pulse was observed in the measured properties of our samples.     

Magnetic and Magnetocaloric Properties of Er2TiMnO7 Compound

Magnetic properties and magnetocaloric effects of the Er₂TiMnO₇ compound have been evaluated by magnetization and heat capacity measurements. The reversible magnetic-entropy change is found to be just above the boiling point of helium and hydrogen liquefaction temperature range of 2 to 50 K. The magnetocaloric behavior estimated from specific heat data is confirmed by adiabatic magnetization above 2 K, and indicates that Er₂TiMnO₇ is a potential candidate for application in magnetic refrigeration in the low-temperature range.     

Probing Inhomogeneities in Type II Superconductors by Means of Thermal Fluctuations, Magnetic Fields, and Isotope Effects

Type II superconductors, consisting of superconducting domains embedded in a normal or insulating matrix, undergo a rounded phase transition. Indeed, the correlation length cannot grow beyond the spatial extent of the domains. Accordingly, the thermodynamic properties will exhibit a finite size effect. It is shown that the specific heat and penetration depth data of a variety of type II superconductors, including cuprates, exhibit the characteristic properties of a finite size effect, arising from domains with nanoscale extent. The finite size scaling analysis reveals essential features of the mechanism. Transition temperature and superfluidity increase with reduced domain size. The combined finite size and isotope effects uncover the relevance of local lattice distortions.