High Negative Temperature Coefficient Research Articles

High negative temperature coefficient of resistance of heteroepitaxial (Ba, Sr) TiO 3 single crystal films prepared by pulsed laser deposition

Heteroepitaxial (Ba, Sr)TiO 3 (BST) films were grown on cleaved single crystal <100> MgO substrates by pulsed laser deposition. Highly <100> oriented BST films with atomic flat surfaces were obtained. By measuring Current–Voltage characteristics, a significant negative temperature coefficient of resistance (about − 4.95% at 300 K) was observed which is different from the positive temperature effect as reported for poly-crystal BST films. As has been suggested the positive temperature coefficient of resistance is attributed to the temperature-dependent barrier height and width at grain boundaries. In this paper, the negative temperature effect of resistance observed in our experiment is discussed and explained by the activation of electrons trapped by oxygen vacancies and absence of grain boundaries. Deep level spectroscopy measurement indicated that the activation energy is approximately 0.31 eV, which is consistent with the value deduced from the slope of ln ⁡ R ∼ 1 T curve.

Preparation, Characterization and Microwave Dielectric Properties of A2BWO6 (A=Sr, Ba; B=Co, Ni, Zn) Double Perovskite Ceramics

The A2BWO6 (A=Sr, Ba; B=Co, Ni, Zn) microwave dielectric ceramics were prepared by conventional solid-state ceramic route. According to the X-ray diffraction (XRD) results, the ceramics have cubic perovskite structure for all compounds. In addition, the Ni-containing compounds showed apparent supperlattice reflections at low angles, whereas the others didn't. The results of Raman spectroscopy show that the degree of B-site ordering is different with different B2+ ions. The dielectric properties of the ceramics were measured in the frequency range 6.5 to 8.5 GHz using resonance methods. The ceramics have dielectric constant in the range 18 to 30, high quality factor (Q×f value up to 56000 GHz) and negative temperature coefficient of resonant frequencies in the range -72.9 to -31.1 ppm/°C. The dielectric constant increased with increasing B2+ ionic polarizability, and the quality factor was enhanced due to the increase in B-site ordering. The relationship between temperature coefficient of dielectric constant and tolerance factor was accordant in comparison with the other perovskites.

Preparation and structure of AlW thin films

Thin films of AlW alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source. The deposition rate at the substrate (glass, fused quartz, and alumina ceramic), positioned 5 cm away from the target surface was 0.1–0.2 nm/s for pure metals, and the final film thickness was a few μm. Completely amorphous films were obtained in the Al 80W 20–Al 67W 33 composition range. At higher tungsten content, the W(Al) solid solution and pure tungsten phases appeared. The amorphous alloys exhibit a high negative temperature coefficient of the electric resistivity, increasing with the aluminum content up to −5.5·10 −4 K −1. Finally, the amorphous AlW alloys exhibit a remarkable microhardness (6–7 GPa), and are structurally stable up to at least 400°C.

Novel atropisomeric binaphthol containing comb‐shaped copolymers forming chiral nematic phases

AbstractNovel comb‐shaped liquid crystalline acrylic copolymers consisting of nematogenic methoxy‐phenylbenzoate acrylic monomers (A) together with novel chiral binaphthol (BN) methacrylic monomers (MB‐m) were synthesized in this study. The prepared copolymers differ in spacer lengths of MB‐m (m = 3, 5, 11) and in their compositions. The homopolymers of the three new chiral BN monomers MB‐m were also prepared. Copolymers with a low concentration of BN monomeric units (less than 16 mol %) display a cholesteric mesophase. The induced chirality in the polymers is due to atropisomerism (chirality due to hindered rotation around single bonds) of the BN molecules. The helical twisting power β (HTP) caused by the atropisomeric units in the synthesized copolymers was shown to decrease significantly on heating. The unusually high negative temperature coefficient of HTP observed above the glass transition temperature (Tg) could be explained in terms of conformational changes of the BN molecules in the copolymers.

Novel atropoisomeric binaphthyl-containing liquid crystalline copolymers forming chiral nematic phases

Novel liquid crystalline (LC) acrylate side group copolymers, which consist of nematogenic phenyl 4-methoxybenzoate acrylate monomer (A) and novel chiral binaphthyl (BN) methacrylate monomers (MB-n) have been synthesized. The copolymers prepared differ in the spacer lengths of MB-n (n 3,5,11) and in their compositions. The homopolymers of the three new chiral binaphthyl monomers MB-n were also prepared. Copolymers with a low concentration of binaphthyl monomer units (less than 16 mol%) display a cholesteric mesophase. The induced chirality in the polymers is due to atropoisomerism (C2-symmetry) of the molecules. The helical twisting powers (beta), caused by the atropoisomeric units in the synthesized copolymers, were determined, and their temperature dependencies studied. The unusually high negative temperature coefficient of beta observed above the glass transition temperature is explained in terms of conformational changes of the BN molecules in the copolymers.

Impact of lone pairs on the conformation and configuration statistics of poly(lactic acid) chains

AbstractNonbonded interaction energies have been calculated for various conformations of the diester giving explicit consideration to the lone‐pair electrons. As suspected from an earlier calculation on the dipeptide, such inclusion of the lone‐pair effect leads to significant differences in the conformational potential energy map. The diester is shown to be more constrained than the dipeptide. The impact of this constrained geometry and the possible occurrence of cis residues on configuration statistics of poly(lactic acid) chains have been studied. Contrary to expectation the inclusion of cis isomers does not explain the paradoxical high negative temperature coefficient of ln C∞.

Resistance material based on silicon carbide

Silicon carbide is used for high-temperature electric heating elements [I]. However, high-density sintered SiC has a relatively low electrical resistivity, on the order of 10 -I ~-cm, and a high negative temperature coefficient of resistivity (TCR) in the temperature range from room to 800-900~ [i], and this makes it unsuitable for the manufacture of fixed network voltage heating elements of small linear dimensions which could replace the shortlived spirals. In this connection, there is interest in possible ways of producing silicon carbide materials of higher resistivity and lower TCR. In the present work the electrical resistivity of silicon carbide was increased by decreasing its density. At the same time, to increase its mechanical strength, decrease its negative TCR, and lower its sintering temperature, silicides of the transition metals Ni, Co, Fe, V, Mn, and Cr were added to it.

An Underwater Thermoelectric Reactor Plant

A conceptual design is presented for an underwater thermoelectric nuclear power plant capable of producing continuous electric power at a 100-kwe level. The inherent advantage of a self-pressurized, natural circulation, pressurized water reactor (PWR) system is combined with a direct-conversion thermoelectric generator to provide a completely static system. The plant is designed for remote startup and completely unattended operation for over five years. Power is produced directly at 40 v; however, utilization of a solid-state inverter and storage batteries provides a peaking capability of several megawatts(e) at higher voltages. A low enriched core, with an inherently high negative temperature coefficient, is used which allows the control rods to be withdrawn completely at plant startup and to remain static throughout design life. No electronic controls, power monitoring, or feedback systems are required for operation or safety. Segmented thermoelectric elements (PbTe and BiTe) are used to obtain optimized performance over the rather small temperature drop available at the couple junctions. Although system weight is strongly dependent on design depth—-80,000 Ib for 18,000 ft'—-advanced pressure balancing concepts would allow weight reductions of 50 to 70% plus higher temperatures for improved thermoelectric conversion efficiencies.

Kinetics of crystallization and a crystal‐crystal transition in poly‐1‐butene

AbstractThe formation and melting of the two crystalline modifications of poly‐1‐butene have been studied by the techniques of differential thermal analysis, infrared spectroscopy, dilatometry, and polarizing microscopy. On cooling the melt, modification 2 (Tm = 120–126°C.) appears within minutes. This form is unstable and gradually transforms to modification 1 (Tm = 135–142°C.). A density change of over 4% accompanies this latter transformation. As torsional damping experiments have shown the amorphous content to decrease by only about 5%, the stable modification must be much more dense than the unstable form. The kinetics of the crystallization of modification 2 from the melt were studied primarily dilatometrically at several temperatures. The data generally followed the Avrami equation with n equal to 4. This crystallization shows a high negative temperature coefficient (T1/2 = 380 min. at 110°C. and 2 min. at 90°C.) and becomes too rapid to measure by this technique at lower temperatures. The kinetics of the transformation of modification 2 to modification 1 were followed in infrared spectral, density gradient tube, and dilatometric studies. At room temperature, halftimes ranging from 250 to 1600 min. were observed, depending on the mechanical and thermal history of the sample. The observed discrepancies have been shown to be due, at least in part, to orientation effects. In dilatometric studies of the temperature dependence, the transition rate showed a pronounced maximum in the region of 15°C. Large increases in the torsional modulus and offset yield point of the specimen were observed to accompany the transition.

Thermistors for Depth Thermometry

limnological studies, accurate temperature measurements at depths in reservoirs are very important. The means used for measuring these temperatures include the reversing thermometer, the depth sampler and ordinary thermometer, and the electrical resistance thermometer (1, 2). Of these, the resistance thermometer offers the greatest advantages, because the temperature-sensing device need not be brought to the surface each time a reading is taken. In the past the most commonly used temperature-sensitive element has been a fine platinum wire with a positive temperature coefficient of resistivity of approximately 0.4 per cent per degree centigrade and a total resistance of about 2.5 ohms at 25 °C. This requires a very accurate and expensive bridge if temperature changes of 0.1 °C are to be measured. The advent of the thermistor in recent years has afforded a much better temperature-sensing device (3-45). Thermistors are thermally sensitive resistors with a high negative temperature coefficient of resistance (approximately 4.4 per cent per degree centigrade at 25°C). They are available in a wide range of types with diverse and stable characteristics. The type most adaptable for depth thermometry is the Western Electric Type 14A thermistor,* which consists of a glass

Molecular Weight Determination by Thermistors

A THERMISTOR is a resistance body which has a high negative temperature coefficient of resistance, and is a valuable circuit element with a large variety of applications. Thermistors can be used as sensitive thermometers in cryoscopic measurements1 and also in place of thermocouples for measuring the lowering of vapour pressure of aqueous colloidal solutions2. At a condition of steady state, the droplets of solution and solvent deposited on the tips of the thermistors have a constant difference in temperature which can be measured as a difference of resistance in an A.C. bridge circuit. The construction and operation of a simple A.C. thermistor bridge, used for measuring the osmotic activities of aqueous solutions, has been described by McGee and Iyengar (submitted for publication elsewhere).

Thermistors as Instruments of Thermometry and Anemometry

Thermistor beads are used as the sensitive elements for measuring temperature and wind speed. Their high resistance makes it unnecessary to correct for the resistance of leads or the change in their resistance due to temperature. Because of the high negative temperature coefficient of resistance of thermistor material slight changes in ambient temperatures caused sufficient changes in the bead resistance to unbalance a bridge circuit and thereby activate a recording milliammeter which had been calibrated to read temperatures. The change in resistance of a hot bead by the wind activated a second milliammeter which was calibrated to read wind speed.

The structure and electrical conductivity of thin films of indium

The high electrical resistance and negative temperature coefficient that thin films of indium, in common with other metals, exhibit when prepared under moderate vacuum conditions (10-4 to 10-5 mm.) and without exhaustive cleaning of the substrate, are shown by electron-diffraction methods to be in no way connected with the existence of any amorphous form of the metal. The lattice of freshly deposited films is found to correspond in form with that of the bulk metal but to have a spacing 0.7 per cent greater than that given by recent X-ray work, gold films here being taken as standards. With slowly deposited films the spacing increases and the axial ratio decreases with rise in temperature up to room conditions, and exposure to air carries this process further until a film showing a cubic indium lattice with spacing 3.4 per cent greater than the bulk metal is obtained. This change is accompanied by slow oxidation, the oxide lattice being 0.6 per cent larger than X-ray values indicate. It is suggested that the high resistance of these films is due to sorption of residual gas, and that the negative temperature coefficient is the result of desorption and resorption processes; changes in crystal-size with temperature are also noted and discussed. The change in lattice constants is ascribed to the action of gas within the lattice, possibly amounting to the production of intermediate stages in oxideformation.