Lattice thermal conductivity of bismuth-antimony alloy single crystals at low temperatures

Thermal conductivity measurements in a number of bismuth-antimony alloy single crystals, with antimony concentrations from 1.72 at.% to 20 at.% are reported. Analysis of the results shows that in this alloy system, below about 25 K, the heat carriers are predominantly phonons. Callaway's theoretical model for lattice thermal conductivity has been utilized for the analysis of the results, using various phonon-scattering processes, and this model has been found to give a good fit to the experimental data, particularly below 25 K, where lattice conduction predominates. The Casimir model for finding the characteristic length for boundary scattering fails to explain the experimental results. This indicates the existence of microscale fluctuations in the composition of the alloy.     

Fatigue behavior of Cu-Al-Be shape memory single crystals

The fatigue behavior of Cu-Al-Be shape memory single crystals is studied in cyclic loading mechanical tests. Based on literature and on tensile tests performed at various temperatures, a model is proposed to explain the mechanism of fatigue. This model is based on the idea that during cyclings, the different zones of the samples spend various lengths of time in the martensitic state. During that time, martensite evolves because of the occurrence of some reordering or other diffusional phenomena inside, and consequently, the value of the martensite start temperature for each zone changes. The kinetics of the change in mechanical behavior along with the cycles, as a function of the test temperature, are accurately described by a Johnson–Mehl relation. Relaxation tests suggest that the mechanism described not only depends on time and temperature but is also re-enforced by the movement of the martensite-matrix interface. From the kinetics of this fatigue behavior, an empirical activation energy related to the mechanism is inferred for this Cu-Al-Be alloy.     

The phenomenon of thermal magnetization of highly anisotropic single crystals

The results of micromagnetic modeling show that the phenomenon of thermal magnetization analogous to that observed in polycrystalline samples is also possible in a highly anisotropic Nd₂Fe₁₄B single crystal. In this case, the thermal magnetization is related to a transition of the single crystal from a metastable to stable state and can take place within a narrow interval of sample dimensions.     

Stress distributions in single shape memory alloy fiber composites

Shape memory alloy (SMA) in the form of wires or short fibers can be embedded into host materials to form SMA composites that can satisfy a wide variety of engineering requirements. The recovery action of SMA inclusions induced by elevated temperature can change the modal properties and hence the mechanical responses of entire composite structures. Due to the weak interface strength between the SMA wire and the matrix, interface debonding often occurs when the SMA composites act through an external force or through actuation temperature or combination of the two. Thus the function of SMAs inside the matrix cannot be fully utilized. To improve the properties and hence the functionality of SMA composites it is therefore very important to understand the stress transfers between SMA fibers and matrix and the distributions of internal stresses in the SMA composite. In this paper, a theoretical model incorporating Brinson’s constitutive law of SMA for the prediction of internal stresses is successfully developed for SMA composites, based on the principle of minimum complementary energy. A typical two-cylinder model consisting of a single SMA fiber surrounded by epoxy matrix is employed to analyze the stress distributions in the SMA fiber, the matrix, and at the interface, with important contributions of the thermo-mechanical effect and the shape memory effect. Assumed stress functions that satisfy equilibrium equations in the fiber and matrix respectively are utilized, as well as the principle of minimum complementary energy, to analyze the internal stress distributions during fiber pull-out and the thermal loading process. The entire range of axisymmetric states of stresses in the SMA fiber and matrix are developed. The results indicate substantial variation in stress distribution profiles for different activation and loading scenarios.     

Effect of thallium doping on the thermal conductivity of PbTe single crystals

The thermal conductivity of thallium-doped PbTe single crystals has been measured in the temperature range 80–310 K, and the electronic and lattice (χ_ph) components of their thermal conductivity have been evaluated. The results indicate that thallium doping markedly reduces the lattice thermal conductivity of PbTe single crystals, by up to ～29%. Heat treatment at ～473 K reduces the density of structural defects in the crystals and increases their χ_ph. Thallium doping has a significant effect on χ_ph starting at 0.05 at % before annealing and starting at 0.01 at % after annealing. We have calculated the density of structural defects in the unannealed samples and the doping-induced thermal resistance Δ W of the unannealed and annealed samples. ΔW has been shown to increase with thallium content.     

Elastic anisotropy and low-temperature thermal expansion in the shape memory alloy Cu-Al-Zn

Cu-based shape memory alloys are known for their technologically important pseudo-elastic and shapememory properties, which are intimately associated with the martensitic transformation. A combination of deformation theory and finite-strain elasticity theory has been employed to arrive at the expressions for higher order elastic constants of Cu-Al-Zn based on Keating's approach. The second- and third-order elastic constants are in good agreement with the measurements. The aggregate elastic properties like bulk modulus, pressure derivatives, mode Grüneisen parameters of the elastic waves, low temperature limit of thermal expansion, and the Anderson-Grüneisen parameter are also presented.     

Low-temperature anisotropy of thermal conductivity of tin single crystals doped with zinc

The thermal conductivity of tin single crystals with zinc admixtures has been measured in the temperature range 3.5–25 K for concentrations up to 0.1 wt%. The anisotropy of thermal conductivity for two orientations, [001] and [010], has been determined. It was found that the influence of zinc admixture on the thermal conductivity anisotropy is of a complex, temperature-dependent character.Nomenclature T ₁ T ₂ Temperature differences in the specimen - Thermal conductivity coefficient - W Thermal resistivity - A, B, C Constants in Eq. (1) - T Temperature - ^th Residual electrical resistivity calculated from W-F law - ₀ Residual electrical resistivity from measurements - L ₀ Lorenz constant - _th Anisotropy coefficient of thermal conductivity - _el Anisotropy coefficient of electrical conductivity - c Admixture concentration     

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co₄₉Ni₂₁Ga₃₀ alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.     

Thermodynamic constitutive model for load-biased thermal cycling test of shape memory alloy

This paper presents a three-dimensional calculation model for martensitic phase transformation of shape memory alloy. Constitutive model based on thermodynamic theory was provided. The average behavior was accounted for by considering the volume fraction of each martensitic variant in the material. Evolution of the volume fraction of each variant was determined by a rate-dependent kinetic equation. We assumed that nucleation rate is faster for the self-accommodation than for the stress-induced variants. Three-dimensional finite element analysis was conducted and the results were compared with the experimental data of Ti–44.5Ni–5Cu–0.5 V (at.%) alloy under bias loading.     

Topology optimization of planar shape memory alloy thermal actuators using element connectivity parameterization

This paper presents the direct application of topology optimization to the design of shape memory alloy (SMA) thermal actuators. Because SMAs exhibit strongly nonlinear, temperature‐dependent material behavior, designing effective multidimensional SMA actuator structures is a challenging task. We pursue the use of topology optimization to address this problem. Conventional material scaling topology optimization approaches are hampered by the complexity of the SMA constitutive behavior combined with large actuator deflections. Therefore, for topology optimization we employ the element connectivity parameterization approach, which offers improved analysis convergence and robustness, as well as an unambiguous treatment of nonlinear materials. A path‐independent SMA constitutive model, aimed particularly at the NiTi R‐phase transformation, is employed, allowing efficient adjoint sensitivity analysis. The effectiveness of the proposed SMA topology optimization is demonstrated by numerical examples of constrained and unconstrained formulations of actuator stroke maximization, which provide insight into the characteristics of optimal SMA actuators. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of the thermal conductivity of molecular crystals

The nonsteady-state heated-wire method was used to measure the coefficients of thermal conductivity of benzene and cyclohexene over the temperature range from–196°C to +30°C. We have attempted to explain the heat-transger mechanism in these substances.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 19, No. 4, pp. 705–709, October, 1970.     

Investigation of spatial inhomogeneities of the thermal properties of a NiTi shape memory alloy device

Local changes of the phase transition on a SM one‐way actuator have been investigated by time‐dependent IR thermography and compared with the local behavior of thermal properties studied by means of scanning thermal microscopy and modulated IR radiometry at room temperature and at 120 °C. The results point towards large spatial inhomogeneities of the shape memory properties developing with increasing temperature and which are the reason that only parts of the material transform into the austenite phase.     

TiNiCu形状记忆合金双向记忆效应研究

研究了热机械训练温度及定型处理温度对TiNiCu形状记忆合金弹簧双向记忆效应的影响。研究结果表明:在纯马氏体状态进行训练时,双向记忆恢复率随训练次数的增加而增加,并在一定的训练次数后达到饱和;在纯奥氏体状态进行训练,双向记忆恢复率随训练次数迅速增加到某一最大值后随训练次数的增加而减小;在马氏体和奥氏体混合相进行训练时,双向记忆恢复率随训练次数先增加而后减小。经过400～550℃×1h/AC定型处理及热机械训练后最大形状记忆恢复率随定型处理温度升高先增大然后减小。由于马氏体再取向时引入的位错有利于双向记忆效应,热诱发和应力诱发的马氏体变体数量不同,引起了在不同状态训练诱发的双向记忆效应随训练次数变化的差异。     

Electrical conductivity of yttria-stabilized zirconia single crystals

The electrical conductivities of YSZ single crystals with various compositions covering FSZ and PSZ regions were measured by a complex impedance method and a four-probe a.c. method. The conductivities changed significantly as a function of composition. A simple conduction model for PSZ showed that the tetragonal phase is a good oxygen ionic conductor having an activation energy for motion of about 0.8 to 0.9 eV. It is promising for low temperature application of a solid state electrolyte.     

Effective thermal conductivity and electrical conductivity of anisotropic solids of low porosity

Equations are derived to determine the effective thermal and electrical conductivities of anisotropic media of low porosity. The influence of porosity on the thermal and electrical conductivities of anisotropic ternary alloys is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 4, pp. 686–692, April, 1976.