Showing papers on "Temperature coefficient published in 1989"

Impedance and modulus spectroscopy of semiconducting BaTiO3 showing positive temperature coefficient of resistance

Abstract: Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain‐boundary and bulk effects may be distinguished from capacitance data: grain‐boundary effects have temperature‐independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain‐boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

Abstract: The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi2Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3

Quantitative infrared methods for the measurement of crystallinity and its temperature dependence: polyethylene

Abstract: Vibrational spectroscopic methods are widely used to characterize semicrystalline polymers in terms of crystallinity. The temperature coefficient of crystallinity, an important and fundamental quantity, is seldom determined for lack of a sensitive method. In this paper, we describe an infrared approach to the measurement of the temperature coefficient of crystallinity. We start from the well-known observation that the integrated intensities of the bands in the spectrum of a semicrystalline polymer change with temperature. It is also known, though less appreciated, that only part of the change is due to changes in crystallinity, the remaining part being due to changes in the intrinsic intensity of the bands. We outline a method for separating these overlapping effects. The method has been applied to a variety of semicrystalline polyethylene samples. The temperature coefficients are found to be highly dependent both on the temperature and on the morphology of the sample. In addition we report crystallinity measurements on a solution crystallized low molecular weight (A& = 13 600) sample, discuss the origin of an apparent anomalous temperature dependence of band intensity cited in the literature, and offer quantitative evidence that the temperature dependence of specific volume is, at temperatures above 0 "C, largely determined by partial melting.

On the non-linear properties of TlInX2 (X = S, Se, Te) ternary compounds

Abstract: Optical and electrical properties of the TlInX 2 (X = S, Se, Te) were studied. The selenides and tellurides are of chain-like structure while the sulfides are layered. The measurements ( I-U characteristics and optical absorption) seem to have a direct connection with the main structural features. The I-U -curves of the first two compounds are S-type with characteristic oscillations in the negative differential resistance (NDR) region while the sulfides show an Ohmic behavior. The optical absorption measurements in the temperature range 15–300 K for Se- and Te-compounds show a positive temperature coefficient of the gap while for the S-compound it is negative. Possible explanations are mentioned.

The design of thin-film polysilicon resistors for analog IC applications

Abstract: The authors describe the design of thin LPCVD polysilicon resistors based on their desired electrical performance (sheet resistance, temperature coefficient, voltage nonlinearity, and matching and uniformity). Based on measured data and an understanding of polysilicon carrier transport phenomena, it is shown how resistor processes can be designed for a wide variety of analog IC (integrated circuit) applications, requiring only ion implantation and standard polysilicon deposition and patterning processes. For a +or-500 p.p.m./ degrees C temperature coefficient of resistance range, the available sheet resistance lies between 2400 and 40 Omega /sq. for a polysilicon thickness ranging from 50-600 nm. Matching and nonlinearity (in a 10-V range) to better than 0.06% has been achieved from a process designed as described. Thus it is demonstrated that performance comparable to thin sputtered resistor films can be achieved without the requirement of a long development time and specialized knowledge. >

Curvature correction of bipolar bandgap references

Abstract: A bipolar bandgap reference circuit employing three resistors of selected nominal resistance values and a method of trimming the values of two of the resistors to cancel the slope and curvature of output voltage due to thermal drift. One of the resistors provides a positive temperature coefficient to counter the temperature dependency of bipolar base-emitter characteristics; this resistor is not trimmed. The other two resistors are thin-film, low TC devices and are "trimmed" (i.e., adjusted) sequentially, to match calculated values intended to minimize the first and second derivatives of the bandgap cell output, as a function of temperature.

Weak Localization in Pregraphitic Carbon-fibers

Abstract: High-resolution resistivity and magnetoresistance measurements have been performed as a function of temperature on pregraphitic carbon fibers. The resistivity exhibits a negative temperature coefficient (d\ensuremath{\rho}/dTl0) from room temperature down to 1.5 K. It is shown that such behavior can hardly be explained by a thermal excitation of carriers in the liquid-helium temperature range within the framework of the simple two-band model, usually applied to graphitic and pregraphitic structures. The magnetoresistance present a negative component more pronounced at low temperature. It is found that the magnetic field acts to restore the expected behavior of the resistivity in the low-temperature range, in the sense that the resistivity becomes gradually temperature independent. These observations as well as the experimental results obtained at higher temperature are consistent with the weak-localization theory for two-dimensional systems.

Low Temperature Properties of Microwave Dielectrics

Abstract: Temperature dependences of resonant frequency and dielectric loss tangent were measured at the temperature range from 20 to 300 K for three kinds of microwave dielectrics, (Zr, Sn)TiO4, Ba(Zr, Zn, Ta)O3, and Ba(Sn, Mg, Ta)O3. The deviations of resonant frequencies from 20 to 300 K for these materials were less than 1200 ppm, which is equivalent to 4.3 ppm/°C of the temperature coefficient. The dielectric loss tangents of (Zr, Sn)TiO4 materials showed a remarkable difference when they were made of the raw materials of different purities. These low temperature characteristics are discussed in connection with the anharmonic terms in the crystal's Hamiltonian.

Dielectric porcelain composition

Abstract: PURPOSE: To obtain a high dielectric constant, a small temperature coefficient and an excellent quality Q by providing a composition which has SrO-CaO-TiO 2 as its main components in such a manner that when an expression is given as xSrO-yCaO- zTiO 2 (x), (y) and (z) are in a specific mol ratio range, and making the composition contain a specific amount of Ta 2 O 5 as its auxiliary component. CONSTITUTION: A composition having as its main component SrO-CaO-TiO 2 in such a manner that when an expression xSrO-yCaO-zTiO 2 is given (x+y+z=1.00) (x), (y) and (z) are in a mol ratio range surrounded by points (a) to (f) in the figure, is made to contain 0. 1 to 10.0 parts by weight of Ta 2 O 5 as its auxiliary component. Since the composition contains Ta 2 O 5 as its auxiliary component, it has the effect of increasing breakdown voltages and, when the amount of the Ta 2 O 5 is less than 0.1 parts by weight against the main component then there is no effect of containing Ta 2 O 5 , while when the amount exceeds 10.0 parts by weight then the quality Q is deteriorated and the temperature coefficient is increased to one side, which is not practical. It is thus made possible to obtain a dielectric porcelain composition of high dielectric constant, high insulation resistance, and high insulation breakdown voltage, being excellent in its quality Q and of small temperature coefficient. COPYRIGHT: (C)1990,JPO&Japio

Positive Temperature Coefficient of Resistivity Effect in Undoped, Atmospherically Reduced Barium Titanate

Abstract: A sharp positive temperature coefficient of resistivity effect for undoped, fluorinated BaTiO3−x samples was observed. The qualitative agreement between the behavior of BaTiO3−x and La-doped BaTiO3, with respect to the trap density at the boundaries, suggests that in the absence of counterdopants such as Mn, the main origin of surface acceptor states is chemisorbed gases. Impedance analysis indicates that, under the conditions of the present work, fluorine seems not to diffuse in significant amounts into the lattice. The treatment of the fluorinated samples in inert/reducing atmospheres did not markedly decrease the resistivity jump, suggesting that fluorine might be chemisorbed more strongly than oxygen. The composition of two different liquid phases, produced by excess titania and silica additions, did not have an important effect on the resulting properties.

Effect of sintering aids on the electrical properties of positive temperature coefficient of resistivity BaTiO3 ceramics

Abstract: The effect of sintering aids SiO2, Al2O3, and TiO2 on the microstructure and electrical properties of Sb3+‐doped BaTiO3 materials is examined. The SiO2 acts as a liquid‐phase former, which not only enhances the atomic diffusion, but also purifies the grain interior and increases the grain conductivity. The room‐temperature resistivity of the materials is lowered. An abundance of the liquid phase due to excess TiO2 will, however, result in a clustering of particles and the formation of a nonuniform microstructure. The Al3+ ions inhibit the grain growth and lead to a small and uniform grain size distribution. These ions also act as electron traps which increase the Schottky barrier height and in turn, raise the high‐temperature resistivity of the samples.

Self limiting electric heating element and method for making such an element

Abstract: This invention relates to a self-limiting electrical heating element comprising resistance components having a positive temperature coefficient and a zero temperature coefficient, arranged in a layered structure with two electrodes placed diagonally within or in contact with two ZTC layers separated by a PTC layer.

Piezoresistivity in semiconducting positive temperature coefficient ceramics

Abstract: The experimental and theoretical aspects of piezoresistivity in semiconducting, positive temperature coefficient (PTC) of resistivity ceramics will be reviewed. Emphasis will be placed on their potential application in sensor technology. Future material and modeling challenges are highlighted.

Temperature compensated stripline structure

Abstract: A stripline structure has a stabilized resonant frequency against temperature variations. It includes a lower and an upper substrate of ceramic materials, each substrate having opposing inner and outer surfaces. Each of the outer surfaces are covered with a layer of conductive material constituting ground planes. Resonator strips of conductive material are situated on each of the inner surfaces, and each have one end connected to the ground, while the opposite end is an open circuit. The upper and lower substrates are bonded together along the length of their respective resonator strips, thereby producing the stripline structure. The length of the resonator determines the resonant frequency of the stripline structure. The two substrates are made of materials having opposite dielectric temperature coefficient. The physical parameters of the stripline structure, such as, thicknesses of the substrates, the widths of the resonator strips, or both can be adjusted in order to produce a net zero, positive or negative frequency temperature coefficient. Furthermore the substrates can be made of dielectric or ferrite materials.

Flexible, elongated positive temperature coefficient heating assembly and method

Abstract: A flexible heating cable and method using positive temperature coefficient conductive (PTC) polymeric material as the primary heat source with the PTC composition material being electrically and mechanically connected to substantially flat, preferably braided, electrical conductors. A covering of dielectric material preferably is used to electrically separate the cable from the environment. The cable construction improves the heat transfer from the PTC composition material to the environment, thereby increasing the power generated by the PTC composition material. Additionally, the cable construction improves the temperature distribution of the cable.

Lattice constant, elastic modulus and electrical resistivity of Ni50Ti50−xAlx

Abstract: Measurements of the lattice constant, the elastic modulus and the electrical resistivity were carried out for Ni50Ti50 − xAlx (x ⩽ 3) alloys in order to investigate the effect of addition of aluminium to a NiTi alloy. At room temperature, the aluminium-containing NiTi alloy has the CsCl structure, from which Ti2Ni precipitates. Each phase forms a solid solution with aluminium. When the aluminium content is increased to 2 at.%, the lattice constant of NiTi decreases while the elastic modulus increases. At higher aluminium content, the lattice constant and the elastic modulus remain practically unchanged. The elastic anomaly was confirmed at temperatures ranging from 293 to 373 K, the elastic modulus showing a positive temperature coefficient. Although the thermal cycle dependence of the transformation behaviour and the elastic anomaly in the CsCl structure for the Ni50Ti50 − xAlx alloys resemble those of binary NiTi, the aluminium addition emphasizes the premartensitic phenomenon and drastically expands the temperature region of the transformation.

Lead zinc niobate pyrochlore: structure and dielectric properties

Abstract: In Pb(B x ′B1−x ″)O3 ceramic compositions, it is customary to find a mixture of cubic pyrochlore and perovskite phases after calcination. Based on X-ray diffraction analysis, we concluded that both phases are made up of the same structural unit of BO6 octahedra. The B′ and B″ ions occupy the B sites randomly to the extent that local charge equilibrium is maintained. Thus a general formula for the pyrochlore phase can be expressed as Pb(B x ′B1−x )O3 · Op where O ⩽p ⩽ 0.5. An extensive study of the Pb(Zn x Nb1−x )O3.5−1.5x pyrochlore system was made by varying the zinc concentration. We interpret changes in the X-ray diffraction pattern and the lattice constant as indicative of the changing occupancy of the seventh oxygen sites in order to maintain local charge balance. The best combination of the dielectric properties, with a dielectric constant of130 and a σ factor greater than 1000 at10 MHz, is achieved at a composition of 0.3 ⩽x ⩽ 0.4 and a sintering temperature of 980° C. The temperature coefficient of the dielectric constant measures −0.75 × 10−3° C−1. It decreases to −0.54 × 10−3° C−1 when5 mol% of PbTiO3 was mixed with the nominal pyrochlore compositions and sintered. Thus, it may be possible to effect a larger change in the temperature coefficient by judiciously including selective amounts of a second phase which has the best compensating temperature coefficient.

JSFR combustion processes of n-heptane and isooctane

Abstract: Combustion processes of n-heptane and isooctane have been studied in a Jet Stirred Flow Reactor (JSFR), which operates according to the Continuous Stirred Tank ideal model of reactor. The range of experimental conditions spanned from 202 to 1212 kPa and from 573 to 840 K. The feed consisted of stoichiometric fuel/air mixtures and flow rates were such to establish a 0.4 s residence time. The analysis of dynamic behavior of combustion processes showed that the same variety of dynamic events is exhibited by both fuels, namely: slow combustion, low temperature multistage ignition, damped and periodic cool flames, “jumps” and high temperature ignitions. Ignition diagrams have been plotted on the temperature/pressure plane. Heat release rates have been measured experimentally and their dependence on temperature clearly shows the region of existence of negative temperature coefficient of the reaction rate. The heat release rate curve provides evidence of hysteresis due to the non linearity of the process. The analyses of reaction intermediates indicate that fuel molecules are not heavily degraded at the sampling conditions which are located in the negative temperature coefficient region. The major differences between the combustion processes of the two fuels are in the pressure limit of the region of low temperature multistage ignition and in the presence of acetaldehyde among the reaction intermediates detected only for the n-heptane/air system. On this basis, the hypothesis is formulated that the different knock resistance of the two fuels could well be caused by the differences of the low temperature mechanisms and by the ability of producing acetaldehyde at an early stage of the combustion process.

Photoconductivity in germanium selenide single crystals

Abstract: The photoconductivity spectral response is measured for GeSe single crystals at different temperatures from room temperature to near liquid nitrogen temperature. Measurements are carried out with plane polarized light with the plane of polarization parallel to the a- and c-crystallographic axes which lie in the plane of cleavage. The photon energy range of measurements extends from 0.5 to 3.0 eV. The photoconductivity energy gap is determined from the spectral response by the Moss rule. The room temperature photoconductivity energy gap is found to be (1.144 ± 0.011) and (1.167 ± 0.025) eV for the a- and c-axes, respectively. The temperature dependence of the photoconductivity energy gap is also deduced. It is found that it is linear between 93 and 250 K with a negative temperature coefficient equal to −(0.35 ± 0.01) meV/K for the a-axis. The results for the c-axis show a large discrepancy which is attributed to crystal lattice bending introduced during the cleavage process. The minority carrier lifetime is determined from the photoconductivity frequency response using square modulated illumination. The photoconductivity frequency response is found to be characterized by two different lifetimes.

Positive temperature coefficient resistor

Abstract: The resistor includes a cooper oxide ceramic with perovskite related structures exhibiting positive temperature coefficient of resistance characteristics in the temperature range of 300°-900° C. Suitable materials are YBa2 Cu3 O7-x, La2 CuO4 and related compounds with other rare earth elements substituted for Y and La and other alkaline earth elements substituted for Ba.

Semiconductor strain detector

Abstract: In a semiconductor strain detector, the temperature characteristic of the output of a zero-point temperature compensating circuit is added to the zero-point temperature characteristic of the output of a bridge circuit composed of strain gauges, to perform a zero-point temperature compensation of the final output. Further, two kinds of diffusion resistors having different surface impurity densities are used in each amplifying circuit to perform a sensitivity-temperature compensation in which a temperature coefficient of sensitivity is considered up to the second-order term.

High frequency dielectric composition

Abstract: A high-frequency dielectric ceramic compound having a perovskite-type crystal structure comprising a chemical composition substantially represented by the formula (Srx Cay Mnz)TiO3 in which 098

Electrical resistivity and structure of liquid Al-Ge alloys

Abstract: Aluminium is very reactive with the quartz glass vessel generally used for electrical resistivity measurement cells. In the paper the authors describe a new geometrical arrangement to measure accurately the electrical resistivity of liquid alloys, using an alumina vessel. They give experimental results for the Al1-xGex liquid alloys. This system presents an anomalous concentration-dependent temperature coefficient of the resistivity, with respect to the calculated curves, when using the Faber-Ziman formula with pseudopotential form factors and hard-sphere partial structure factors. The authors have measured total structure factors of four Al-Ge alloys by neutron scattering on the 'Orphee' reactor at Saclay, using sapphire monocrystal cells. The temperature dependence of the total experimental structure factor explains qualitatively the anomalous resistivity temperature coefficient.