Dielectric and electromechanical properties of rare earth calcium oxyborate piezoelectric crystals at high temperatures

The electrical resistivity, dielectric, and electromechanical properties of ReCa(4)O(BO(3))(3) (ReCOB; Re = Er, Y, Gd, Sm, Nd, Pr, and La) piezoelectric crystals were investigated as a function of temperature up to 1000 °C. Of the studied crystals, ErCOB and YCOB were found to possess extremely high resistivity (p): p > 3 × 10(7) ω.cm at 1000 °C. The property variation in ReCOB crystals is discussed with respect to their disordered structure. The highest electromechanical coupling factor κ(26) and piezoelectric coefficient d(26) at 1000°C, were achieved in PrCOB crystals, with values being on the order of 24.7% and 13.1 pC/N, respectively. The high thermal stability of the electromechanical properties, with variation less than 25%, together with the low dielectric loss (<46%) and high mechanical quality factor (>1500) at elevated temperatures of 1000 °C, make ErCOB, YCOB, and GdCOB crystals promising for ultrahigh temperature electromechanical applications.     

X-ray topography of piezoelectric La(3)Ta(14)Ga(5.5)O(14) crystal grown by Czochralski Method.

We performed synchrotron X-ray topography on a La(3)Ta0(0.5)Ga (5.5)O(14) (LTG) crystal grown by the Czochralski method. Since a synchrotron X-ray source can provide high-energy X-rays, one can detect bulk structures by X-ray topography. LTG is one of the most attractive piezoelectric crystals along with La(3)Ga(5)SiO(14) (LGS) because of its excellent acoustic properties (temperature compensation of acoustic losses). Since LTG single crystals can be grown from a stoichiometric melt, it was expected that single crystals with better quality than the LGS crystal, which cannot be grown from a stoichiometric system but only from a congruent melt, can be obtained. However, 60 keV X-ray topography revealed that the LTG crystal quality was not as high as the LGS crystal quality. The crystal quality of the central region was lower than that of the surrounding region.     

The stability of zinc oxide transparent electrodes fabricated by R.F. magnetron sputtering

The stability of the electrical, optical and mechanical properties of sputtered zinc oxide (ZnO) thin films with resistivities from 10^?2 to 10^?4 ωcm were investigated. No significant changes in these properties are observed for ZnO films exposed to air at room temperature for 10 months. A change in the electrical resistance of the ZnO films with temperature up to 400 °C is observed in various ambients such as vacuum, inert gases and air. After heat treatment in these ambients at 400 °C, the resistivity of the films increased by one to ten orders of magnitude. The increased resistivity can be returned to the resistivity of the virgin state, within one order of magnitude, by heat treatment in a hydrogen ambient at temperatures near 400 °C. For practical use of ZnO films at high temperatures, the increase in the resistivity might become a disadvantage for ZnO transparent electrodes fabricated by r.f. magnetron sputtering.     

炭基体的结构对C/C复合材料导电性能的影响

采用CVI+PIC工艺制备以2D碳纤维预制体为增强体、由不同炭基体结构组成的C/C复合材料,随后在不同温度对其进行热处理得到不同石墨化度的炭基体结构,研究了PyC/ReC比值和石墨化度对材料电阻率的影响。结果表明,随着PyC/ReC比的提高低密度C/C复合材料的电阻率在27.3×10^-6~28.0×10^-6 Ω·m间基本不变,因为石墨微晶的尺寸和结构完整性的增大与材料孔隙率的提高对电阻的影响相反。随着PyC/ReC比的提高,高密度C/C复合材料的电阻率从24.9×10^-6 Ω·m降低到20.5 ×10^-6 Ω·m。其可能的原因是,材料内部的孔隙较少,孔隙率的轻微提高使阻碍载流子在导电网络中的有效传递的作用显著下降。随着热处理温度从1800℃提高到2500℃,C/C复合材料的石墨化度明显提高,电阻率明显降低,其主要原因是载流子浓度的提高和晶界散射的减弱。     

Structural evolution in polyolysed hybrid organic-inorganic alumina gels

New material gels obtained by reaction of the aluminum sec-butoxide with propan-1,2-diol at room temperature without using a catalyst or adding water, have been pyrolyzed at different temperatures under argon and in air. The obtained products have been characterised by various methods (X-ray, IR and Raman spectroscopies, SEM and density measurements) to determine their structures. Corundum is formed at 1450°C when the material is heat-treated under argon, but this phase is obtained rapidly at temperatures above 1000°C for the xerogel pyrolysed in the air.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

The Influence of Heat Treatment on the Electrical Characteristics of Semi-Insulating SiC Layers Obtained by Irradiating <Emphasis Type="Italic">n</Emphasis>-SiC with High-Energy Argon Ions

Irradiation of crystalline n-type silicon carbide (n-SiC) with high-energy (53-MeV) argon ions was used to create near-surface semi-insulating (i-SiC) layers. The influence of subsequent heat treatment on the electrical characteristics of i-SiC layers has been studied. The most high-ohmic ion-irradiated i-SiC layers with room-temperature resistivity of no less than 1.6 × 10¹³ Ω cm were obtained upon the heat treatment at 600°C, whereas the resistivity of such layers heat-treated at 230°C was about 5 × 10⁷ Ω cm.     

Electrical resistivity of Si-Ti-C-O fibres after rapid heat treatment

Two types of Si-Ti-C-O fibres were heat treated in a preheated graphite furnace at temperatures between 1273 and 1973 K, and the change in the electrical resistivity was measured after removing the fibres from the furnace. The resistivity of the fibres decreased monotonically with increasing heat-treatment temperature, but showed a significant increase of the order of 10¹–10² in the temperature range of gas evolution from the fibres. The resistivity of the fibre which has an amorphous character began to increase at a lower temperature than that of the fibre with a crystalline character. This increase in resistivity did not occur during heat treatment in a pure oxygen atmosphere, because the oxide layer formed on the fibre surface suppressed gas evolution from the fibres. The X-ray diffraction patterns of heat-treated fibres in nitrogen or oxygen atmospheres revealed that -SiC crystals began to precipitate from the amorphous state as the heat-treatment temperature increased. The -SiC crystal growth, however, did not always correspond with the decrease in the fibre resistivity.     

The stability of aluminium-doped ZnO transparent electrodes fabricated by sputtering

The stability of the electrical, optical and mechanical properties of sputtered aluminium-doped ZnO (AZO) films with a resistivity from 10^-3 to 10^-4 Ω cm was investigated. No significant change in these properties of AZO films was observed for use in air at room temperature for 1 year. It is shown that stable properties for use in vacuum and inert and nitrogen gases at temperatures as high as 400°C can be attained for AZO films sputtered with an Al₂O₃ content above 1.0 wt.%. After heat treatment in air at 400°C, the resistivity of AZO films increases by about five orders of magnitude. The resistivity can be returned to that of the as-deposited state by heat treatment in hydrogen gas at a temperature near 400°C.     

Microwave hydrothermal synthesis of nanosize PbO added Mg-Cu-Zn ferrites

Lead oxide added Mg-Cu-Zn ferrite powders were prepared by a co-precipitation method in a microwave-hydrothermal (M-H) system. The synthesized ferrite samples were characterized by powder X-ray diffraction (XRD), Infrared spectroscopy (IR), transmission electron microscopy (TEM) and Differential scanning calorimeter (DSC). Nanophase ferrites (~10–20 nm) with high surface area were synthesized at a low temperature of 160°C after a treatment time of 1 hour. The nano-powder was sintered at 900°C/4h in air atmosphere. The variations of the sintered density, electrical resistivity, initial permeability, and saturation magnetization with additive concentration have been investigated and the obtained results were compared with one prepared by the conventional ceramic method. It is found that the addition of PbO improves sintered density, electrical resistivity and permeability.     

Biomimetic calcium phosphate coating on Ti-7.5Mo alloy for dental application

Titanium and its alloys have been used as bone-replacement implants due to their excellent corrosion resistance and biocompatibility. However, a titanium coating is a bioinert material and cannot bond chemically to bone tissue. The objective of this work was to evaluate the influence of alkaline treatment and heat treatment on the formation of calcium phosphate layer on the surface of a Ti-7.5Mo alloy after soaking in simulated body fluid (SBF). Thirty six titanium alloy plates were assigned into two groups. For group I, samples were immersed in a 5.0-M NaOH aqueous solution at 80°C for 72 h, washed with distilled water and dried at 40°C for 24 h. For group II, after the alkaline treatment, samples were heat-treated at 600°C for 1 h in an electrical furnace in air. Then, all samples were immersed in SBF for 7 or 14 days to allow the formation of a calcium phosphate coating on the surface. The surfaces were characterized using SEM, EDS, AFM and contact angle measurements.     

X-ray Topography of Piezoelectric La/sub 3/Ta/sub 0.5/Ga/sub 5.5/O/sub 14/ Crystal Grown by Czochralski Method

We performed synchrotron X-ray topography on a La₃Ta_0.5Ga_5.5O₁₄ (LTG) crystal grown by the Czochralski method. Since a synchrotron X-ray source can provide high-energy X-rays, one can detect bulk structures by X-ray topography. LTG is one of the most attractive piezoelectric crystals along with La₃Ga₅SiO₁₄ (LGS) because of its excellent acoustic properties (temperature compensation of acoustic losses). Since LTG single crystals can be grown from a stoichiometric melt, it was expected that single crystals with better quality than the LGS crystal, which cannot be grown from a stoichiometric system but only from a congruent melt, can be obtained. However, 60 keV X-ray topography revealed that the LTG crystal quality was not as high as the LGS crystal quality. The crystal quality of the central region was lower than that of the surrounding region.     

CuInGaSe2 crystals synthesis by selenization of Cu–In–Ga alloy in H2Se atmosphere for solar cell applications

Copper indium gallium diselenide (CuInGaSe₂) crystals were synthesized using two step growth strategy. A facile solution route was employed as a primary step to synthesize Cu–In–Ga (CIG) metallic precursor using ethylenediamine as a solvent. Thin films of CIG metallic precursor have been deposited using spray deposition technique on to molybdenum coated soda lime glass substrate under inert atmosphere. The subsequent step involved the selenization of metallic precursor thin films in H₂Se atmosphere at 450 °C for 90 min followed by annealing in Ar thus yielding solar cell applicable dense CuInGaSe₂ crystals. The surface morphology, phase structure and composition of the deposited films were analyzed by field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and electrical resistivity measurement respectively. The results revealed that annealed films were crystalline in nature exhibiting homogeneous single chalcopyrite phase.     

表面处理对第二代单晶高温合金胞状再结晶的影响

对第二代镍基单晶高温合金进行表面处理并在1000～1200℃保温,研究表面加工工艺和热处理条件对胞状再结晶行为的影响.结果表明:磨削、吹砂、喷丸试样热处理后表面均发生了胞状再结晶,其生成厚度依次增加.喷丸试样经1200℃/50h后胞状再结晶区厚度可达0.1mm,晶团之间及与基体之间存在取向差小于5°的小角晶界和35～4...     

Heat-treatment effect of dispersed particles on the preparation of thick silica films by using electrophoretic sol-gel deposition

Thick silica films with no cracks were prepared on a stainless steel sheet by electrophoretic sol-gel deposition. The sols for electrophoresis were prepared by the re-dispersion of silica particles which were separated from the original sol made from a silicon alkoxide. The sols were then heat-treated at various temperatures in air. The heat treatment of the particles, separated by centrifugation from the original sol before the electrophoresis, prevented the generation of microcracks on the surface of the films during the drying process. The maximum thickness of the as-deposited films containing no cracks was about 10 m when the particles were heat-treated at 600 °C for longer than 12 h.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.     

Dependence of the electrical and optical properties of sputter-deposited ZnO:Ga films on the annealing temperature,time, and atmosphere

Effects of annealing process parameters such as annealing temperature, time, and atmosphere on the electrical resistivity and transmittance properties of Ga-doped ZnO (ZnO:Ga) thin films deposited on glass by rf magnetron sputtering were investigated. The electrical resistivity of a ZnO:Ga thin film is effectively decreased with increasing annealing temperature and time in a reducing atmosphere such as N₂ + 5%H₂. This is attributed to passivation of grain boundaries and zinc ions by hydrogen atoms resulting in increases in carrier concentration and mobility. Also the resistivity of 4.9 × 10⁻⁴Ω cm was obtained by annealing at 200°C for 15 h in the same atmosphere, which is not bad for a transparent conductor for solar cell applications. However, annealing at a temperature higher than 400°C is less effective. The lowest resistivity of 2.3 × 10⁻⁴Ω cm was obtained by annealing at 400°C for 1 h in an N₂ + 5%H₂ atmosphere. The optical transmittance of the ZnO:Ga film is improved by annealing regardless of the annealing atmosphere. Annealing in N₂ + 5%H₂ atmosphere widens the optical band gap, while annealing in an O₂ atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.     

Characterization of ITO/CdO/glass thin films evaporated by electron beam technique

Abstract

A thin buffer layer of cadmium oxide (CdO) was used to enhance the optical and electrical properties of indium tin oxide (ITO) films prepared by an electron-beam evaporation technique. The effects of the thickness and heat treatment of the CdO layer on the structural, optical and electrical properties of ITO films were carried out. It was found that the CdO layer with a thickness of 25 nm results in an optimum transmittance of 70% in the visible region and an optimum resistivity of 5.1×10^?3 Ω cm at room temperature. The effect of heat treatment on the CdO buffer layer with a thickness of 25 nm was considered to improve the optoelectronic properties of the formed ITO films. With increasing annealing temperature, the crystallinity of ITO films seemed to improve, enhancing some physical properties, such as film transmittance and conductivity. ITO films deposited onto a CdO buffer layer heated at 450 °C showed a maximum transmittance of 91% in the visible and near-infrared regions of the spectrum associated with the highest optical energy gap of 3.61 eV and electrical resistivity of 4.45×10^?4 Ω cm at room temperature. Other optical parameters, such as refractive index, extinction coefficient, dielectric constant, dispersion energy, single effective oscillator energy, packing density and free carrier concentration, were also studied.     

Effect of heat pretreatment on electrical conductance changes by NO2 adsorption of lead phthalocyanine thin film

The sensing characteristics of an NO₂ gas sensor using lead phthalocyanine thin film are influenced by heat-pretreatment time and ambient gas. The response behaviour of conductance for the change in ambient gas was characterized using Elovich's equation. The first stage in the adsorption and desorption kinetics reflects surface phenomena and the second stage the film diffusion. Both components were improved by heat treatment in air. The rising time in the NO₂ adsorption process was within 2 min, and the recovery time within 5 min at 130 °C for the film annealed in air for 1 h. The heat pretreatment induced the formation of a continuous layer of fine particles, ∼ 0.1 μm in size, and larger single crystals isolated from each other, which formed on the continuous layer. The formation of the larger single crystals is not preferable in fabricating a sensor with fast rising and recovery times.