Low-Temperature Thermal Conductivity and Specific Heat of Plastically Deformed High-Purity Tantalum Single Crystals

The thermal conductivity and the specific heat of plastically deformed, high-purity tantalum single crystals have been measured together with an amorphous SiO₂ specimen in the temperature range between 50 mK and about 2 K. After plastic deformation, the thermal conductivity was reduced by a factor of more than 100 and had a magnitude comparable to that of the amorphous SiO₂ specimen. However, the specific heat measurements revealed a T³-relationship for the phonon contribution down to the lowest temperatures with a magnitude as in the case of undeformed crystalline solids. Thus, it must be concluded that the scattering of thermal phonons introduced by the plastic deformation has to be attributed to intrinsic properties of dislocations rather than to the interaction of phonons with tunneling systems. In the present paper the scattering mechanism is related to oscillations of geometrical kinks in non-screw dislocations.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.     

Investigation of the heat conductivity of niobium in the temperature range 0.05–23 K

We report thermal conductivity measurements on a single-crystal niobium specimen of resistivity ratio 33,000 over the temperature range 0.05–23 K in the superconducting state and above 9.1 K in the normal state. The axis of the niobium rod was [110] oriented. The surface roughness was varied by sandblasting of the sample. The values of the thermal conductivity in the range from the lowest temperatures up to the maximal value covered a range of six orders of magnitude (=2×10^–5 W cm^–1 K^–1 at 50 mK to =22 W cm^–1 K^–1 at 9 K). Above 2 K the results for the untreated and the sandblasted sample are in accord, whereas below 2 K the influence of the sample surface is discernible. The various conduction and scattering mechanisms are discussed.     

Internal friction in copper and α-brasses during plastic deformation

The internal friction Q ^–1 of annealed copper and -brass wires containing 10, 20, 30 and 35 at. % of zinc was studied by a torsional oscillation method during plastic deformation. The results are interpreted in terms of two theoretical models ascribing the amplitude-dependent internal friction, observed in the pre-yield stage, to coupling of the cyclic stress with the creep component of the deformation, and the amplitude-independent internal friction at larger, plastic, strains to losses arising from contributions of the torsional stress to the plastic deformation. Up to the maximum tensile strain of 1 % used in the experiments, the influence of zinc content on Q ^–1 is not pronounced.     

Quality factor of polycrystalline Nb between 0.4 and 10 K

The audiofrequency mechanical quality factor of niobium has been measured in the temperature regime 0.4–10 K. The results show that below 2 K the acoustic lossQ ^–1 reduces exponentially asT ^–1 . The data imply that the activation energy for thermally activated relaxation losses is finite atT=0 and that arbitrarily highQ factors may be achievable at very low temperatures.     

Theory of helium under heat flow near the λ point. II. Dynamics of phase change

The He I-He II interface is a crucial aspect in the transformation processes between the superfluid and normal fluid phases. Its motion is investigated when temperatures and heat flows at boundaries deviate from those of a stationary coexistence state. As a unique feature, the heat flow to the interface from the He I side can be mostly transmitted to the He II side by thermal counterflow, and the latent heat generation (or absorption) at the interface becomes negligibly small. In any case the interfacial motion is so slow that the temperature on the He II sideT is still given by the stationary relationT –T Q ^3/4, whereT is the critical temperature and Q is the heat flow. The temperature profile and the interfacial position are calculated in some nonstationary cases. To this end a simple approximation scheme is developed. First, the interface can propagate with a constant velocity and the superfluid region can expand as a shock wave. Second, if the heat flow at the warmer boundaryQ _w and that in the He II regionQ _– are fixed at different values, the length of the He I region y_i changes in time as (d/dt)y _i ^1+p = const Q _w – Q_–, wherep=1/(1–x), and x is the critical exponent of the thermal conductivity. In particular, ify _i=0 att=0 andQ _w>Q_–, the normal fluid region emerges asy _it ^1/(1+p) at the warmer boundary. Third, ifQ _– and the temperature at the warmer boundary are fixed, the interfacial position approaches an equilibrium position exponentially in time. The uniqueness of the problem arises from the superfluidity on the He II side and the strong critical singularity of the thermal conductivity on the He I side.     

Non-linear response of internal friction to tensile strain rate and frequency during plastic deformation of high-purity aluminium

The internal friction of high-purity aluminium during the process of plastic deformation was measured by a middle torsion pendulum on a modified tensile testing machine. The effects of tensile strain rate, , in the range of 0.73×10^–6 to 50×10^–6s^–1, and frequency of internal friction measurement, f, in the range of 0.38 to 2.6 Hz were studied. The results showed a non-linear dependence of internal friction, Q ^–1, on and f ^–1 or on (=2 f). The interrelationship between internal friction during the process of plastic deformation and dislocation motion, and the effect of non-linearity on the dynamic behaviour of dislocations are discussed.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Galvanomagnetic properties of plastically deformed InSb single crystals

The dc Hall effect, dc conductivity and mobility have been studied on deformed and undeformed samples ofn-type InSb from liquid nitrogen temperature to room temperature. These studies have shown that the Hall coefficient values of deformed samples do not differ much from undeformed sample, but a considerable amount of change was observed in mobility, suggesting that equal number of donor and acceptor type dislocations are introduced during the deformation process. In addition the mobility variation of the deformed samples with temperature has shown a peak in 170–300°K range. The dislocation mobility (μ _D) is deduced from the observed mobilities of deformed and undeformed samples. The plotμ _D vs T has two regions, region 1 being independent of temperature and region 2 having a linear increase with temperature. Theβ factor obtained from region 2 is found to be almost equal to the one calculated from Dexter and Seitz model. The dislocation densities at room temperature are also calculated for the deformed samples using the above model.     

Synthesis and characterization of (Sr1−x′K2x)Zr4(PO4)6 ceramics

Single phase (Sr_1–x K_2x )Zr₄(PO₄)₆, where x lies between 0.0 and 1.0, ceramic powder with a submicron scale particle size has been synthesized successfully at calcination temperatures as low as 650–750°C by a sol-gel technique. The formation of the powder strongly depends on calcination temperature, but is independent of solution pH in the studied range. Dilatometric measurement shows an ultra-low linear coefficient of thermal expansion of 0.1×10^–6°C^–1 when x=0.5 at temperature intervals of 25–1000°C. Thermal conductivity and flexural strength of the materials were determined at ambient temperature to be 1.0 Wm^–1K^–1 and as high as 280 MPa, respectively, indicating that this material can be an excellent candidate in many applications, especially those subjected directly to severe environments.     

The thermal conductivity of propane

This paper presents thermal conductivity measurements of propane over the temperature range of 192–320 K, at pressures to 70 MPa, and densities to 15 mol · L^–1, using a transient line-source instrument. The precision and reproducibility of the instrument are within ±0.5%. The measurements are estimated to be accurate to ±1.5%. A correlation of the present data, together with other available data in the range 110–580 K up to 70 MPa, including the anomalous critical region, is presented. This correlation of the over 800 data points is estimated to be accurate within ±7.5%.Nomenclature a _n, b_ij, b_n, c_n Parameters of regression model - C Euler's constant (=1.781) - P Pressure, MPa (kPa) - P _cr Critical pressure, MPa - Q ₁ Heat flux per unit length, W · m^–1 - t time, s - T Temperature, K - T _cr Critical temperature, K - T ₀ Equilibrium temperature, K - T _re Reference temperature, K - T _r Reduced temperature = T/T _cr - T _TP Triple-point temperature, K Greek symbols Thermal diffusivity, m² · s^–1 - T _i Temperature corrections, K - T Temperature difference, K - T _w Temperature rise of wire between time t ₁ and time t ₂, K - T ^* Reduced temperature difference (T–T _cr)/T_cr - _corr Thermal conductivity value from correlation, W · m^–1 · K^–1 - _cr Thermal conductivity anomaly, W · m^–1 · K^–1 - _e Excess thermal conductivity, W · m^–1 · K^–1 - ^* Reduced density difference - Thermal conductivity, W^–1 · m^–1 · K^–1, mW · m^–1 · K^–1 - _bg Background thermal conductivity, W · m^–1 · K^–1 - ₀ Zero-density thermal conductivity, W · m^–1 · K^–1 - Density, mol · L^–1 - _cr Critical density, mol · L^–1 - _re Reference density, mol · L^–1 - _r Reduced density Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Microstructure,chemistry, elastic properties and internal friction of Silceram glass-ceramics

The temperature dependence of both Young's modulus (E) and internal friction (Q ^–1) from room temperature to 700° C has been determined by Förster's forced-resonance method for three Silceram glass-ceramics, produced by the direct controlled cooling of glass melts in the quaternary system CaO-MgO-Al₂O₃-SiO₂. These results are correlated with microstructural and phase chemistry data as well as calculated viscosity against temperature data. In particular, the viscosity of the residual glass is shown to predominate over its volume fraction in deter mining the temperature dependence ofE andQ ^–1 for a given Silceram. A simple model which enables the residual glass content of Silceram glass-ceramics to be estimated from a know ledge of the proportions of silicon, iron and magnesium in the corresponding glass melts is also proposed. Furthermore, the room-temperature bulk modulus (K) and Poisson's ratio of two Silceram glass-ceramics are calculated using experimentalE and shear modulus (G) values obtained using both Förster's method and another forced-vibration technique.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

Nuclear magnetic resonance techniques can be used to monitor in situ the dynamical behaviour of point and line defects in materials during deformation. These techniques are non-destructive and non-invasive. We report here the atomic transport, in particular the enhanced diffusion during deformation by evaluating the spin lattice relaxation time in the rotating frame, T ₁,in pure NaCl single crystals as a function of temperature (from ambient to about 900K) and strain-rate (to 1.0s^–1) in situ during deformation. The strain-induced excess vacancy concentration increased with the strain-rate while in situ annealing of these excess defects is noted at high temperatures. Contributions due to phonons or paramagnetic impurities dominated at lower temperatures in the undeformed material. During deformation, however, the dislocation contribution became predominant at these low temperatures. The dislocation jump distances were noted to decrease with increase in temperature leading to a reduced contribution to the overall spin relaxation as temperature is increased. Similar tests with an improved pulse sequence (CUT-sequence), performed on ultra-pure NaCl and NaF single crystals revealed slightly different results; however, strain-enhanced vacancy concentrations were observed. The applicability of these techniques to metallic systems will be outlined taking thin aluminium foils as an example.     

Anomalous Conductivity in Random Magnetic (Ga,Cr) As

The electrical conductivity of molecular beam epitaxy grown zinc blende Ga_1–x Cr _x As, x = 0.1, exhibits anomalous behavior below room-temperature. The room-temperature resistivity is small, = 0.1 cm, and comparable to (Ga,Mn)As. Near room temperature, the conductivity is activated, following = ₀ exp(–E _A/kT ), with an activation energy of E _A = 63 meV. In this activated region, Hall measurements also show activated behavior in the hole concentration where p 10²⁰ cm^–3, indicating that Cr can also act as an acceptor similar to Mn. For decreasing temperature, the resistivity increases rapidly because of hopping or tunneling conduction, and becomes strongly insulating at low temperatures. From T = 20 to 200 K the conductivity follows exp[–(T ₁/T)^1/2] over eight orders of magnitude range of conductivity, possible evidence of tunneling between metallic-like polarons.     

Thermal expansion of niobium in the range 1500–2700 K by a transient interferometric technique

The linear thermal expansion of niobium has been measured in the temperature range 1500–2700 K by means of a transient (subsecond) interferometric technique. The basic method involves rapidly heating the specimen from room temperature up to and through the temperature range of interest in less than 1 s by passing an electrical current pulse through it and simultaneously measuring the specimen temperature by means of a high-speed photoelectric pyrometer and the shift in the fringe pattern produced by a Michelson-type interferometer. The linear thermal expansion is determined from the cumulative shift corresponding to each measured temperature. The results for niobium may be expressed by the relation (l-l ₀)/l ₀=5.4424×10^–3–8.8553×10^–6 T+1.2993×10^–8 T ² –4.4002×10^–12 T ³+6.3476×10^–16T⁴ where T is in K and l ₀ is the specimen length at 20°C. The maximum error in the reported values of thermal expansion is estimated to be about 1% at 2000 K and not more than 2% at 2700 K.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

High-temperature thermoelectric properties of the sintered Bi2Sr2Ca1 − xYxCu2Oy (x = 0 – 1)

Electrical conductivity and Seebeck coefficient were measured in a temperature range of 320–1073 K for sintered samples of Bi₂Sr₂Ca_{1 – x} Y _x Cu₂O _y (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). It has been found that the conduction behavior changes from n-type metallic to p-type semiconducting with increasing yttrium concentration. The power factors were in a range of 1.7–3.0 × 10^–5 Wm^–1 K^–2 for the sample with x = 0.8, being maximized by the optimization of the yttrium concentration. The thermal conductivity for the sample with x = 0.8 was 0.73 Wm^–1 K^–1 at 310 K, and decreased with increasing temperature. The values of thermoelectric figure of merit were estimated to be in a range of 3.4–4.8 × 10^–5 K^–1 at temperatures of 320–673 K for the sample with x = 0.8.     

Temperature dependence of dynamic Young's modulus and internal friction in LPPS NiCrAlY

The piezoelectric ultrasonic composite oscillator technique (PUCOT), operating near 80 kHz, was used to measure the temperature dependence, in the range 23–1000 °C, of dynamic Young's modulus,E, and internal friction,Q ^–1 in three compositions of low-pressure plasma-sprayed NiCrAlY: Ni-15.6Cr-5.2Al-0.20Y (16-5), Ni-17.2Cr-11.6Al-0.98Y (17–12), and Ni-33Cr-6.2 Al-0.95 Y (33–6). Ambient temperature (23 °C) dynamic Young's moduli for the three alloys were 205.0, 199.8, and 231.0 GPa, respectively. In each case, dE/dT was found to be — 0.06 GPa °C^–1 over temperature ranges 23–800, 23–400 and 600–900, and 23–700 °C, respectively. Internal friction was essentially independent of temperature to about 600 °C (700 °C for the 16–5 alloy), at which point a temperature dependence of the formQ ^–1 =A exp (C/RT) was observed. The constantA for the three alloys was determined to be 62.7, 555, and 2.01 × 10⁶, respectively. The constantC for the three alloys was determined to be 82.8, 111, and 170 kJ/mol^–1, respectively. While the physical mechanism is not fully understood, both the pre-exponential constantA and the activation energyC correlate with durability in thermal barrier coatings (TBCs) wherein these alloys are used as bond coats.     

Correlation of electrical properties and internal friction in mixed conduction glasses containing ionic and electronic conduction (1) Fe2O3-Na2O-P2O5 glasses

The electrical properties and internal friction in (40–x)Fe₂O₃·xNa₂0.60P₂O₅ glasses were measured. Two or three peak on internal friction were observed in the temperature range of –100 to 300° C at a frequency of about 1 Hz. The peak area of internal friction could be explained quantitatively by the additivity law of diffusion of Na⁺ ion and hopping of electrons which are carriers similar to those of dielectric loss. Activation energy, peak temperature of dielectric loss and internal friction showed almost the same value. Both relaxation phenomena have the same mechanism which is due to the diffusion of Na⁺ion and the hopping of electrons between Fe²⁺ Fe³⁺. The high-temperature peak is assumed to result from the interaction between protons or alkali ions and non-bridging oxygen.     

Plasticity of columnar-grained CoSi2 with the C1 structure

Columnar-grained CoSi₂ crystals with the C1 structure (CaF₂ type) have been deformed in compression in vacuum at high temperatures up to 1400 K. The yield stress increases steeply with decreasing temperature, and the fracture precedes yielding below 700 K. Above 900 K, specimens can be compressed to a strain above 80% without fracture. From the strain-rate sensitivity determined by the stress relaxation test and the temperature dependence of the yield stress, the activation volume and the activation enthalpy of plastic deformation have been analysed. The activation volumes at high stresses (^*>100MNm^–2) are less than 10b ³ (whereb is the magnitude of the Burgers vector), indicating that the deformation is controlled by the Peierls mechanism. The total activation enthalpy is about 3 eV. The possibility of the dissociation of 1/2110 dislocations is proposed.