Search

Lattice effects related to metal–insulator (M–I) and charge-ordered (CO) transitions have been studied through the thermal expansion and the thermal conductivity measurements from 77 to 450 K in La 0.7 Ca 0.3 Mn 0.9 Cr 0.1 O 3 and La 0.4 Ca 0.6 Mn 0.99 Cr 0.01 O 3 . Thermal-expansion coefficient shows an obvious contraction near the M–I transition temperature T MI , but a sharp increase around the CO transition temperature T CO . Thermal conductivity is enhanced below both T MI and T CO . These findings are attributed to the strong charge–lattice coupling associated with the different electron states in manganites.

The dc magnetization, the electrical resistivity and the thermal conductivity have been measured in the temperature range of 5–300 K for Cr 67 Fe 23 Mn 10 and Cr 55 Fe 25 Mn 20 alloys. Curie temperature was confirmed and the ferromagnetic phase is not long-range order; Neel temperature was recorded and the antiferromagnetic phase is of spin-density wave (SDW). Our results are discussed on the basis of the magnetic nesting associated with paramagnetic and ferromagnetic cluster and SDW antiferromagnetic in this system.

The temperature dependence of the thermal conductivity κ(T) and electrical resistivity ρ(T) has been measured for perovskite Nd0.7Sr0.3Mn1−xCrxO3 (0.01≤x≤0.15) in the temperature range of 10–300 K. The double-bump feature in ρ(T) is observed for x = 0.08 and 0.10 samples. Below the insulating-metallic transition temperature TIM, κ(T) shows a sharp increase and such an increase is depressed gradually by the increasing Cr content. While κ(T) displays an anomalous decrease around the temperature where the second bump of ρ(T) emerges. We suggest that the enhancement of κ(T) below TIM is related to both phonons and magnons, the ferromagnetic interaction of Mn3+-O-Cr3+ should be super-exchange interaction rather than double-exchange interaction, and the Cr doping just weakens the DE interaction in this system.

The magnetic, electrical and thermal transport properties of the perovskite La0.7Ca0.3Mn0.9Cr0.1O3 have been investigated by measuring dc magnetization, ac susceptibility, the magnetoresistance and thermal conductivity in the temperature range of 5–300 K. The spin glass behaviour with a spin freezing temperature of 70 K has been well confirmed for this compound, which demonstrates the coexistence and competition between ferromagnetic and antiferromagnetic clusters by the introduction of Cr. Colossal magnetoresistance has been observed over the temperature range investigated. The introduction of Cr causes the ``double-bump'' feature in electrical resistivity ρ(T). Anomalies on the susceptibility and the thermal conductivity associated with the double-bumps in ρ(T) are observed simultaneously. The imaginary part of ac susceptibility shows a sharp peak at the temperature of insulating–metallic transition where the first resistivity bump was observed, but it is a deep-set valley near the temperature where the second bump in ρ(T) emerges. The thermal conductivity shows an increase below the temperature of the insulating–metallic transition, but the phonon scattering is enhanced accompanying the appearance of the second peak of double-bumps in ρ(T). We relate those observed in magnetic and transport properties of La0.7Ca0.3Mn0.9Cr0.1O3 to the spin-dependent scattering. The results reveal that the spin–phonon interaction may be of more significance than the electron (charge)–phonon interaction in the mixed perovskite system.

The thermal diffusivities of nanostructured NiAl (n-NiAl) alloys with different relative densities and the polycrystalline NiAl alloy are measured by using photoacoustic technique in which the signal is detected at the front surface while the rear-surface is excited. It is shown that the values of thermal diffusivity of the nanostructured NiAl alloy are lower than that of the polycrystalline NiAl alloy. The thermal diffusivities of n-NiAl alloys decrease with the decrease of their relative densities when they are over 70%. When the relative density of n-NiAl alloys is lower than 70%, an anomalous increase of thermal diffusivity is observed, which indicates there exists a critical excess volume ΔVc (or critical relative density) in n-NiAl alloys near 70%. The results are discussed.

The temperature dependence of the thermal conductivity in Bi2Sr2Ca1−xCexCu2Oy x = 0.1, 0.2, 0.3, 0.4 is presented. With increasing Ce-doping level, the thermal conductivity peak under TC is suppressed then disappears, while another peak appears at low temperatures for the non-superconducting compounds. The numerical analysis shows that the thermal conductivity peak under TC can be well described by the normal electron relaxation-time contribution model, and the phonon-induced thermal conductivity peak could be well described within the Debye approximation of the phonon spectrum. The existence and variation of these two thermal conductivity peaks indicate the adjustability between the superconducting and insulating components in the samples with different Ce-doping levels.

Using electrophoretic deposition it is possible to prepare massive area thick film which shows good superconducting properties in the microwave field; it opens perspectives for the application of polycrystalline YBCO layer in RF cavity. A coaxial resonator with an inner conductor which is coated in YBCO layer by electrophoresis is used to measure the surface resistance of YBCO at 500MHz. The recent results are as follows: R 300 K = 124 mΩ , R 77 K = 0.6 mΩ .

Direct current resistivity and its temperature dependence for nanostructured NiAl (n‐NiAl) was investigated at temperatures from 77 to 300 K. The resistivity of n‐NiAl (size 5–6 nm) was higher than that of polycrystalline NiAl and increased with decreasing density. For the n‐NiAl with relative density D≳70%, the resistivity decreased with decreasing temperature, manifesting metallic behavior. However, its temperature coefficient of resistivity decreased monotonically with decreasing density, and changed sign from positive to negative at densities 68%–70%, below which it displays on nonmetallic behavior. These results can be well interpreted by dominant grain boundary scattering.

Besides the Kondo effect observed in dilute magnetic alloys, the Cr-doped perovskite manganate compounds La0.7Ca0.3Mn1−xCrxO3 also exhibit Kondo effect and spin-glass freezing in a certain composition range. An extensive investigation for the La0.7Ca0.3Mn1−xCrxO3 (x=0.01, 0.05, 0.10, 0.3, 0.6, and 1.0) system on the magnetization and ac susceptibility, the resistivity and magnetoresistance, as well as the thermal conductivity is done at low temperature. The spin-glass behavior has been confirmed for these compounds with x=0.05, 0.1, and 0.3. For temperatures above Tf (the spin-glass freezing temperature) a Curie–Weiss law is obeyed. The paramagnetic Curie temperature θ is dependent on Cr doping. Below Tf there exists a Kondo minimum in the resistivity. Colossal magnetoresistance has been observed in this system with Cr concentration up to x=0.6. We suppose that the substitution of Mn with Cr dilutes Mn ions and changes the long-range ferromagnetic order of La0.7Ca0.3MnO3. These behaviors demonstrate that ...

The temperature dependence of thermal conductivity (K),resistivity (ρ) and low field ac susceptibility (χ) of perovskite nick elates Nd＿2-xSr＿xNiO＿4(x =0.33,1.35) have been measured at temperatures from 77 to 300 K. The thermal conductivity an omalies associated with charge_order (CO) and spin_order (SO) are observed simultaneously in Nd＿1.67Sr0.33NiO＿4 for the firs t time so far as we know. The thermal conductivity is enhanced belowT_CO , but is suppressed near T＿SO. There is also response to CO and S O in the χ-Τ curve. Only CO is observed in the ρ-Τ curve. In contrast, th ere are no anomalies in K-Τ, ρ-Τ andχ-Τ of Nd＿1.67Sr0.33NiO＿4. The anomalous phonon thermal conductivi ty in Nd＿1.67Sr0.33NiO＿4 indicates that there are strong charg e_phonon interaction and the spin_phonon interaction in this compound.

The photoacoustic spectra from ptype αporous silicon carbides prepared under UV photoassisted process and under darkcurrent condition have been measured，the original silicon carbides were also measured. Combined with RosencwaigGersho theory, the photoaconstic spectra are turned into the absorption spectra. It is found that the energy band gaps of the porous silicon carbide were lower than that of the original silicon carbide, and the causes were analyzed in detail. The absorptions near the absorption edge were studied as well.

The temperature dependence of thermal conductivity and electronic conductivity in superconductor MgB 2 , MgCNi 3 and Bi 2 Sr 2 Ca 0.9 Ce 0.1 Cu 2 O 8+y are presented. The thermal conductivity of Bi 2 Sr 2 Ca 0.9 Ce 0.1 Cu 2 O 8+y shows a peak under T c observ ed in cuprates, but those of MgB 2 and MgCNi 3 decrease monotonously. We calculate the elec tron thermal condu ctivity and the phonon thermal conductivity by Wiedemann-Franz law. The electron s contribute a large fraction to the thermal conductivity in normal state of MgB 2 and MgCNi 3 because of the non-localigation of mutual eff ect of electrons. The analysis on electronic thermal conductivity indicates that the scattering by impurities prevails in electronic thermal resistance of both samples.

The thermal conductivity of MgB2 polycrystalline sample in the mixed state has been investigated as a function of magnetic field (0-7 T) and temperature (5-45 K). The thermal conductivity increased rapidly in low magnetic fields, and became saturation in higher fields. This behaviour correlates with the two-gap electronic structure of MgB2. Analysis indicates that the remarkable en hancement of thermal conductivity of MgB2 polycrystalline sample in the mixed state cannot be completely attributed to the electronic contribution.

The low\|frequency internal friction in Cr 75 Fe 16 Mn 9 alloy has been measured in temperature range of 100—600?K and at frequencies lower than 10?Hz. It is found that an internal fraction peak at 450?K and corresponding jump of modules exist in the paramagnetic phase.This indicates there is a disorder\|order transition on coupling states between individual spins for the Cr and Fe components. There is a broad internal friction peak in the temperature range from 200 to 300?K,which consists of a peak at 280?K and the extended broad peak at 220—270?K. We suggest that this effect is the reflection of the spin density wave antiferromagnetic transition and the coexistence of different magnetic phases in the alloy. Our results reflect the fact that internal friction method would be a sensitive and effective probe for investigating the change of microstructure including electric structure. PACC: 6240; 7530F; 7550E

The thermal conductivity and the electrical conductivity of Cr75FexMn25-x(x=16,18) alloys have been measured over the temperatures range from 77K to 300K. The change of slope on thermal conductivity-temperature curve and the maximum on electrical conductivity-temperature curve have been found near the transition temperature of spin density wave type antiferromagnetism, which is related to the opening of the energy gap. Both alloys exhibit a rise in the phonon thermal eonductivity with decreasing temperature as being characteristic of umklapp processes.

The photoluminescence (PL) spectra from p-type α-porous silicon carbides prepared under UV photo-assisted process and under dark-current condition are investigated in detail. Emission bands with peak energies of 2.35, 2.50,2.70, and 3.43 are resolved. The PL stability in time and the PL difference arising from different tuning excitation energies are studied. It is found that the PL spectra of the α-porous silicon carbide depend strongly on the preparation conditions for electrochemical etching. The PL spectrum of the sample prepared under photo-assisted process has an enhancement on the lower-energy side of the emission; on the contray, another one under dark-current condition has an enhancement at the higher energy side, and the former stability is better than the latter one, and the latter PL intensity decreases with the increase of the time in the air. The reasons about these differences are discussed.

The thermal conductivity and thermal diffusivity of ZrO2 coatings were studied using arc-photothermal evaluation technique in which the sample was heated at front surface and detected at back surface, and finite-difference thermal flow model was applied. The values of thermal conductivity and thermal diffusivity of ZrO2 are ｋ＝０．００６９Ｊ／ｃｍ·Ｋ·ｓ and α＝０．００２８ｃｍ2／ｓ, respectively. It is shown that, first, the finite-difference thermal flow model is an available and useful method for the multilayer coating system, and, second, the arc photothermal evaluation technique has low cost of equipment, and more simple and convenient measurement.

The thermal properties for PTS are studied by photoacoustic effect in temperature range 175-225 K. It is found that there is a peak of C3K at 200 K (here C3 and K are the specific heat and thermal conductivity of PTS, respectively), which shows crearly the existence of a second order phase transition in view of thermal propertis for the first lime. An anomalous thermal conductivity in the above temperature range is deduced from experiment results.

In the amorphous alloys Nb_(1-x)Ni_x (x = 0.564, 0.598, 0.65), we observed the double Curie temperature phenomenon of itinerant electron magnetism and also the typical susceptibility cusp for spin glass freezing. In this alloy system, both itinerant electron magnetism and localized magnetic moments appear very clearly.

The photoacoustic signal and phase angle have been measured for the single crystal of Polybis-(p-toluence sulfonate) of 2,4-(hexadiyne)-l,6-diol(PTS) in the temperature range of 175-225 K based on the photoacoustic effect (PAE.) The jump of photoacoustic signal and phase angle at 200 ±0.5 K was observed. We suggested that the jump can be attributed to the second order structural phase transition in PTS.

The valence-band spectra of Nb-Ni glasses during crystallization have been studied systematically by means of UPS technique and X-ray diffraction. The double peaks in valence-band spectra of NbNi2 are discussed.

The electron structures of amorphous Nb100-xNix (x= 65, 59.8, 56.4) alloys are studied by means of UV-Photoelectron spectroscopy (UPS). It is found that the position of the Fermi level is located near the minimum of the density of states, which explains the double Curie-point phenomena in Nb-Ni alloys. Analysing the electron structure of Nb-Ni alloys, it is pointed out that the hybridization of two d-bands with energies close to each other may form a new hybridizated band near EF and the hybridization is related to the valence difference. The charge transfer makes the peaks of bands shifting. The density of states of TE-TL. alloys at EF should include two parts of contributions, i.e., that from 3d electron stsates, which is of advantage to magnetism, and that from 4d electron states, being of advantage to superconductivity. Different ratio of these contributions at Er influences the magnetism and superconductivity of the alloys greatly. The coexistence of localized magnetic moments and superconductivity has also been discussed.