Small Negative Temperature Coefficient Research Articles

Piezoresistive and thermoresistive responses of carbon nanotube-based strain gauges with different grid geometric parameters

Arrays of carbon nanostructures have gained relevance in current strain sensor technology, pursuing high sensitivity, enhanced stretchability and tunability in the temperature coefficient of resistance. The design and fabrication parameters such as the grid geometry and polymer coating of a strain gauge based on a dense array of carbon nanotubes (CNTs) comprising the grid, deposited over a polyimide (Kapton®) substrate, are investigated herein. It is found that varying the geometrical parameters of the CNT grid led to a 13% increase in the gauge factor. Polysulfone coating lowers the sensitivity compared to polyurethane coating, but increases the repeatability and linearity of the response. The fabricated gauges were characterized regarding their piezoresistive response at room and higher temperatures. A small negative temperature coefficient of resistance (−5.63 ×10−2 %/°C) was found. The fabricated gauges show repeatability and linearity when using as a strain sensor bonded to a 4-point bending coupon during 50 loading-unloading cycles.

Phase precipitation and magnetic properties of melt-spun ternary Gd2Fe14B alloy and advantages of gadolinium substitution in Y2Fe14B alloy

To take the advantage of gadolinium (Gd) in developing and manufacturing RE-permanent magnets, the magnetic properties and phase precipitation behavior of Gd2Fe14B alloys prepared by melt spinning were investigated in this work. The results show that optimally direct quenched nanocrystalline Gd2Fe14B alloy exhibits the magnetic properties with remanence Jr of 0.51 T, coercivity Hc of 187 kA/m, and maximum energy product (BH)max of 33.1 kJ/m3. It also shows excellent thermal stability with a small negative temperature coefficient of Jr (α = −0.066%/K) and a positive temperature coefficient of Hc (β = 0.171%/K). The phase precipitations of as spun Gd2Fe14B alloy during melt spinning and over-quenched amorphous Gd2Fe14B alloy during heat treatment were clarified. The magnetic properties of the crystallized alloys annealed at 800 °C are close to those of the optimized directly quenched as-spun alloys, indicating good performance stability under various processes. In addition, the role of Gd substitution in (Y1–xGdx)2Fe14B alloys was systematically studied. Gd substitution can significantly enhance the thermal stability and decrease the irreversible remanence loss of Y2Fe14B alloy. With increasing Gd content, Gd-doped Y2Fe14B alloys show continuous enhancement of Hc at elevated temperatures. The highest β value of 0.22%/K was obtained in (Y0.2Gd0.8)2Fe14B alloy at 300–400 K. Gd substitution is not only effective in improving the exchange coupling effect and corrosion resistance of Y2Fe14B alloy but also superior to Nd in terms of improving the oxidation resistance. The present work has certain instructions for designing and developing new low cost RE permanent magnets with improved thermal and environment stability.

The Singular NMR Fingerprint of a Polyproline II Helical Bundle.

Polyproline II (PPII) helices play vital roles in biochemical recognition events and structures like collagen and form part of the conformational landscapes of intrinsically disordered proteins (IDPs). Nevertheless, this structure is generally hard to detect and quantify. Here, we report the first thorough NMR characterization of a PPII helical bundle protein, the Hypogastrura harveyi "snow flea" antifreeze protein (sfAFP). J-couplings and nuclear Overhauser enhancement spectroscopy confirm a natively folded structure consisting of six PPII helices. NMR spectral analyses reveal quite distinct Hα2 versus Hα3 chemical shifts for 28 Gly residues as well as 13Cα, 15N, and 1HN conformational chemical shifts (Δδ) unique to PPII helical bundles. The 15N Δδ and 1HN Δδ values and small negative 1HN temperature coefficients evince hydrogen-bond formation. 1H-15N relaxation measurements reveal that the backbone structure is generally highly rigid on ps-ns time scales. NMR relaxation parameters and biophysical characterization reveal that sfAFP is chiefly a dimer. For it, a structural model featuring the packing of long, flat hydrophobic faces at the dimer interface is advanced. The conformational stability, measured by amide H/D exchange to be 6.24 ± 0.2 kcal·mol-1, is elevated. These are extraordinary findings considering the great entropic cost of fixing Gly residues and, together with the remarkable upfield chemical shifts of 28 Gly 1Hα, evidence significant stabilizing contributions from CαHα ||| O═C hydrogen bonds. These stabilizing interactions are corroborated by density functional theory calculations and natural bonding orbital analysis. The singular conformational chemical shifts, J-couplings, high hNOE ratios, small negative temperature coefficients, and slowed H/D exchange constitute a unique set of fingerprints to identify PPII helical bundles, which may be formed by hundreds of Gly-rich motifs detected in sequence databases. These results should aid the quantification of PPII helices in IDPs, the development of improved antifreeze proteins, and the incorporation of PPII helices into novel designed proteins.

AlGaN/GaN Lateral CRDs With Hybrid Trench Cathodes

A novel AlGaN/GaN-on-Si lateral current regulating diode (CRD) featuring a hybrid trench-gated cathode is proposed and experimentally demonstrated. By tailoring the recess depth and the length of the gate trench, the device characteristics including the knee voltage, the current density, and the current steadiness can be flexibly modulated. The CRD is capable of delivering an excellent steady current in a large voltage range of 0–200 V and a wide temperature range from −50 °C to 250 °C. Meanwhile, the regulating current exhibits small negative temperature coefficients less than −0.3 %/°C. Fast response capability of the CRD is validated by the pulse I–V measurements. The proposed CRD with such a new functionality together with the high performance is of great potential to expand the applications of AlGaN/GaN electronic devices.

Correlation between Electrical Resistivity and the Structural Evolution of Zr-Based Bulk Amorphous Alloys

The electrical resistivity of three kinds of Zr-based bulk amorphous alloys as a function of continuous increasing temperature was measured by the direct current four-probe method. Combining with Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis, the correlation between the electrical resistivity and structural evolution of these amorphous alloys has been demonstrated. The experimental results showed that three amorphous alloys all had a small negative temperature coefficient of resistivity (TCR<-10-4 K-1) before crystallization. Little change of the electrical resistivity related with the glass transition process was observed, while the electrical resistivity decreased sharply once the crystallization occurred. The onset of crystallization determined by the electrical resistivity measurement was far lower than that by DSC. The different stages of crystallization behavior could be evidently identified by the change of the electrical resistivity.

High Temperature Properties of Highly Electrical Resistive (Fe0.81Co0.19)84Ta9B7Alloy Cast in Air

This paper presents results from in situ X-ray diffraction (XRD), electrical resistivity, and magnetic measurements on an air-cast, amorphous (Fe0.81Co0.19)84Ta9B7 alloy as a function of temperature. The high temperature magnetic measurements revealed that the Curie temperature of the amorphous phase is 327 °C and crystallization begins at 483 °C. The crystallite sizes determined from the Scherrer analysis of XRD data increased from 13.4 nm at 525 °C to 31.3 nm at 600 °C. XRD data also indicated the presence of two minor metastable phases; one in an as-cast state, which decomposes at temperatures above 400 °C and a second one that starts forming at temperatures above 400 °C and decomposes before the temperature reaches 525 °C. Permeability increased with temperature first, showing a peak value at 125 °C and steadily decreased afterward to values comparable with those of dust cores. Coercivity increased significantly above 200 °C and a peak value of 130 Oe was observed at $\sim 400~^{\circ }\text{C}$ . In the as-cast state, the material exhibited an electrical resistivity value of 180 $\mu \Omega $ -cm, which was significantly reduced after crystallization. The material also exhibited a small negative temperature coefficient of resistivity.

Transition temperatures and upper critical fields of NbN thin films fabricated at room temperature

NbN thin films were deposited on thermally oxidized Si substrate at room temperature by using reactive magnetron sputtering in an Ar-N2 gas mixture. Total sputtering gas pressure was fixed while varying N2 flow rate from 1.4 sccm to 2.9 sccm. X-ray diffraction pattern analysis revealed dominant NbN(200) orientation in the low N2 flow rate but emerging of (111) orientation with diminishing (200) orientation at higher flow rate. The dependences of the superconducting properties on the N2 gas flow rate were investigated. All the NbN thin films showed a small negative temperature coefficient of resistance with resistivity ratio between 300 K and 20 K in the range from 0.98 to 0.89 as the N2 flow rate is increased. Transition temperature showed non-monotonic dependence on N2 flow rate reaching as high as 11.12 K determined by the mid-point temperature of the transition with transition width of 0.3 K. On the other hand, the upper critical field showed roughly linear increase with N2 flow rate up to 2.7 sccm. The highest upper critical field extrapolated to 0 K was 17.4 T with corresponding coherence length of 4.3 nm. Our results are discussed with the granular nature of NbN thin films.

Forward current transport mechanisms in Ni/Au-AlGaN/GaN Schottky diodes

The forward current transport mechanisms in Ni/Au-AlGaN/GaN Schottky diodes are studied by temperature dependent current-voltage (T-I-V) measurements from 298 to 473 K. The zero-bias barrier height qϕBn and ideality factor values determined based on the conventional thermionic-emission (TE) model are strong functions of temperature, which cannot be explained by the standard TE theory. Various transport models are considered to analyze the experimental I-V data. The fitting results indicate that the increased current at low bias is due to the trap-assisted tunneling with an effective trap density of about 8.8 × 106 cm−2, while the high-bias current flow is dominated by the TE transport mechanism, accompanied by a significant series resistance effect. By fitting the high-forward-bias I-V characteristics, the effective qϕBn values with a small negative temperature coefficient are obtained. The temperature dependence of the saturation tunneling current and qϕBn is finally explained by considering the thermally induced band gap shrinkage effect.

Magnetic and transport properties of Sm7Rh3 single crystal

A Sm7Rh3 single crystal with Th7Fe3-type hexagonal structure was grown by the Czochralski method. The magnetic and transport measurements revealed a uniaxial magnetocrystalline anisotropy in the magnetic susceptibility, magnetization and electrical resistivity. Sm7Rh3 was found to exhibit antiferromagnetic transition at TN=54.0K and another magnetic transition at Tt=25.0K. The specific heat data clearly showed the bulk nature of paramagnetic to ordered magnetic phase transition by the presence of a sharp peak at TN and a small anomaly at Tt. The paramagnetic susceptibility does not obey the Curie–Weiss law, attributing to the temperature independent Van Vleck contribution and Pauli paramagnetism of conduction electrons. Metamagnetic phase transitions were observed along the c-axis in the ordered states. The magnetic field H–temperature T phase diagram was constructed. Anisotropic paramagnetic electrical resistivity showed the small negative temperature coefficients.

Experimental and modeling study of thermal rate coefficients and cross sections for electron attachment to C60

Thermal electron attachment to C(60) has been studied by relative rate measurements in a flowing afterglow Langmuir probe apparatus. The rate coefficients of the attachment k(1) are shown to be close to 10(-6) cm(3) s(-1) with a small negative temperature coefficient. These results supersede measurements from the 1990s which led to much smaller values of k(1) with a large positive temperature coefficient suggesting an activation barrier. Theoretical modeling of k(1) in terms of generalized Vogt-Wannier capture theory shows that k(1) now looks more consistent with measurements of absolute attachment cross sections sigma(at) than before. The comparison of capture theory and experimental rate or cross section data leads to empirical correction factors, accounting for "intramolecular vibrational relaxation" or "electron-phonon coupling," which reduce k(1) below the capture results and which, on a partial wave-selected level, decrease with increasing electron energy.

Temperature offset drift of GMI sensors

Temperature offset stability of magnetoimpedance sensor is discussed based on experiments accomplished with soft magnetic amorphous material. An amorphous ribbon of Co 67Fe 4Cr 7Si 8B 14 composition was tested in a magnetic shield with adjustable internal temperature. DC resistance and complex components of impedance at frequencies up to 30 MHz were investigated in the temperature range of −20 to +75 °C. The measured data were used for the evaluation of temperature dependence of transverse permeability that is assumed to be a major contributor to the temperature variations of impedance. We also show that thermal treatments of the amorphous ribbon significantly affect the temperature behaviour of GMI. In case of tested alloy, which has very small negative temperature coefficient of resistivity, the achieved equivalent temperature offset drift is −0.27 A/m/K (−340 nT/K).

Crystal structure and microwave dielectric properties of a new A 4B 3O 12-type cation-deficient perovskite Ba 3LaTa 3O 12

A new microwave dielectric ceramic Ba 3LaTa 3O 12 has been prepared by conventional solid-state ceramic route. Through Rietveld refinement of X-ray powder diffraction data, the compound is identified as an A 4B 3O 12-type B-site cation-deficient perovskite with space group R 3 ¯ m and lattice constants a = 5.7573(2) Å, c = 28.2386(2) Å, V = 810.62(4) Å 3 and Z = 3. The microwave dielectric properties of Ba 3LaTa 3O 12 were investigated. The compound exhibits a relative dielectric constant ( ɛ r) of 36.8, a quality factor Q u × f of 21,965 at 6.4040 GHz and a small negative temperature coefficient of resonant frequency ( τ f) of −49.6 ppm/°C.

Zn 0.9 Mg 0.1 O ∕ ZnO p – n junctions grown by pulsed-laser deposition

The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.

Abnormal temperature dependence of intrinsic coercivity in sintered Sm–Co-based permanent magnets

In search of high-temperature permanent magnet materials, sintered Sm–Co-based magnets with abnormal temperature dependence of intrinsic coercivity have been obtained. These magnets have small negative, or near-zero, or large positive temperature coefficients of intrinsic coercivity. A model of coercivity mechanism has been proposed to explain this abnormal temperature dependence. According to this model, the coercivity in 2:17-type permanent magnets is controlled by two conflicting mechanisms. The first mechanism is thermal activation, which leads to decreased coercivity with increasing temperature. The second mechanism can be explained by a pinning process in which increasing temperature may result in an increased difference of crystalline anisotropy between the 2:17 cell phase and 1:5 cell boundary phase and, in turn, a higher coercivity at high temperature.

Effects of SnO<sub>2</sub> and Glass Particle Size on the Microstructure and the Electrical Properties of SnO<sub>2</sub>-Glass Composites

The effects of the particle size of SnO2 and glass on the densification behavior, the microstructure and the electrical properties of SnO2-glass composites were examined. Moreover, the relationship between the electrical properties of the glass composites and the particle dispersion of SnO2 in a glass matrix was discussed following quantitative analysis of SnO2 particle dispersion by computer-assisted image analysis on SEM photographs. The results are summarized as follows: (1) when coarse grained SnO2 particles were used, the densification of the glass composites proceeded at low firing temperatures. Therefore, the use of coarse SnO2 particles was useful to get the glass composite having a high electrical conductivity and a small negative temperature coefficient of resistance (TCR) at low firing temperatures. However, after the completion of densification, the glass composite prepared using fine SnO2 particles showed a higher electrical conductivity and a smaller negative TCR because SnO2 particles were more highly crowded in a glass matrix and thus the SnO2 interparticle spacings were smaller than those of the sample dispersed with coarse SnO2 particles. (2) when fine glass particles were used, the densification of the glass composites proceeded at low firing temperatures and the particle dispersion of SnO2 in a glass matrix was homogeneous. Besides, the glass composite prepared using fine glass particles showed a high electrical conductivity and a small negative TCR at low firing temperatures. On the other hand, at high firing temperature, the glass composite having a higher conductivity and a smaller negative TCR was obtained when coarse glass particles were used, because the densely aggregated SnO2 particles formed highly conductive networks in a glass matrix.

Temperature and Magnetic Field Dependence of Electrical Resistivity in Al–Mn Quasicrystal at Low Temperatures

The temperature dependence of electrical resistivity and the effect of magnetic field on it have been measured in a quasicrystalline Al 79.5 Mn 20.5 alloy of almost single phase. This phase has large resistivity (∼800 µΩcm) and a small negative temperature coefficient above 30 K like high-resistive amorphous metals, though the crystallized phase has a positive coefficient like usual metals. Below 30 K, the resistivity of the quasicrystalline phase increases rapidly with decreasing temperature, exhibiting exact logarithmic temperature dependence from 1 K to 15 K. The resistivity increases with increasing magnetic field (≤ 140 kG) in this temperature range, and the origin of the logarithmic temperature dependence is not considered to be the ordinary Kondo effect. Some discussion is made by comparing the present results with those of amorphous metals.

Hall Coefficients of Liquid Se–Te Alloys

The Hall coefficients of liquid Se–Te alloys have been measured by an alternating current and a direct current magnetic field for operation temperature from their melting point to about 750°C. The signs are negative for liquid Te and liquid Se x Te 1- x , alloys with x =0.2, 0.3, 0.4 and 0.5. The absolute values of Hall coefficient for liquid Se–Te alloys exhibit the negative temperature coefficients and increase with increasing Se concentration. The Hall mobilities of these alloys have small negative temperature coefficients. The relationship between the Hall coefficient and the density of states at the Fermi level has been discussed on the basis of his formula given by Friedman.

Amorphous X films (X = C, Si, OR Re) prepared by extracting Ag with I from AgX films

A new technique to make amorphous (a) materials is described and successfully applied to C, Si and Re. The basic scheme consists in depositing a random solid solution or an amorphous XAg mixture where X is totally inert to I. The Ag atoms are then removed by exposing the XAg film to I, leaving an amorphous X film with a density appreciably lower than bulk. The technique was tested on CAg films since the resulting a-C films could be compapared with a-C films deposited by conventional means. X-ray fluorescence and neutron activation analysis established that the Ag extraction by I in a-C films several thousand Å thick is extremely good, with Ag residues ranging from 10 −4 to 10 −2 of the original Ag content. The resistivity at 300 K of these a-C films was as high as 10 7 Ω cm, 5 orders of magnitude above that of evaporated or sputtered a-C films. It is likely that this effect is due to the removal of defect states such as dangling bonds either by surface reconstruction or by the trapped I similarly to a-C:H films. The most rewarding application of this technique was the preparation of essentially pure (except for a few at% I) a-Re films, since Re films sputtered at 77 K are crystalline. The properties of these a-Re films fall in three classes depending on the original Re content of the starting ReAg film. At 90 at% Re, the a-Re films are superconducting at ⋍ 6 K; the resistivity is about 2 × 10 −4 Ω cm at 300 K and has a small negative temperature coefficient [ α = (d ϱ/ ϱ)d T = −3 × 10 −4 K −1] typical of amorphous metals. For Re concentrations between 43.5 and 59 at% the density of the a-Re films is about half that of bulk. The resistivity of these films displays two different types of temperature dependence: at intermediate resistivities [ϱ(300 K) ⋍ 2 × 10 −2 Ω cm] α = −3 × 10 −3 K −1 while for high room temperature resistivities (10 −1 to 10 4 Ω cm) ϱ ∼- exp T −1 2 . In other words, above 60 at% Re the a-Re films displays an essentially metallic conductivity while below 60 at% Re they are characterized by a semiconducting conductivity.

Absorption of ultrasound in tissues

The primary absorption of ultrasound in most soft tissues occurs at the macromolecular level. The attenuation observed when a tissue sample is interposed between a source and receiver may be affected by reflection, refraction, and phase distortion in the wave transmitted through the sample. However, when these effects are minimized, the differences between absorption and attenuation coefficients are small. Most soft tissues are characterized by small negative temperature coefficients up to physiological temperatures. At high intensities, nonlinear phenomena become important and can create losses in tissues which are mechanistically distinct from small amplitude absorption processes.

Isotope exchange reactions of hydroxy(1-) or hydroxy-d(1-) ions with hydrogen and water

Exchange between gaseous hydroxyl ion and hydrogen and water has been investigated by using two mass spectrometers and two modes of ionization. Exchange of OH/sup -/ and D/sub 2/ is a relatively slow reaction (collision efficiency = 2.9 x 10/sup -2/), and its rate constant exhibits a small negative temperature coefficient, k = CT/sup -0/ /sup 7/. The exchange of OD/sup -/ with H/sub 2/ is 30% slower than OH/sup -/ + D/sub 2/, the difference becoming less as the temperature increases. Exchange of OD/sup -/ with H/sub 2/O is fast (collision efficiency approx. 1). Exchange of OD/sup -/ with CH/sub 4/ and i-C/sub 4/H/sub 10/ has also been examined, but these reactions are very slow (k < or = 3 x 10/sup -13/ cm/sup 3//molecule s) and may well be zero.