Large Positive Temperature Coefficient Research Articles

Phase transitions and microwave dielectric behaviors of the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4 ceramics

AbstractMicrowave dielectric ceramics with intrinsic low sintering temperatures are potential candidates for low temperature co‐fired ceramics technology. In the present work, the (Li0.5Y0.5)MoO4 ceramic with tetragonal scheelite structures was selected to improve microwave dielectric properties of BiVO4 ceramics. As proved by X‐ray diffraction (XRD) results, scheelite structured solid‐solution ceramics were formed with x value ≤0.1 in the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4. In situ XRD results further confirmed that the addition of (Li0.5Y0.5)MoO4 also lowered transition temperature from distorted monoclinic to tetragonal scheelite structure. When x value increased further, zircon phase was detected by XRD. Room and high‐temperature Raman spectra also supported the XRD results. Differences of thermal expansion coefficients of both monoclinic and tetragonal scheelite phases lead to an abnormality at phase transition temperature. Good microwave dielectric properties with permittivity above 70 and Qf (Q = quality factor = 1/dielectric loss and f = frequency) value above 8000 GHz were obtained in the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4 solid‐solution ceramics with x value ≤0.1 sintered below 800°C. However, permittivity peak values at phase transition temperatures lead to large positive or negative temperature coefficient of resonant frequency, and this needs to be modified via composite technologies in the future.

Sb-doping effects on twin-shift option and microwave dielectric properties of Ba8CoNb6O24 eight-layer hexagonal perovskite ceramics

B-site deficient eight-layer hexagonal perovskites possess in general high quality factors and large positive temperature coefficients of resonant frequency (τf). Recently we successfully lowered τf of shifted perovskite Ba8CoNb6O24 down to near zero through one-third Ta5+ substitution but raised the cost of the materials. In this work, Ba8CoNb6O24 was doped with cheaper elements Sb5+ on Nb5+ sites using conventional solid-state reactions, and near-zero τf of 0.66(2) ppm/°C, modest εr of 27.99(3), Q × f of 1.016(6) × 104 GHz, and significantly reduced cost of the ceramics were achieved on the composition Ba8CoNb5.4Sb0.6O24. The lower polarizability of Sb5+ than Nb5+ results in smaller εr and therefore τf values. About 15 % Sb5+ substitution for Nb5+ in Ba8CoNb6-xSbxO24 dramatically transforms the shifted structure to twinned structure, in contrast with 50 % Ta5+ substitution in the Ta5+ case. This is ascribed to reduced SOJT effects and ionic size of d10 Sb5+ compared with d° Nb5+ and Ta5+.

Effect of CuO and TiO2 on the sintering temperature and dielectric properties of BaWO4 for LTCC applications

A series of (1-x) BaWO4-xCuO low-temperature sintered composite ceramics were prepared via co-firing the mixture of BaWO4 and CuO in the first part of this study. The sintering temperature of BaWO4 was decreased from 1150 °C to 950 °C when a little amount of CuO was used as sintering aids. The 0.95BaWO4-0.05CuO sample heated at 950 °C for 4 h had good dielectric performance (Q × f ~ 31,847 GHz, εr ~ 7.99, τf ~ −81.7 ppm/°C). However, the negative τf was too large to apply to practice. To solve this problem, the rutile phase TiO2 nano-particle with a large positive temperature coefficient of +450 ppm/°C was added as τf compensator in the second part of the study. TiO2 powders not merely improved the temperature stability, but promoted the grain growth. However, the εr value increased from 10.5 to 12.6 and Q×f value decreased slightly as TiO2 content increased from 0.30 to 0.45. The 0.65 (0.95BaWO4-0.05CuO) −0.35TiO2 composite ceramic displayed an optimum performance (Q × f ~ 22,012 GHz, εr ~ 11.21, τf ~ −2.9 ppm/°C). Such a sample was chemically compatible with silver, implying that it can be implemented on LTCC applications.

Universal Control on Pyroresistive Behavior of Flexible Self‐Regulating Heating Devices

AbstractSmart heating devices with reliable self‐regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self‐regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self‐regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 106. Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained.

The factors affecting MTC of thorium–plutonium-fuelled PWRs

Plutonium loading in a plutonium–thorium (Pu–Th) mixed oxide (MOX) fuelled pressurized water reactor (PWR) core is typically constrained by large maximum radial form factors (RFF) and positive moderator temperature coefficient (MTC). The large form factors in higher Pu content fuels stem from the large differences in burnup, and thus reactivity, between fresh and burnt fuel, while positive MTC can potentially be the result of the high soluble boron concentrations needed to maintain criticality for such reactive fuel. The conventional solution to these problems is the use of burnable poisons (BPs). While BPs are able to reduce RFF, the positive MTC is not entirely due to a large critical boron concentration (CBC) requirement. In fact, analysis shows a positive MTC in Th–Pu fuel is mainly caused by fissioning in the epithermal–fast energy range. A reduction in epithermal–fast fissioning through the use of certain BPs and the strategic employment of loading patterns that encourage leakage are more effective in attaining negative MTC, as a reduction in CBC has a negligible effect on MTC. This paper examines the contributions to positive MTC by isotope and energy and identifies characteristics of BPs that are able to mitigate positive MTC in a Pu–Th MOX PWR core.

The investigation of the zero temperature coefficient point of power MOSFET

The paper investigates the zero temperature coefficient (ZTC) point of power MOSFET, based on the output characteristic of power MOSFET, the temperature coefficient of threshold voltage and the carrier mobility. It is found that the gate voltage has a big effect on the ZTC point. The result indicates that there are three types of temperature coefficient under different gate voltage. When the gate voltage is near the threshold voltage, both the linear region and saturation region shows a large positive temperature coefficient. With the increase of gate voltage, the temperature coefficient of the linear region changes from positive to negative, when the saturation region still remains positive, giving rise to the ZTC point. When the gate voltage is high enough, the negative temperature coefficient is present on both the linear and saturation region, resulting in no ZTC point. According to the experimental result, the change of ZTC point as a function of temperature is larger when the gate voltage is higher. The carrier mobility is also discussed, displaying a positive temperature coefficient at low gate voltage due to the free charge screen effect.

Effect of ball milling and heat treatment process on MnBi powders magnetic properties

The metallic compound MnBi has high intrinsic coercivity with large positive temperature coefficient. The coercivity of MnBi exceeds 12 kOe and 26 kOe at 300 K and 523 K, respectively. Hence MnBi is a good candidate for the hard phase in exchange coupled nanocomposite magnets. In order to maximize the loading of the soft phase, the size of the MnBi particle has to be close to 500 nm, the size of single magnetic domain. Low energy milling is the common method to reduce MnBi particle size. However, only 3–7 μm size particle can be achieved without significant decomposition. Here, we report our effort on preparing submicron MnBi powders using traditional powder metallurgy methods. Mn55Bi45 magnetic powders were prepared using arc melting method, followed by a series of thermal-mechanical treatment to improve purity, and finished with low energy ball milling at cryogenic temperature to achieve submicron particle size. The Mn55Bi45 powders were decomposed during ball milling process and recovered during 24 h 290 °C annealing process. With increasing ball-milling time, the saturation magnetization of MnBi decreases, while the coercivity increases. Annealing after ball milling recovers some of the magnetization, indicating the decomposition occurred during the ball-milling process can be reversed. The coercivity of Mn55Bi45 powders are also improved as a result of the heat treatment at 290 °C for 24 h. The world record magnetization 71.2 emu/g measured applying a field of 23 kOe has been achieved via low energy ball mill at room temperature.

Effect of mixed fillers on positive temperature coefficient of conductive polymer composites

This paper investigates the trade-off between low percolation threshold and large positive temperature coefficient (PTC) intensity in conductive polymer composites (CPCs). Conductive particles with low aspect ratios and large dimensions have been demonstrated to induce large PTC intensity in CPCs. Conversely high aspect ratio conductive (nano)particles like carbon nanotubes (CNTs) are desirable because of their extremely low percolation threshold (typically well below 1 wt.%), providing benefits in terms of reduced density, brittleness, costs and improved processability. Herein we report on combinations of different conductive fillers to explore the possibility to obtain both low percolation threshold and high PTC intensity. For the first time we use model systems in which at least one of the two conductive fillers is of relatively homogenous size and shape to facilitate unraveling some of the complicated inter-relationships between (mixed) conductive fillers and the PTC effect.

Microwave dielectric properties of novel glass-free low temperature firing ACa2Mg2V3O12 (A=Li, K) ceramics

Two novel low-firing microwave dielectric ceramics ACa2Mg2V3O12 (A=Li, K) were prepared using the solid state reaction method. The phase composition, sintering behavior, and microwave dielectric properties were investigated. X-ray diffraction (XRD) analysis showed that both ceramics crystallized into a cubic garnet structure. Both ceramics were well densified at temperatures lower than 960°C. LiCa2Mg2V3O12 ceramic sintered at 940°C with relative density of ~96.3% obtained the optimum microwave dielectric properties with εr ~9.8, Q×f ~24,900GHz (at 11.0GHz), τf ~+259.2ppm/°C. For KCa2Mg2V3O12, the ceramic sintered at 900°C had a relative density of ~96.1%, a relative permittivity (εr) ~10, a quality factor (Q×f) ~30,330GHz and a large positive temperature coefficient of resonance frequency τf ~+190.9ppm/°C. Both ACa2Mg2V3O12 (A=Li, K) ceramics were chemically compatible with Ag electrodes. The large positive τf of LiCa2Mg2V3O12 ceramic could be compensated by forming sold solution with NaCa2Mg2V3O12, and improved properties with a near-zero τf=+2ppm/°C, εr=9.9, Q×f=45,500GHz was obtained for 0.16LiCa2Mg2V3O12–0.84NaCa2Mg2V3O12 ceramic sintered at 920°C for 4h.

Mechanical and electrical properties of low temperature phase MnBi

Low temperature phase (LTP) manganese bismuth (MnBi) is a promising rare-earth-free permanent magnet material due to its high intrinsic coercivity and large positive temperature coefficient. While scientists are making progress on fabricating bulk MnBi magnets, engineers have begun considering MnBi magnets for motor applications. Physical properties other than magnetic ones could significantly affect motor design. Here, we report results of our investigation on the mechanical and electrical properties of bulk LTP MnBi and their temperature dependence. A MnBi ingot was prepared using an arc melting technique and subsequently underwent grinding, sieving, heat treatment, and cryomilling. The resultant powders with a particle size of ∼5 μm were magnetically aligned, cold pressed, and sintered at a predefined temperature. Micro-hardness testing was performed on a part of original ingot and we found that the hardness of MnBi was 109 ± 15 HV. The sintered magnets were subjected to compressive testing at different temperatures and it was observed that a sintered MnBi magnet fractured when the compressive stress exceeded 193 MPa at room temperature. Impedance spectra were obtained using electrochemical impedance spectroscopy at various temperatures and we found that the electrical resistance of MnBi at room temperature was about 6.85 μΩ m.

Effect of composition and heat treatment on MnBi magnetic materials

The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26kOe at 300 and 523K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn–Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55at.% Mn exhibits both the highest LTP content (93wt.%) and magnetization (74emug−1 with 9T applied field at 300K).

Improvement on Heating Head Performance

We have been improving the heating head structure for thermal printing processes. The major structural change is that the heating element is on the back side of ceramic substrate in a conventional sense. There are several advantages in this configuration such as the increased thermal capacity of the heating head, much higher mechanical anti-abrasion and scratch resistance ability, simplified substrate manufacturing process.We will discuss how the new heating head functionality can be improved from an application point of view. There are two areas we are addressing in this report: Heating head driving method and heating surface coating.The first area is maintaining the heating temperature constant. The heating head has the temperature sensing element alongside of the heating element which is made of resistive material having a large positive temperature coefficient of resistance. The temperature change is reflected in the resistance change of sensing element which is fed into the heating element control circuit. The heating element energy can be controlled by varying the driving voltage with continuous on-time or varying the on time with constant supply voltage. We employed the later control method for this paper with the pulse width modulation circuit for a simplified way of maintaining the desired process heating head temperature.The second area is the ability to overcoat the heating surface with a material suitable for the heating process. Unlike the existing heating heads where the heating surfaces are overcoated by an insulative material such as glass which gives a limited freedom for the coating process on top, the new heating surface is ceramic and various coating materials which can be conductive and/or lower coefficient of friction can be used for overcoating. This capability enhances the width of heating process application range as the wider variety of media can be heated as well as the applications beyond printing field.

Nanostructured bulk MnBi magnets fabricated by hot compaction of cryomilled powders

Cryomilling was employed for Mn50+xBi50−x (x = 0,5) alloys to synthesize nanocrystalline powders with particle size of 400–500 nm without much increase of the unwanted secondary Bi phase. At room temperature, the cryomilled powders showed large coercivity (Hc) values of 18.5 kOe and 20.7 kOe for Mn50Bi50 and Mn55Bi45, respectively. Cryomilled powders were hot compacted to form a bulk nanostructured MnBi magnet. The Mn50Bi50 hot compacted magnet exhibits Hc of 12.9 kOe with remanence value of 26 emu g−1 at room temperature. The microstructure of hot compacted magnet illustrates uniform grains with an average size of 40 nm. Furthermore, the magnet shows a large positive temperature coefficient of Hc which reaches a value of more than 30 kOe above 450 K.

Back-illuminated separate absorption and multiplication AlGaN solar-blind avalanche photodiodes

This letter reports the fabrication and performance of back-illuminated separate absorption and multiplication AlGaN solar-blind avalanche photodiodes. Devices with a 60-μm-diameter active area and a double-mesa structure exhibit a low dark current density of 1.06 × 10−8 A/cm2 at the reverse bias of 20 V and a maximum multiplication gain up to 3000 at the reverse bias of 91 V. The temperature dependence of avalanche voltage shows a large positive temperature coefficient of 0.05 V/K, confirming that avalanche multiplication is the dominant gain mechanism in the photodiodes.

Structure and dielectric properties of A-site-deficient perovskite Nd(1 − x)/3M x NbO3 (M = Li, Ag; 0 ≤ x ≤ 0.20) ceramics

Crystal structure and dielectric properties of the A-site-deficient perovskites Nd(1 − x)/3MxNbO3 (M = Li, Ag; 0 ≤ x ≤ 0.20) ceramics were investigated with X-ray diffractometer (XRD), network analyzer, and impedance analyzer. XRD results showed that no secondary phase was observed for Ag-doped specimens. However, a second phase LiNdNb2O7 appeared for Li-doped ones when x ≥ 0.16. The crystal structure at room temperature transformed from orthorhombic (0 ≤ x < 0.16) into tetragonal symmetry (0.16 ≤ x ≤ 0.20) with the increase of x. Complex impedance analysis and the dielectric properties measured at low frequency showed that the dielectric loss in Nd(1 − x)/3MxNbO3 (M = Li, Ag; 0 ≤ x ≤ 0.20) at low frequency was mainly caused by the lithium or silver ionic conduction, which increased with the increase of temperature. However, the dielectric loss originated from the ionic conductivity was frozen at microwave frequency. The dielectric constant (er) increased with the addition of Ag, whereas it decreased with the increase of Li addition. The pure and Ag-doped specimens exhibited large positive temperature coefficient of resonant frequency (τf), while the τf value changed from positive into negative with the increase of Li addition. The Q × f value increased from 437 to 1,110 GHz in the case of Li doping and 765 GHz in the case of Ag doping, respectively, when x increased from 0 to 0.12. Further increase in x decreased the Q × f values both for the Li- and Ag-doped specimens. Good combined microwave dielectric properties with er = 153, Q × f = 1,110 GHz and τf = −45 ppm/°C could be obtained for the Li-doped specimen when x = 0.12.

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.

Cross-comparison of fast reactor concepts with various coolants

Four fast reactor concepts using lead (LFR), liquid salt, NaCl–KCl–MgCl 2 (LSFR), sodium (SFR), and supercritical CO 2 (GFR) coolants are compared. Since economy of scale and power conversion system compactness are the same by virtue of the consistent 2400 MWt rating and use of the S-CO 2 power conversion system, the achievable plant thermal efficiency, core power density and core specific powers become the dominant factors. The potential to achieve the highest efficiency among the four reactor concepts can be ranked from highest to lowest as follows: (1) GFR, (2) LFR and LSFR, and (3) SFR. Both the lead- and salt-cooled designs achieve about 30% higher power density than the gas-cooled reactor, but attain power density 3 times smaller than that of the sodium-cooled reactor. Fuel cycle costs are favored for the sodium reactor by virtue of its high specific power of 65 kW/kgHM compared to the lead, salt and gas reactor values of 45, 35, and 21 kW/kgHM, respectively. In terms of safety, all concepts can be designed to accommodate the unprotected limiting accidents through passive means in a self-controllable manner. However, it does not seem to be a preferable option for the GFR where the active or semi-passive approach will likely result in a more economic and reliable plant. Lead coolant with its superior neutronic characteristics and the smallest coolant temperature reactivity coefficient is easiest to design for self-controllability, while the LSFR requires special reactivity devices to overcome its large positive coolant temperature coefficient. The GFR required a special core design using BeO diluent and a supercritical CO 2 reflector to achieve negative coolant void worth—one of the conditions necessary for inherent shutdown following large LOCA. Protected accidents need to be given special attention in the LSFR and LFR due to the small margin to freezing of their coolants, and to a lesser extent in the SFR.

Investigation of (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 Microwave Dielectrics

The microwave dielectric properties and microstructures of (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 ceramics were investigated using a network analyzer and scanning electron microscope (SEM). In order to achieve a temperature-stable material, we used a method of combining a positive-temperature-coefficient material with a negative one. SrTiO3 has a large positive temperature coefficient of resonant frequency (τf) of 1,700 ppm/°C. (Mg0.6Zn0.4)0.95Co0.05TiO3 possesses a negative temperature coefficient of resonant frequency of -65 ppm/°C. The dielectric constant and density increase with increasing SrTiO3 content. By appropriately adjusting x in the (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 ceramic system, a near-zero τf value can be obtained. A new microwave material of 0.96(Mg0.6Zn0.4)0.95Co0.05TiO3–0.04SrTiO3 exhibits excellent dielectric properties including of a dielectric constant of 23.5, a Q ×f value of 92,000 GHz, and a temperature coefficient of resonant frequency of -2.5 ppm/°C, and can be utilized in the fabrication of microwave devices.

Study of (1 − x)(Mg 0.6Zn 0.4) 0.95Co 0.05TiO 3– xCa 0.61Nd 0.26TiO 3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg 0.6Zn 0.4) 0.95Co 0.05TiO 3– xCa 0.61Nd 0.26TiO 3 ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca 0.61Nd 0.26TiO 3 has a large positive temperature coefficient of resonant frequency. (Mg 0.6Zn 0.4) 0.95Co 0.05TiO 3 possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg 0.6Zn 0.4) 0.95Co 0.05TiO 3– xCa 0.61Nd 0.26TiO 3 ceramic system, a near-zero τ f value can be obtained. A new microwave dielectric material of 0.8(Mg 0.6Zn 0.4) 0.95Co 0.05TiO 3–0.2Ca 0.61Nd 0.26TiO 3 possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °C.

Effect of temperature on the electrical resistivity of YBa2Cu3O7−x/polyethylene composite materials

AbstractHeterogeneous materials composed of conducting YBa2Cu3O7−x particles randomly dispersed inside an insulating low density polyethylene (LDPE) matrix were prepared. The resistivity of the composites was measured as a function of temperature between 293 and 400 K. A large positive temperature coefficient (PTC) of resistance was observed near 395 K. The mechanism of the PTC effect in the materials is related to the percolation behavior of the YBa2Cu3O7−x/LDPE composites and the thermal expansion of the LDPE matrix. The intensity of the PTC effect for these composites was found to be as large as nine orders of magnitude. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers.