Absolute Value Of Temperature Coefficient Research Articles

Magnetic properties and microstructure evolution of in-situ Tb-Cu diffusion treated hot-deformed Nd-Fe-B magnets

In this work, Tb70Cu30 alloy is employed as the addition for enhancing the hard magnetic properties of hot-deformed Nd-Fe-B magnets through in-situ diffusion. The magnets with and without Tb-Cu addition were prepared by hot pressing at 700 °C followed by hot deformation at different temperatures using melt spun Nd-Fe-B based powder as the raw materials. Tb and Cu elements were found to diffuse into the magnet after preparation. By adding 2 wt% Tb70Cu30, the coercivity and thermal stability of hot-deformed magnet were significantly improved due to the formation of local high-anisotropy (Nd,Tb)2Fe14B phase and the reduction in grain size. The intrinsic coercivity Hcj increased from 1281 kA/m to 1681 kA/m and the absolute value of temperature coefficient of coercivity |β| decreased from 0.479%/°C to 0.431%/°C for the magnets hot-deformed at 750 °C. It was confirmed that the diffused Tb atom prefers to substituting Nd in Nd2Fe14B type main phase rather than entering into RE-rich phase, and Cu distributes mainly at the grain boundaries. The reduction in crystal orientation was observed for diffused magnets, which is related to the increased rare earth-rich phase, resulting in the decreased remanence. The evolutions of the grain orientation, microstructure and magnetic properties of the magnets prepared at different hot-deformation temperatures were discussed as well.

Magnetic Properties, Phase Transition Temperatures, Intergranular Exchange Interactions and Microstructure of Ta-Doped Nd-Ce-Fe-B Nano ribbons

The effects of Ta doping on the magnetic properties, temperature stabilities, phase constituents, intergranular exchange interactions, and microstructures of (Nd0.8Ce0.2)13.5Fe80.5−xTaxB6 (x = 0–1.5) nanoribbons are investigated in this paper. It is found that Ta doping increased the coercivity of the alloys, while the remanence remained nearly unchanged. The temperature stability of the alloys improved significantly as the absolute value of temperature coefficient of coercivity β enhanced from 0.48%/K for Ta-free alloy to 0.38%/K for the alloys with 0.3–1.5 at.% Ta addition. The lattice constants (a, c), unit cell volume, and phase transition temperatures (Tsr, TC) of the alloys changed by adding Ta into the alloys. Comprehensive best magnetic properties of Hcj = 16.0 kOe, Br = 7.65 kG, and (BH)max = 12.60 MGOe are obtained in (Nd0.8Ce0.2)13.5Fe79.6Ta0.9B6 ribbon. The improvement in magnetic properties is mainly attributed to refined and uniform microstructure of the Ta-doped alloys.

High temperature characteristics of AlN film SAW sensor integrated with TC4 alloy substrate

This paper reports the high temperature characteristics of AlN film SAW sensor integrated with TC4 alloy substrate. The AlN films were directly sputtered on the TC4 metal substrate. SAW resonators were fabricated on the AlN films/TC4 structure by lift-off photolithography techniques. X-ray diffraction measurement shows that the AlN film is highly c-axis oriented. To explore the temperature behavior of the layered AlN/TC4 SAW devices, the samples with different thicknesses of AlN film were prepared and measured at different temperature. The results show that the resonance frequency of the AlN film SAW device decreases with increasing temperature. The dependence of temperature behaviors of the devices on the thickness of AlN films are also presented and discussed. The absolute value of temperature coefficient of frequency decreases from 89.4ppm/°C to 66.2ppm/°C, when the thickness of AlN film increases from 1.5μm to 3.0μm. From room temperature to 350°C, the electromechanical coupling coefficient (K2) of the devices increases by about 30%. The thermal hysteresis errors of the SAW devices are less than 3.87%. The prepared AlN film SAW sensor integrated with TC4 alloy has great potential applications in high temperature harsh environment.

Improving the structure, magnetic properties and thermal stability of rapidly quenched TbCu7-type SmCo6.4Si0.3Zr0.3 alloy by carbon addition

The effects of carbon addition on the structure and magnetic properties of rapidly quenched TbCu7-type SmCo6.4Si0.3Zr0.3 alloy have been investigated. The alloys with a small amount of C addition (x≤0.2) showed single Sm(Co,M)7 phase, while ZrC phase appeared in the alloys with x=0.3 and 0.4. With the increase of C content, the grain size decreased from approximately 850nm for x=0.1 to approximately 300nm for x=0.4. The coercivity Hc and maximum magnetic energy product (BH)max increased with the C content from x=0 to 0.2 and then decreased with excessive C addition. The optimal magnetic properties of Hc=1577kA/m, Jr=0.53T and (BH)max=52.1kJ/m3 were achieved for SmCo6.4Si0.3Zr0.3C0.2 alloy with a grain size of 600–700nm, which is close to the single domain size of TbCu7-type Sm(Co,Zr)7 alloy. Furthermore, C addition also improved the thermal stability by reducing the absolute values of temperature coefficients of remanence and coercivity.

Metal silicide/poly-Si Schottky diodes for uncooled microbolometers

Nickel silicide Schottky diodes formed on polycrystalline Si 〈P〉 films are proposed as temperature sensors of monolithic uncooled microbolometer infrared focal plane arrays. The structure and composition of nickel silicide/polycrystalline silicon films synthesized in a low-temperature process are examined by means of transmission electron microscopy. The Ni silicide is identified as a multi-phase compound composed of 20% to 40% of Ni3Si, 30% to 60% of Ni2Si, and 10% to 30% of NiSi with probable minor content of NiSi2 at the silicide/poly-Si interface. Rectification ratios of the Schottky diodes vary from about 100 to about 20 for the temperature increasing from 22℃ to 70℃; they exceed 1,000 at 80 K. A barrier of around 0.95 eV is found to control the photovoltage spectra at room temperature. A set of barriers is observed in photo-electromotive force spectra at 80 K and attributed to the Ni silicide/poly-Si interface. Absolute values of temperature coefficients of voltage and current are found to vary from 0.3%℃ to 0.6%/℃ for forward bias and around 2.5%/℃ for reverse bias of the diodes.

Effect of Hydrogen Absorption on Electrical Transport Properties for Ni<sub>36</sub>Nb<sub>24</sub>Zr<sub>40</sub> Amorphous Alloy Ribbons

Electrical resistivity measurements in the temperature range from 6 to 300K were carried out in order to investigate the effects of the hydrogen absorption on the transport properties of Ni36Nb24Zr40 amorphous alloy ribbon. The electrical resistivity basically behaved negative temperature dependence in all prepared specimens of (Ni0.36Nb0.24Zr0.40)100¹xHx with 0 ¯ x < 15. The absolute value of temperature coefficient of the resistivity, TCR, increased with increasing the amount of the absorbed hydrogen. In addition, it was also clear that the electrical resistivity gradually increased with the time just after electrochemically charging, and tended to saturate after about 800ks at 300K. The behavior of the gradual increase in the electrical resistivity with the time was explained by the model including two kinds of the relaxation times, being associated with the migration of the hydrogen from the sample surface. [doi:10.2320/matertrans.MF201305]

Preparation and characterization of isotactic polypropylene/MgO composites as dielectric materials with low dielectric loss

The dielectric and thermal properties of isotactic polypropylene/magnesium oxide (0–30 vol %) composites were investigated. The dielectric constant of the composite material increased from 2.3 to 3.2 on increasing the MgO content and the observed variation of the dielectric constant with the MgO fraction was in good agreement with the Bruggeman mixing model. Low dielectric losses at microwave frequencies, on the order of 10−4, were obtained using highly crystalline MgO as the filler, and it was also found that the use of larger MgO particle sizes led to further reductions in the dielectric loss. The absolute value of temperature coefficient of the dielectric constant as well as the coefficient of thermal expansion decreased with increasing MgO content, while thermal conductivity increased with increasing MgO content in a manner which could be predicted using the Maxwell-Garnett mixing model.

Reducing Dy Content by Y Substitution in Nanocomposite NdFeB Alloys With Enhanced Magnetic Properties and Thermal Stability

Dy has been employed as an additive in NdFeB magnets to improve their coercivity and thermal stability, but the high cost of the Dy has become a major concern. In this work, the effects of Y substitution for Dy on the magnetic properties and thermal stability of melt-spun nanocomposite [Nd0.8(Dy1-xYx)0.2]10Fe84B6 alloys have been systematically studied. The results show that Y substituting Dy can not only improve the remanence and energy product but also improve their thermal stability. [Nd0.8(Dy0.5Y0.5)0.2]10Fe84B6 alloy with 50% Dy reduction exhibits a high maximum energy product of 139 kJ/m3 and low absolute values of temperature coefficients α and β, -0.090%/°C and -0.394%/°C, respectively. It is suggested that Y substitution be a feasible way to reduce the cost of the NdFeB magnets and maintain their high temperature performance.

Electrical and optical properties of reactive sputtered TiO x thin films for uncooled IR detector applications

Nanostructured titanium oxide (nano-TiO x ) thin films for uncooled IR detectors were fabricated by dc reactive magnetron sputtering and post-deposition annealed in oxygen atmosphere. The crystalline structure and surface morphology were characterized by glancing incidence X-ray diffraction (GIXRD) and field emission scanning microscopy. The results of GIXRD measurements indicate that TiO x thin film deposited at room temperature is amorphous. A mixture of anatase and rutile nanocrystalline structure phase were present in oxygen annealed TiO x thin film. A weak absorption peak around 438 cm−1 corresponding to Ti–O stretching vibration is observed by Fourier transform infrared spectroscopy with annealed TiO x thin film. The X-ray photoelectron spectra reveals Ti3+ and Ti4+ ions are coexisting in TiO x films. The optical spectra of the films indicate that the optical absorption edge of the nano-TiO x film exhibits a red shift compared to the as-deposited film. Furthermore, compared to bulk TiO x , a blue shift was observed in both of the deposited and annealed films due to quantum size effect. The dependence of resistivity on temperature reveals both the absolute value of temperature coefficient of resistivity (TCR) and activation energy of TiO x thin film increase significantly after annealing in oxygen.

Enhancement of magnetic properties and thermal stability of nanocrystalline (NdDyTb) 12.3(FeZrNbCu) 81.7B 6.0 alloy with Co substitution

The major drawbacks of Nd-Fe-B magnets are relatively low Curie temperature and poor thermal stability. Ribbons with the near stoichiometric 2:14:1 composition of Nd 10.8Dy 0.75Tb 0.75Fe 79.7– x Co x Zr 0.8Nb 0.8Cu 0.4B 6.0 ( x=0, 3, 6, 9, 12, 15) were prepared by rapid quenching and subsequent heat treatment. The effect of Co element on the magnetic properties, thermal stability, and microstructure of the ribbons was systematically studied by vibrating sample magnetometer (VSM), thermal magnetic analysis, atomic force microscopy (AFM), and transmission electron microscopy (TEM). It was found that Co substitution was significantly effective in improving the magnetic properties and the thermal stability of nanocrystalline ribbons. Although the intrinsic coercivity decreased from 1308.7 kA/m for x=0 to 817.4 kA/m for x=15, the remanence polarization and maximum energy product increased from 0.839 T and 116.5 kJ/m 3 for the Co-free samples to 1.041 T and 155.1 kJ/m 3 for the 12at% Co-substituted samples, respectively. About 10 K increase in Curie temperature was observed for the 2:14:1 phase with 1at% Co substitution. The absolute values of temperature coefficients of induction and coercivity were significantly decreased with Co substitution, which may be attractive for high operational temperature applications. The microstructure of nanocrystalline ribbons was slightly refined with Co substitution.

Influence of zirconium addition on microstructure, magnetic properties and thermal stability of nanocrystalline Nd 12.3Fe 81.7B 6.0 alloy

Effect of Zr addition on microstructure, magnetic properties and thermal stability of Nd 12.3Fe 81.7– x Zr x B 6.0 ( x=0–3.0) ribbons melt-spun and annealed was investigated. Magnetic measurement using vibrating sample magnetometer (VSM) revealed that Zr addition was significantly effective in improving the magnetic properties at room temperature. The intrinsic coercivity H ci of the optimally processed ribbons increased monotonically with increasing Zr content, from 751.7 kA/m for x=0 to 1005.3 kA/m for x=3.0. Unlike the coercivity, the remanence polarization J r increased first with Zr addition, from 0.898 T up to 1.041 T at x=1.5, and then decreased with further Zr addition. The maximum energy product ( BH) max behaved similarly, increasing from 103.1 kJ/m 3 to a maximum of 175.2 kJ/m 3 at x=1.5. Microstructure studies using atomic force microscopy (AFM) and transmission electron microscopy (TEM) had shown a significant microstructure refinement with Zr addition. The absolute values of temperature coefficients of induction and coercivity were significantly increased with increasing Zr content, indicating that Zr was detrimental to thermal stability of the melt-spun Nd 2Fe 14B-type material.

Improvement of Temperature Coefficient of Resistance by Co-Implantation of Argon or Xenon or Fluorine in Boron Implanted Polysilicon Resistors

This paper proposes a new method to decrease the absolute value of temperature coefficient of resistance (TCR) in P-type boron implanted polysilicon resistors, at a given intermediate sheet resistance values, by selecting an optimized combination of boron doping implant conditions with co-implantation conditions. The co-implantation ion species that were investigated are fluorine, argon and xenon. Each of the co-implantation species was studied at three different co-implantation conditions and two different boron doping implant conditions of dose and energy. The stopping and range of ions in matter (SRIM) Monte Carlo simulation and thermawave measurements were used in order to set the experimental conditions and to evaluate the crystal damage and the amorphous layer depth created by the co-implantation. Amorphous layer is defined in this work as highly disordered material with vacancies concentration above 10% of the crystal density. Electrical measurements show that co-implantation causes a remarkable change in sheet resistance and TCR. Deeper co-implanted amorphous layer increases TCR and decreases sheet resistance. Typical polysilicon TCR is negative, while the TCR of a single crystal is positive. An improvement of TCR is defined at this work as minimum TCR in absolute values, which is lower sensitivity of sheet resistance to operational temperatures. Co-implantation that created deep amorphous layer in the polysilicon, decreases the TCR absolute values by up to ~75 % at the lower boron doping implant samples. Deep co-implanted amorphous layer also decreases sheet resistance by up to ~40% in both of the boron doping implants. Co-implanted xenon and fluorine exhibit similar effect on sheet resistance and TCR, while the co-implanted argon effect is weaker in comparison to samples without co-implantation. The increase of TCR and the decrease of sheet resistance are attributed to grains regrowth during the rapid thermal anneal (RTP). The regrowth of the co-implanted amorphous layer creates larger grains with less grain boundaries and less defects in comparison to the polysilicon samples without co-implantation. Analysis by X-ray diffractometer (XRD) and atomic force microscope (AFM) supports the suggested model.

Measurement of the Density of Ni-Cr Alloy by a Modified Pycnometric Method

A modified pycnometric method has been developed to obtain the accurate density value of liquid nickel and Ni-Cr alloy in the liquid and solid-liquid coexistence states. The density of liquid nickel decreases linearly with increasing temperature in the range from the melting point to 1873 K. The density at the melting point and the volume thermal expansion coefficient of liquid nickel are 7.91 Mgm � 3 and 1:87 � 10 � 4 K � 1 , respectively. The density of Ni-Cr alloys in the liquid and solid-liquid coexistence states was found to decrease linearly with increasing temperature and chromium concentration in the alloy. The temperature coefficient of density of Ni-Cr alloys was found to change at the liquidus temperature. The absolute value of temperature coefficient of density in the solid-liquid coexistence state is larger than that of the liquid alloy.