Temperature Dependence Of Resistivity Research Articles

Response of the Verwey transition in magnetite to controlled point-like disorder induced by 2.5 MeV electron irradiation

Controlled point-like disorder induced by low temperature 2.5 MeV electron irradiation was used to probe the nature of the Verwey transition in magnetite, Fe3O4. Two large single crystals, one with optimal transition temperature, TV≈121 K, and another with TV≈109 K, as well as magnetite magnetosome nanocrystals harvested from the lysed cells of the marine magnetotactic vibrio Magnetovibrio blakemorei strain MV-1, TV≈110 K, were examined. Temperature-dependent resistivity is consistent with the semiconductor-to-semiconductor (insulator) sharp, step-like Verwey transition from a state with a small bandgap of around 60 meV to a state with a large bandgap of about 300 meV. The irradiation causes an up-shift of the resistivity curves above the transition without transition smearing or broadening. It also causes an apparent down-shift of the resistivity maximum at high temperatures. In the lower TV crystal, the electron irradiation drives the transition temperature into a “forbidden” interval of TV , believed to separate the first order from the second order phase transition. Contrary to this belief, the transition itself remains sharp and hysteretic without a significant change in the hysteresis width indicating the strong 1st order character of the Verwey transition for all TV values. The separate 2nd order - looking transition is likely due to sample inhomogeneities. We conclude that the sudden change of the bandgap accompanied (or driven) by the monoclinic distortion and the change of magnetic anisotropy is the reason for the Verwey transition in magnetite and the effect of additional disorder is mostly in the smearing of the sharp gap edges near the Fermi level.

Power generation of annular thermoelectric generator with silicone polymer thermal conductive oil applied in automotive waste heat recovery

The annular thermoelectric generator (ATEG) matches the shape of the exhaust pipe. To improve the power generation efficiency of the ATEG in automobiles, a scheme is proposed to use a silicone polymer-based thermal conductive oil to transfer heat from the exhaust. Based on this concept, a new type of concentric tube heat exchanger is designed. A finite element model of the new system is established by considering temperature gradients, temperature dependence, and fluid resistance characteristics. Numerical simulations are performed to investigate the effects of exhaust parameters, thermal oil parameters, and heat exchanger structure on thermoelectric performance. The findings demonstrate a 15.2% increase in maximum output power of the new generator compared to a conventional ATEG, accompanied by a 19% decrease in the optimal thermoelectric module area. Furthermore, the net power first increases and then decreases with increasing thermal oil mass flow rate. The thermal oil mass flow rate can be adjusted in real-time based on fluctuations in exhaust parameters to achieve peak net output power. Finally, optimal fitting relationships between the heat exchanger dimensionless diameter and the total diameter, as well as between exhaust parameters and the optimal thermal oil mass flow rate, are obtained through parameter analysis and derivation.

Coexisting cubic and tetragonal phases with ferrimagnetism in equiatomic quaternary Heusler alloys: CuTCrAl (T = Co and Fe)

We have investigated two equiatomic quaternary Heusler alloys CuTCrAl (T = Co and Fe) theoretically and experimentally. Both the alloys have been characterized using X-ray Diffraction (XRD) and neutron powder diffraction (NPD). Refined results of XRD and NPD show the presence of A2/B2 type disorder with the coexistence of mixed-phase of cubic and tetragonal symmetries in CuFeCrAl and CuCoCrAl. NPD magnetic structure refinement indicates the presence of ferrimagnetism with a very low moment at low temperatures. DC magnetization results reveal the presence of weak itinerant ferromagnetic behaviour. Temperature dependence resistivity reflects the metallic character with the presence of weak localization at low temperatures. The coexistence of cubic/tetragonal and ferrimagnetism along with metallic nature in a single system is quite rare, revealing the potential of these alloys for multifarious applications.

Revealing the Intrinsic Decay of Mechanoluminescence for Achieving Ultrafast‐Response Stress Sensing

AbstractConverting mechanical energy into photon emission provides a promising route for intelligent sensing, self‐powered lighting, and distributed energy harvesting, which is of great significance for finding a feasible solution to the current sensing technical bottleneck and energy crisis. As the basis for understanding the conversion mechanism and realizing high‐frequency mechanical energy utilization, elucidating the dynamic process of intensity variation in the mechano‐to‐photon conversion remains a great challenge. Herein, a time‐domain characterization scheme that enables to unravel the intrinsic decay of mechanoluminescence (ML) with lifetimes from milliseconds down to tens of microseconds is constructed. It is demonstrated that ML decay characterization is an important tool to reveal the dynamics of charge migration in ML materials. The ML decay in a typical self‐reproducible ML material ZnS:Mn2+ shows temperature dependence and stress fluctuation resistance, which opens up a new reliable approach for self‐powered and remote temperature sensing. Finally, benefiting from the shortest ML lifetime recorded to date, an ultrafast‐response stress sensor that enables to detect individual pulses of ultrasonic waves with ML sensing technology is developed.

Structure, magnetic and electrical properties of Tb0.5Nd0.5CoO3 perovskite solid solutions with carbon material flakes (CMF)

In this paper, we study the ceramics of cobaltite [Tb0.5Nd0.5CoO3]1−yCy with equiatomic contents of terbium and neodymium and with the addition of a carbon material flakes (СMF) with the content of carbon in the range of weight fraction 0 < y < 0.01. The ceramics were obtained by the method of solid-phase reactions according to the standard ceramic technology in air. Using X-ray diffraction and energy-dispersive analysis, it was established that the initial (undoped) solid solution is single-phase with perovskite structure. It was shown that Co3+ ions in undoped ceramics under study are in a non-magnetic low-spin state over the entire temperature range under consideration (2–320 K), and the entire contribution to the magnetization is due to Tb3+ and Nd3+ ions. An analysis of the temperature dependences of the electrical resistance R(T) shows that the initial solid solution behaves like a dielectric with a reduced activation energy of about 0.3–0.35 eV in the temperature range of 280–320 K, due to hopping conduction with a constant activation energy (constant range hopping) by defects in Tb0.5Nd0.5CoO3 matrix. At lower temperatures (200 – 275 K) in the initial (undoped) Tb0.5Nd0.5CoO3 samples, the Mott-type hopping conductivity mechanism with a variable range hopping and high values of the characteristic temperature To ∼ 109 K was observed. Simultaneously, at 300 K, we observed hopping conductivity on alternating current according to the law σ(ω) ∼ ω-α(ω), where the exponent α, which determines a hopping probability, depends on the frequency. In [Tb0.5Nd0.5CoO3]1−yCy samples, heavily doped with СMF with y = 0.01, carrier transport in the entire studied temperature range of 2–320 K mainly occurs along highly conductive carbon-based channels, formed inside of Tb0.5Nd0.5CoO3 matrix by СMF. At temperatures below 15–20 K, these samples show hopping behavior of R(T) with a variable range hopping over localized states, described by the Mott–Kirkpatrick law. In the temperature range 20–100 K electrical resistance versus temperature R(T) along carbon-based channels obey the law R(T) ∼ Ln T. In this case, the relative magnetoresistance MR(T, B) is characterized by a negative sign at temperatures below 10 K in magnetic fields less than 1 T. This behavior of the R(T, B) dependences were described on the basis of the theory of quantum corrections to the Drude conductivity for two-dimensional samples under conditions of weak localization, which is consistent with the behavior of layered carbon-based materials known from the literature. At T > 150 K curves R(B) follow to Lorentz-like positive magnetoresistive effect in Tb0.5Nd0.5CoO3 matrix in the whole magnetic field range.

Temperature-Dependent Sheet Resistance and Surface Characterization of Thin Copper Films Bonded to FR4 Composite under Mechanical Vibrations

Electrical boards, also called printed circuit boards, constitute the basis of most electronic devices. These boards are mainly fabricated of thin copper films bonded to fiber epoxy laminates, such as FR4. Being the most important functional component of these devices, they sometimes undergo mechanical stresses such as shock and vibration during transport and operation that can induce electrical failure and malfunction; hence, studies addressing the effects of vibrations on their electrical properties have important applications. In this paper, small cantilever samples made of bare copper bonded to FR4 with three isolated rectangular zones were studied to analyze, for the first time, variations in electrical properties such as sheet resistance and resistivity before and after 200 k, 500 k, and 800 k vibration cycles at three different temperatures (25, 35, and 45 °C). A significant rise in resistance equivalent to 1657% of the initial value was observed from 0 to 800 k vibration cycles. These changes were accompanied by a 95% decrease in conductivity, from 4.1 × 107 to 2.3 × 106 S/m, whereas very little change in the electrical properties was observed due to temperature rise. Surface analysis by ESEM showed cracks ~1 µm in width and several millimeters in length with a crack density of ~8 cracks per mm after 800 k cycles. The surface composition (100% copper) was not altered even upon a high number of vibration cycles, and static drop contact angle measurements of 117–119 degrees indicated an increase in the hydrophobicity of the surface attributed to increased surface roughness and the accumulation of very small air bubbles on the cracks.

Tunable electron scattering mechanism in plasmonic SrMoO3 thin films

4d transition metal perovskite oxides serve as suitable testbeds for the study of strongly correlated metallic properties. Among these, SrMoO3 (SMO) exhibits remarkable electrical conductivity at room temperature. The temperature-dependent resistivity (ρ(T)) exhibits a Fermi-liquid behavior below the transition temperature T∗, reflecting the dominant electron-electron interaction. Above T∗, electron-phonon interaction becomes more appreciable. In this study, we employed the power-law scaling of ρ(T) to rigorously determine the T∗. We further demonstrate that the T∗ can be modified substantially by ∼40 K in epitaxial thin films. It turns out that the structural quality determines T∗. Whereas the plasma frequency could be tuned by the change in the electron-electron interaction via the effective mass enhancement, we show that the plasmonic properties are more directly governed by the electron-impurity scattering. The facile control of the electron scattering mechanism through structural quality modulation can be useful for plasmonic sensing applications in the visible region.

The Electronic Structure, Thermoelectric, and Optical Properties of Heusler Alloys Mn2MeAl (Me = Ti, V, Cr)

The results of calculations of the electronic structure, thermoelectric characteristics, and experimental study of the thermoelectric, electrical, and optical properties of Mn2MeAl Heusler alloys (Me = Ti, V, Cr) are presented. The agreement between theory and experiment in the sign of the Seebeck coefficient is shown. The obtained pattern of the band spectrum allows us to qualitatively explain the peculiarities of the temperature dependences of the electrical resistance and the permittivity dispersion.

The Electronic Structure, Thermoelectric, Optical Properties of Heusler Alloys Mn2MeAl (Me = Ti, V, Cr)

The results of calculations of the electronic structure, thermoelectric characteristics, and an experimental study of the thermoelectric, electrical, and optical properties of Mn2MeAl Heusler alloys (Me = Ti, V, Cr) are presented. The agreement between theory and experiment in the sign of the Seebeck coefficient is shown. The obtained pattern of the band spectrum allows us to qualitatively explain the peculiarities of the temperature dependences of the electrical resistance and the permittivity dispersion.

Studies of structural, dielectric, and electrical properties of polycrystalline barium bismuth tungstate for thermistor application

In this communication, preliminary investigation of structural and detailed analysis of electrical proprties of a new barium bismuth tungstate of a chemical composition Ba3Bi2WO9, produced via a standard high-temperature solid-state reaction method, is presented. The basic crystal structure of the compound is monoclinic (a = 8.4621 Å, b = 6.7358 Å, c = 6.9064 Å and β = 109.94°). The homogeneous distribution of grains (average grain size = 1.717 μm) is studied via surface morphology demonstrating the feasibility of fabricating a high-density sample under experimental conditions. This compound shows ferroelectric and related properties, which have been experimentally verified by examination of the frequency-temperature dependence of capacitance, impedance, and other related behaviors. The variation of ac conductivity of the material under alternating current varies according to Jonscher's universal power law. The ferroelectric property of the material (Pr = 0.331C/cm2, Ec = 0.207 kV/cm) is demonstrated by the electric field-dependent polarisation (P-E loop) study. The temperature dependence of resistance yields a thermistor constant (β) and temperature coefficient of resistance (TCR), which may be useful for fabricating thermistor-based device applications.

Ordered L10-FeNi (111) epitaxial thin film on Al2O3 (0001) substrate: Molecular beam epitaxy growth and characterizations

Ordered hard magnetic L10-FeNi has attracted attention as a candidate for rare-earth-free permanent magnet with a large maximum magnetic energy product. Fabricating L10-FeNi is a big challenge using conventional methods due to its low order-disorder transition temperature, 320 °C. In this work, high-quality ordered L10-FeNi films were fabricated by using molecular beam epitaxy directly on Al2O3 (0001) substrate with a two-step co-deposition method below 250 °C. The films were epitaxially grown with [111] orientation, confirmed by reflection high energy electron diffraction and x-ray diffraction patterns. An order-disorder phase transition at 320 °C was observed in temperature dependent electrical resistivity curve. The two-step molecular beam epitaxy grown film exhibited high uniaxial magnetic anisotropy constant, Ku, up to 1.15 × 106 J/m3. The obtained Ku value is comparable to previous result for FeNi along [100] growth direction, indicating an isotropic Ku. This work provides an effective molecular beam epitaxy growth method of high-quality ordered L10-FeNi (111) film for the development of the next-generation rare-earth-free permanent magnet.

LaNi9Si4: CRYSTAL STRUCTURE AND ELECTRICAL PROPERTIES

The structure of LaNi9Si4 compound has been investigated by X-ray powder ans sing­le crystal method. This silicide crystalize in CeNi8.5Si4 structure type: space group I4 mcm, Pearson symbol tI56, а = 7.86415(6), с = 11.5101(1) Å, RB = 0.0653; а = 7.83933(17), с = 11.4472(5) Å, R = 0.0220, wR = 0.0734 for X-ray single crystal data. Unlike the prototype CeNi8.5Si4.5, where the Wyckoff position 4d is occupied by mixture of Ni and Si atoms, in the structure of the ternary silicide LaNi9Si4, the atoms are ordered in all Wyckoff positions. Additionally, the electrical properties for the compound were investigated. The temperature dependence of the electrical resistivity exhibits metal behaviour (ρ0 = 121.60 μΩ∙cm, θD = 191 K, A = 2.2·10-9 μΩ∙m∙K-3).&#x0D;

Noncollinear commensurate antiferromagnetic structure in metallic Pr2PdAl7Ge4

We report the crystal structure, magnetic structure, magnetic properties, electrical- and magnetotransport properties, and heat capacity of the rare-earth transition-metal compound ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$. ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ crystallizes in the noncentrosymmetric tetragonal structure $P\overline{4}{2}_{1}m$ (No. 113) with unit cell parameters $a=6.0059(3)\phantom{\rule{4pt}{0ex}}\AA{}$ and $c=15.2278(13)\phantom{\rule{4pt}{0ex}}\AA{}$. The N\'eel temperature ${T}_{\mathit{N}}$ of ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ was determined to be 6 K by the temperature dependences of magnetization, heat capacity, and electrical resistivity. The magnetic-field-induced antiferromagnetic to ferromagnetic transition occurs for magnetic fields $H$ along both the $ab$ plane and the $c$ axis. The electrical resistivity shows a metallic behavior with an upturn at the ordering temperature due to a superzone gap. Negative linear magnetoresistance is observed since the magnetic field increases the order degree of the antiferromagnetic state, reducing the magnetic correlation scattering. Neutron powder diffraction experiments reveal that metallic ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ has an unusual noncollinear commensurate antiferromagnetic structure with the propagation vector $\mathbit{k}=(0,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}0.5)$. Magnetic Pr atom layers stack with the ABBA sequence along the $c$ axis. Within the layers, the Pr moments lie in the $ab$ plane, and the angle between the nearest-neighbor spins is $67.{1}^{\ensuremath{\circ}}$. This magnetic structure can well explain the magnetic properties of ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$.

Thermoelectric properties of Ru2TiGe Heusler phase

We report a study of structural, magnetic, heat capacity and thermoelectric properties of a Ru-based Heusler alloy, Ru2TiGe. The magnetic measurements reveal that at higher temperatures, diamagnetic and Pauli paramagnetic contributions dominate the magnetic behaviour whereas, at lower temperatures (T ≤ 20 K), superparamagnetic interaction among clusters is observed. The development of such magnetism, however, does not exhibit any noticeable influence in thermoelectric properties. Though the temperature dependence of electrical resistivity exhibits a metal-like nature, the large value of Seebeck coefficient leads to an appreciable power factor of the order of 0.54 mW/mK2 at 300 K. Large power factor as well as low thermal conductivity results in a value of zT = 0.025 at 390 K for Ru2TiGe that is orders of magnitude higher than that of the other pure Heusler alloys and point towards its high potential for practical thermoelectric applications.

Giant dielectric constant, magnetocaloric effect and spin-phonon coupling in EuTbCoMnO6 semiconductor

Transport, dielectric, magnetic and Raman studies of polycrystalline double perovskite EuTbCoMnO6 have been performed. Temperature-dependent resistivity follows the variable range and small polaron hopping mechanism. The temperature-dependent dielectric study shows the usual frequency-dependent step-like behavior. It also shows thermally activated relaxor peak in loss curves with giant dielectric constant near room temperature. The temperature-dependent magnetization demonstrates a second-order phase transition around 113 K. The critical behavior near the magnetic transition has been systematically investigated. The Widom scaling relation and magnetic entropy change calculation have verified the reliability of critical parameters. Maximum magnetic entropy change around the magnetic transition temperature shows the magnetocaloric effect. The estimated values of critical parameters are close to the value of the theoretical mean-field model. Temperature-dependent Raman study shows spin-phonon coupling in the system. The appearance of the magnetocaloric effect and colossal dielectric constant shows that these materials are interesting for real-life applications.

Tetragonal Kondo Insulator EuCd2Sb2 Discovered via High Pressure High Temperature Synthesis

AbstractMagnetic and electronic properties of quantum materials heavily rely on the crystal structure even in the same chemical compositions. In this study, it is demonstrated that a layered tetragonal EuCd2Sb2 structure can be obtained by treating bulk trigonal EuCd2Sb2 under high pressure (6 GPa) and high temperature (600 °C). Magnetization measurements of the newly formed layered tetragonal EuCd2Sb2 confirm an antiferromagnetic ordering with Neel temperature (TN) around 16 K, which is significantly higher than that (TN ≈ 7 K) of trigonal EuCd2Sb2, consistent with heat capacity measurements. Moreover, bad metal behavior is observed in the temperature dependence of the electrical resistivity and the resistivity shows a dramatic increase around the Neel temperature. Electronic structure calculations with local density approximation dynamic mean–field theory (LDA+DMFT) show that this material is strongly correlated with well‐formed large magnetic moments, due to Hund's coupling, which is known to dramatically suppress the Kondo scale.

Insulating Nature of Iridium Oxide Ca5Ir3O12 Probed by Synchrotron-Radiation-Based Infrared Spectroscopy

Ca5Ir3O12 shows semiconducting behavior, although it is expected to have metallic conduction because of its 1/6-filled band. To investigate the insulating nature of Ca5Ir3O12, synchrotron-radiation-based infrared spectroscopy and electrical resistivity measurements were performed. Optical conductivity obtained using Kramers–Krönig transformation is compared with ab initio calculations based on density-functional and random-phase-approximation calculations, and their differences are carefully discussed. A detailed analysis of the temperature dependence of electrical resistivity along the c-axis reveals that ln ρ ∝ T−2/3. This temperature dependence indicates that the electrical conduction of Ca5Ir3O12 along the c-axis follows Efros–Shklovskii variable-range hopping. The optical conductivity with polarization parallel to the c-axis shows no clear onset at 1,300 cm−1 (0.16 eV) of the energy gap obtained by assuming the Arrhenius law. This feature of the optical conductivity is consistent with the fact that Ca5Ir3O12 follows Efros–Shklovskii variable-range hopping.

Increased phase coherence length in a porous topological insulator

The surface area of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surfacelike. Weak antilocalization was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

Absence of Weak Localization Effects in Strontium Ferromolybdate

Sr2FeMoO6-δ (SFMO) double perovskite is a promising candidate for room-temperature spintronic applications, since it possesses a half-metallic character (with theoretically 100% spin polarization), a high Curie temperature of about 415 K and a low-field magnetoresistance (LFMR). The magnetic, resistive and catalytic properties of the double perovskite SFMO are excellent for spintronic (non-volatile memory), sensing, fuel cell and microwave absorber applications. However, due to different synthesis conditions of ceramics and thin films, different mechanisms of electrical conductivity and magnetoresistance prevail. In this work, we consider the occurrence of a weak localization effect in SFMO commonly obtained in disordered metallic or semiconducting systems at very low temperatures due to quantum interference of backscattered electrons. We calculate the quantum corrections to conductivity and the contribution of electron scattering to the resistivity of SFMO. We attribute the temperature dependence of SFMO ceramic resistivity in the absence of a magnetic field to the fluctuation-induced tunneling model. We also attribute the decreasing resistivity in the temperature range from 409 K to 590 K to adiabatic small polaron hopping and not to localization effects. Neither fluctuation-induced tunneling nor adiabatic small polaron hopping favors quantum interference. Additionally, we demonstrate that the resistivity upturn behavior of SFMO cannot be explained by weak localization. Here, the fitted model parameters have no physically meaningful values, i.e., the fitted weak localization coefficient (B′) was three orders of magnitude lower than the theoretical coefficient, while the fitted exponent (n) of the electron–electron interaction term (CnTn) could not be assigned to a specific electron-scattering mechanism. Consequently, to the best of our knowledge, there is still no convincing evidence for the presence of weak localization in SFMO.

Experimental investigation of density, electrical resistivity and DSC of AlNiCoCuZr equiatomic alloy

Equiatomic Al–Ni–Co–Cu–Zr alloy was produced by arc melting. Rapidly quenched rods were prepared by suction casting method. Density, electrical resistivity and thermal analysis of the alloy were investigated experimentally. Basing on X-ray diffraction it was shown that the structure of the alloy consists of two competing solid solutions: BCC-ZrNi2Al (Heusler-type phase) and γ-phase Cu9Al4. In addition, pure cooper and Cu3Al, Cu5Zr7Ni5 intermetallic compounds were determined. Rapid quenching of this alloy leads to the formation of solid solutions based on BCC-ZrNi2Al and high-temperature Cu3Al phase. It is shown that the alloy in crystalline state has a linear density change up to solidus temperature. A region of resistivity decrease with increasing temperature is found out for the first time. The melting process goes in a wide temperature range (above 250 K) where 4 thermal reactions were detected. Density and electrical resistivity changes here non-linearly. In liquid state there are no thermal effects in the alloy and temperature dependencies of density and electrical resistivity can be fitted by linear functions.