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Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 2002"


Journal ArticleDOI
TL;DR: In this article, the growth of crack-free GaN-based light emitting diodes (LEDs) on silicon on patterned Si(111) with areas of 100 μm×100 μm is reported.
Abstract: Cracking of GaN on Si usually occurs due to the large thermal mismatch of GaN and Si when layer thicknesses exceed approximately 1 μm in metalorganic chemical vapor deposition (MOCVD) preventing the realization of device-quality material. The thermal stress can be reduced significantly by a combination of different concepts such as the insertion of low-temperature AlN interlayers, introducing multiple AlGaN/GaN interlayers, and growing on prepatterned substrates. The growth of crack-free GaN-based light emitting diodes (LEDs) on silicon on patterned Si(111) with areas of 100 μm×100 μm is reported

302 citations


Journal ArticleDOI
Xiangyong Zhao1, Bijun Fang1, Hu Cao1, Yiping Guo1, Haosu Luo1 
TL;DR: In this paper, a detailed study of the dependence of the dielectric and piezoelectric performance on the composition, poling field and crystallographic direction for Pb(Mg 1/3 Nb 2/3 )O 3 PbTiO 3 (PMN-PT) single crystal plates with compositions around the rombohedral-tetragonal morphotropic phase boundary (MPB).
Abstract: This paper presents a detailed study of the dependence of the dielectric and piezoelectric performance on the composition, poling field and crystallographic direction for Pb(Mg 1/3 Nb 2/3 )O 3 PbTiO 3 (PMN–PT) single crystal plates with compositions around the rombohedral–tetragonal morphotropic phase boundary (MPB). The Tmax of the plates lies in the range of 130–160 °C. Influence of poling field in a range of 0–3 kV mm −1 on their properties was investigated. Under appropriate poling conditions and with proper compositions, the plates show excellent piezoelectric property with d 33 larger than 1800 pC N −1 along 〈001〉 and 〈011〉, and 1300 pC N −1 along 〈111〉, respectively. It is also suggested that the relaxor nature of PMN, the domain structure modulation and the MPB effect all play important roles in influencing the piezoelectric performance. The possibility of the existence of the induced orthorhombic or monoclinic phase is also discussed in this paper.

161 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present a brief derivation of the temperature distribution in the sample and the die in spark plasma sintering, and point out that temperature difference is inevitable in the samples of SPS system.
Abstract: The paper presents a brief derivation of the temperature distribution in the sample and the die in spark plasma sintering. It aims to point out that temperature difference is inevitable in the samples of SPS system. Under certain conditions, the difference may reach a few hundreds of degrees of centigrade. Experiments also proved the existence of such a difference, which is larger than the one under steady-state temperature distribution.

157 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the correlation between the heat treatment and the observed photoactivities and found that an increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatments duration, led to a decrease in the activities of the catalyst particles.
Abstract: Magnetic iron oxide–titania photocatalysts (Fe 3 O 4 –TiO 2 ) were prepared using a coating technique in which the photoactive titanium dioxide was deposited onto the surface of a magnetic iron oxide core. These particles had a core–shell structure. The prepared particles were heat treated at high temperature in order to transform the amorphous titanium dioxide into a photoactive crystalline phase. The heat treatment temperature and duration were varied, and the correlation between the heat treatment and the observed photoactivities was investigated. An increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatment duration, led to a decrease in the activities of the catalyst particles. A decrease in surface area due to sintering, along with the diffusion of Fe ions into the titanium dioxide coating are seen as contributing factors to the decline in photoactivity which accompanied an increase in the heat treatment. Differential scanning calorimetry analysis (DSC) results confirmed that the presence of the iron oxide core did not have an effect on the phase transformation of titania under the experimental conditions investigated. In this study we also present surface charge measurements which show that the surface of the particles became more positive as the heat treatment was increased. This is an indication of changing surface properties as heat treatment is applied. For single-phase TiO 2 powders, this is postulated to be due to a decrease in the surface hydroxyl (OH) groups and/or residual organics (OR) groups. For the Fe 3 O 4 –TiO 2 powders, in addition to the loss of OH and OR groups, the diffusion of the Fe into the titania shell is postulated to also play a role in the changing surface properties with applied heat treatment. Experiments aimed at reducing the duration of the heat treatment revealed that a heat treatment duration of 20 min at 450 °C was sufficient to transform amorphous titanium dioxide into a photoactive crystalline phase. This does not only minimise loss of surface hydroxyl groups but it also has the potential to limit the oxidation of the magnetic core, which occurs due to the porosity of the coating. This has practical implications in terms of separating the magnetic particles from the treated waste waters under the application of an external magnetic field. It also presents an opportunity to produce photoactive composite particles while limiting the interactions between the core and the shell during the heat treatment.

155 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of Zn 2+ Ti 4+ substitution, annealing temperature on complex permeability, permittivity and microwave absorption had been studied for Ba(ZnTi) x Fe 12−2 x O 19 ferrite in the frequency range from 100 MHz to 6.0 GHz.
Abstract: Ba(ZnTi) x Fe 12−2 x O 19 ( x varies from 0.2 to 1.0 in steps of 0.2) hexaferrites were prepared by citrate sol–gel process, the formation temperature is about 800 °C. The samples were characterized by XRD, scanning electron microscopy. The effect of Zn 2+ Ti 4+ substitution, annealing temperature on complex permeability, permittivity and microwave absorption had been studied for Ba(ZnTi) x Fe 12−2 x O 19 ferrite in the frequency range from 100 MHz to 6.0 GHz. All the ferrites exhibit significant dispersion in complex permeability. The dispersion in complex dielectric constant is not significant. The variations of reflection loss have been studied as a function of frequency, Zn 2+ Ti 4+ content, and thickness of the absorber.

111 citations


Journal ArticleDOI
TL;DR: In this paper, the interfacial stability of (PEO)10LiCF3SO3 composite polymer electrolytes (CPEs) with titanium oxide (TinO2n−1, n=1, 2) prepared by ball milling as ceramic filler is presented.
Abstract: Electrochemical properties and interfacial stability of (PEO)10LiCF3SO3 composite polymer electrolytes (CPEs) with titanium oxide (TinO2n−1, n=1, 2) prepared by ball milling as ceramic filler are presented. The amount of titanium oxide powders introduced was between 5 and 15 wt.% into the (PEO)10LiCF3SO3 polymer electrolyte. The addition of titanium oxide which consisted of plate-like spherical shape ranging from sub-micron to several microns increases the ionic conductivity by an order of magnitude compared with (PEO)10LiCF3SO3 polymer electrolyte without titanium oxide, and also have the higher ionic conductivity at low temperature. Li/CPEs/50% S cells have a initial discharge capacity of between 1400 and 1600 mA h g−1-sulfur with current rate of 100 mA g−1-sulfur at 90 °C and show the higher initial charge/discharge performance than without titanium oxide. The interfacial stability was remarkably improved by the addition of titanium oxide into the (PEO)10LiCF3SO3 polymer electrolyte.

105 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the coercivity, H c, of Ba-ferrite particles is mainly controlled by the crystalline anisotropy, and that the substitution of Fe 3+ ions by Zn 2+, Ni 2+ and Ti 4+ ion will affect the saturation magnetization σ s, coercivity H c and their temperature coefficients.
Abstract: The Ba-ferrite particles have a positive temperature coefficient of d H c /d T . From the viewpoint of recording system applications, the d H c /d T should be reduced in order to improve the operating margin for temperature change. It was shown that the coercivity, H c , of Ba-ferrite particles is mainly controlled by the crystalline anisotropy. In the Ba-ferrite there are five distinct crystallographic sites or sublattices, and the Fe 3+ ion located on different site has different contribution to crystalline anisotropy. The replacement of Fe 3+ ions by Zn 2+ , Ni 2+ , and Ti 4+ ion will effect on the saturation magnetization σ s , coercivity H c and their temperature coefficients. The Zn–Ti, Ni–Ti, and Ni–Zn–Ti substituted Ba-ferrite particles were investigated. It was found that the Zn–Ti substituted Ba-ferrite particles have higher σ s , but large temperature coefficient of coercivity d H c /d T . In the case of Ni–Ti and Ni–Zn–Ti substitution the d H c /d T could be reduced to a small value, near to zero or negative value, while the σ s decreases very slowly with increasing the amount of substitution.

105 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of metal ion dopants on photocatalytic kinetics were investigated by the degradation of 2-chlorophenol under an ultraviolet light source, and the results showed that the TiO 2 nanoparticles with the metal ion nanoparticles had higher photocatalysis activity than undoped TiO2.
Abstract: TiO2 nanoparticles were synthesized using the metallorganic chemical vapor deposition process. Particles with and without metal ion dopants were obtained. X-ray photoelectron and energy dispersive X-ray spectroscopic measurements confirmed the stoichiometry of the TiO 2 nanoparticles. X-ray diffraction patterns showed a polycrystalline anatase structure of TiO 2. Transmission electron microscopy revealed that these particles are of nanoscale dimensions. Exact particle size and size distribution analyses were carried out by dynamic light scattering. The average particle size was determined to be 22 nm. The nanosize particles provided large surface area for photocatalysis and a large number of free surface-charge carriers, which are crucial for the enhancement of photocatalytic activity. To improve the photocatalytic activity, metal ions, including transition metal ions (Pd 2+ , Pt 4+ , Fe 3+ ) and lanthanide ion (Nd 3+ ) were added to pure TiO2 nanoparticles. The effects of dopants on photocatalytic kinetics were investigated by the degradation of 2-chlorophenol under an ultraviolet light source. The results showed that the TiO 2 nanoparticles with the metal ion dopants have higher photocatalytic activity than undoped TiO 2. The Nd 3+ ion of these dopant metal ions showed the highest catalytic activity. The difference in the photocatalytic activity with different dopants is related to the different ionic radii of the dopants.

102 citations


Journal ArticleDOI
TL;DR: In this article, nanosized nickel aluminate spinel particles have been synthesized with the aid of ultrasound radiation by a precursor approach using an aqueous solution of nickel nitrate, aluminium nitrate and urea.
Abstract: Nanosized nickel aluminate spinel particles have been synthesized with the aid of ultrasound radiation by a precursor approach. Sonicating an aqueous solution of nickel nitrate, aluminium nitrate and urea yields a precursor which on heating at 950 °C for 14 h yields nanosized NiAl 2 O 4 particles with a size of ca. 13 nm and with a surface area of about 108 m 2 g −1 . The nanosized nickel aluminate particles as well as the precursor have been characterized by elemental analysis, powder X-ray diffraction, Transmission electron microscopy, Thermogravimetric analysis, DSC, FT-IR, Diffuse reflectance spectroscopy and BET surface area measurements.

94 citations


Journal ArticleDOI
TL;DR: In this article, the performance of a gas-sensing device based on group III-nitride materials such as platinum (Pt)-GaN Schottky diodes as well as high-electron-mobility transistors with catalytically active platinum gates were investigated.
Abstract: The paper reports on novel gas-sensing devices based on group III-nitride materials Both platinum (Pt)–GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AlGaN heterostructures with catalytically active platinum gates were investigated The performance of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NOx was tested Test gas concentrations were chosen to simulate exhaust gas emissions from lean-burn 4-stroke petrol engines We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO and NO2 These performance characteristics are similar to those of comparable SiC devices With GaN devices this performance, however, can be obtained at a reduced complexity of the device processing and a greater freedom in the choice of sensor architectures

91 citations


Journal ArticleDOI
TL;DR: In this article, the state of the art of different classes of silicon-based optoelectronic devices (waveguides, detectors, light sources etc.) is given, and the major obstacles to be overcome are presented.
Abstract: Si based optoelectronic devices have recently successfully entered the market of optical communications. This fact is stimulating renewed efforts in the design of optoelectronic components based on the well-established Si technology. In this paper, the motivations supporting these efforts and the major obstacles to be overcome are presented. Moreover, an overview of the state of the art of different classes of silicon based optoelectronic devices (waveguides, detectors, light sources etc.) is given.

Journal ArticleDOI
TL;DR: The use of surface voltage and surface photovoltage measurements has broadened from initial application of minority carrier diffusion length measurements to a wide variety of semiconductor characterization, including surface voltage, surface barrier height, flatband voltage, oxide thickness, oxide charge density, interface trap density, mobile chargedensity, oxide integrity, generation lifetime, recombination lifetime and doping density.
Abstract: Surface voltage and surface photovoltage measurements have become important semiconductor characterization techniques, largely because of their contactless nature and the availability of commercial equipment. The use of these contactless measurement techniques has broadened from initial application of minority carrier diffusion length measurements to a wide variety of semiconductor characterization, including surface voltage, surface barrier height, flatband voltage, oxide thickness, oxide charge density, interface trap density, mobile charge density, oxide integrity, generation lifetime, recombination lifetime and doping density. It is likely that this range of applications will broaden further. As with all characterization techniques, there are limitations but they are frequently compensated by the contactless nature of the measurement thereby simplifying test structure fabrication.

Journal ArticleDOI
TL;DR: In this paper, a low energy plasma enhanced CVD (LEPECVD) was proposed for SiGe heteroepitaxy, which can achieve a wide range of epitaxial growth rates ( −1 up to at least 10 nm s −1 ), independent of the substrate temperature in the range of 500 −750 °C.
Abstract: We present a state-of-the-art growth technique, introduced recently for SiGe heteroepitaxy. At present, its most important application is the fast fabrication of high-quality relaxed SiGe buffer layers. The process is based on chemical vapor deposition (CVD), enhanced by a high intensity, low energy plasma, which is why we call it ‘low energy plasma enhanced CVD’ (LEPECVD). The key features of the process are a wide range of epitaxial growth rates ( −1 up to at least 10 nm s −1 ), independent of the substrate temperature in the range of 500–750 °C and easy control of film composition, governed solely by the mixture of the precursor gases (SiH 4 and GeH 4 ). Possible applications of LEPECVD include electronic devices, such as modulation doped SiGe FETs (SiGe-MODFETs) or strained-layer SiGe MOSFETs, all based on relaxed buffer layers and targeting the fast-growing RF device market. Another field of applications is high-performance SiGe solar cells. For economic production, all these devices rely on fast, high-quality epitaxy, which is the strength of LEPECVD. The process, developed on the experimental system for 4-inch wafers, is now being transferred to a 300-mm single wafer reactor for production.

Journal ArticleDOI
TL;DR: In this article, a milling procedure was used to produce nanocrystalline powders of CaCu3Ti4O12 (CCTO) for the first time, using two different experimental procedures.
Abstract: Mechanical alloying has been used successfully to produce nanocrystalline powders of CaCu3Ti4O12 (CCTO), for the first time, using two different experimental procedures. The milled CCTO were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. For two different milling procedures, CCTO was obtained after a couple of hours of milling (in average 30 h of milling, depending on the reaction procedure). The X-ray diffraction (XRD) patterns indicate that the crystallite size is within the range of 20–35 nm. After 100 h of milling the formation of CCTO was confirmed by X-ray powder diffraction in both procedures, with good stability. We also prepare the CCTO ceramic using the traditional procedure described in the literature and compared the physical properties of these samples with those ones obtained by milling process and good agreement was observed. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time leads to the formation of nanocrystalline CCTO, as seen by XRD analysis. These materials are attractive for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the decrease of the size of the devices are crucial. This milling process presents the advantage that melting is not necessary and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material can be compacted and transformed in solid ceramic samples or used in others procedures of film preparation. The high efficiency of the process opens a way to produce commercial amount of nanocrystalline powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.

Journal ArticleDOI
TL;DR: In this paper, the optimal conditions for elaboration of vanadium oxide films were studied as a function of spray solution concentrations, using X-ray diffraction (XRD) and optical measurements.
Abstract: Vanadium oxide thin films have been successfully deposited in the first time by spray pyrolysis technique at two substrate temperatures, 200 and 250 °C. In view of defining the optimal conditions for elaboration of vanadium oxide films, the as-deposited films were studied as a function of spray solution concentrations, using X-ray diffraction (XRD) and optical measurements. V2O5 and V4O9 polycrystalline films with an orthorhombic structure were easily obtained under the different spraying conditions. The AFM result indicates a grain size of approximately 50 and 80 nm for V2O5 and V4O9, respectively. The optical results show that the optical gap is equal to 2.44 eV for V2O5 and 2.34 eV for V4O9.

Journal ArticleDOI
TL;DR: In this article, the vibrational spectra in the region of internal vibrations of ions corroborates the X-ray data recently published by Janczak (Acta Crystallogr. 57 (2001) 1120).
Abstract: Room temperature powder infrared and Raman measurements for the new melaminium salt, 2,4,6-triamino-l,3,5-triazine-1-ium chloride hemihydrate, C 3 H 8 N 6 + ·Cl − · 0.5H 2 O were carried out. The vibrational spectra in the region of internal vibrations of ions corroborates the X-ray data recently published by Janczak (Acta Crystallogr. 57 (2001) 1120) Some spectral features of this new crystal are referred to corresponding one for melamine crystal as well as other melamine complexes in crystalline form.

Journal ArticleDOI
TL;DR: In this article, the dielectric constant and the loss factor for the Li2B4O7 (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectrics mixture rule formalism.
Abstract: Transparent glasses in the system (100−x)Li2B4O7–x(SrOBi2O3Nb2O5) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi2Nb2O9 (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (er) and the loss factor (D) for the lithium borate, Li2B4O7 (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.

Journal ArticleDOI
TL;DR: In this paper, the authors focused on low temperature ceramic co-fired with copper and showed that the final structure of the sintered thick film appeared as three dimensional network structure.
Abstract: Low temperature cofired ceramics (LTCC) technology has been widely used for high frequency components. Thick film copper is one of critical technology in microelectronic packaging. This paper focuses on low temperature ceramic cofired with copper. The difficulty with LTCC/Cu is organic material removal and copper circuit protection. By the careful control of the oxygen partial pressure in different sintering procedure, a quality multiplayer/copper system can be achieved. Different shrinkage characteristics of copper thick film and LTCC sheet result in delamination, cracking, and camber in the final products. Sintering promoters play an important role in the electrical conductivity and adhesion strength of copper thick film. The final structure of the sintered thick film appeared as three dimensional network structure. Approximately 8 wt.% glass promoter can match the shrinkage rate of the substrate and silver paste. LTCC has a low thermal conductivity, which limits its application in some fields. Thermal vias buried in LTCC substrate can resolve this problem.

Journal ArticleDOI
TL;DR: In this paper, the effect of microstructural features on the electrical properties of TiO2 (rutile) was studied using impedance spectroscopy in conjunction with X-ray diffraction and scanning electron microscopy.
Abstract: The effect of microstructural features on the electrical properties of TiO2 (rutile) was studied using impedance spectroscopy in conjunction with X-ray diffraction and scanning electron microscopy. The electrical properties of TiO2 were measured at 350 °C after sintering in air at different temperatures. It was found that the electrical resistivity increased significantly after the interconnected pores closed in the TiO2 due to sintering. However, further decrease of porosity in the TiO2 had little effect on its resistivity prior to the closure of pores. On the other hand, after the sintered specimens were annealed in vacuum, the resistivity of TiO2 decreased considerably without apparent change of the microstructure in TiO2. Moreover, the resistivity, measured at room temperature, decreased after the interconnected pores closed due to sintering. These results suggest that the electrical conduction in TiO2 after sintering in air is mainly due to ionic conduction, while the conduction in the sintered TiO2 after annealing in vacuum was mainly due to electronic conduction. The closure of the open pores in the TiO2 reduced the surface ionic conduction but increased the electronic conduction route. Overall, the microstructural features of TiO2 have characteristic effects on its electrical properties.

Journal ArticleDOI
TL;DR: In this article, the authors provide an overview on Random Telegraph Signals (RTSs) in solid-state devices and more in particular in scaled silicon MOSFETs.
Abstract: This paper provides an overview on Random Telegraph Signals (RTSs) in solid-state devices and more in particular in scaled silicon MOSFETs. It tries to answer the following questions: what is an RTS? How does it behave as a function of the operation conditions (temperature, bias)? And what can we learn from it? It will be shown that when properly analysed, RTS is a sensitive local probe for the study of single defects and their microscopic environment. In this way, new insights into trap dynamics and device physics can be gathered which are relevant for submicron and nano-electronic devices.

Journal ArticleDOI
TL;DR: In this article, cubic nickel oxide (bunsenite) films with a preferred (200) orientation and an average grain size of 15 nm were deposited on Si(111) substrate by a pulse ultrasonic spray pyrolysis method.
Abstract: Cubic nickel oxide (bunsenite) films with a preferred (200) orientation and an average grain size of 15 nm were deposited on Si(111) substrate by a pulse ultrasonic spray pyrolysis method. Aqueous ethanol solutions of nickel nitrate were ultrasonically nebulized and thermally deposited on Si(111) using an aerosol generator, which was operated at 2.5 MHz. The resulting films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. Highly crystalline NiO films with (200) orientation were obtained at 360 °C substrate temperature from 0.05 M solution and 5 l min −1 carrier gas flow rate. All grains were seen to be elongated along one direction by AFM. The ratio (64.58%) of the oxygen adsorbed to the total O1s intensity indicated that the film deposited by this method was a good candidate for electrochromic films.

Journal ArticleDOI
TL;DR: In this paper, an unhampered version of the Weight Induced Epitaxial Lift-Off (WI-ELO) was used to free single crystalline films from the GaAs substrates on which they have been deposited by etching a sacrificial AlAs release layer.
Abstract: The ‘Weight Induced Epitaxial Lift-Off’ (WI-ELO) process is used to free single crystalline films from the GaAs substrates on which they have been deposited by etching a sacrificial AlAs release layer. The lateral etch rate Ve of this process is influenced by the weight induced radius of curvature R of the film. Bulk-etch experiments of AlxGa1−x As layers were conducted to compare an unhampered etching process with WI-ELO. It is found that standard WI-ELO etching demonstrates etch rates exceeding the bulk etch rate. Further experiments have shown that the WI-ELO etch rate is not constant in time, but consists of a slow initial etch rate Ve,i, followed by the faster nominal etch rate Ve,n. The latter part is influenced by the applied radius of curvature R via Ve,n=3.1+293R−1.2 with R in mm and Ve,n in mm h−1. This result implies an etch rate consisting of a constant plus a radius-induced part, resulting in both a qualitative and quantitative discrepancy with established theory. The explanations could be the different reaction kinetics and the occurrence of stress or strain in the film.

Journal ArticleDOI
TL;DR: In this article, a tetrapod ZnO nanopowders with four needle-like legs was used for co-sintering oxides instead of adding amorphous lead zinc borosilicate frit.
Abstract: Varistors based on ZnO–lead zinc borosilicate glass were prepared from tetrapod ZnO nanopowders, every one of which had four needle-like legs and each one was about 20 nm or less in diameter and from several hundreds of nanometers to several micrometers in length by the method of direct co-sintering synthesis of oxides instead of adding amorphous lead zinc borosilicate frit. The compact green disks were conventionally sintered in air for 2 h at a temperature of 900–1170 °C. The varistors with nonlinear coefficient α=38.7 and leakage current IL=1.7 μA were obtained. The results showed that the sintering temperature was lowered to 900 °C, and there was very little influence of the sintering temperature on the nonlinear coefficient α at a range 900–1170 °C.

Journal ArticleDOI
TL;DR: In this article, the structural homogeneity of the melt spun ribbon and plastic deformation of the hot consolidated body were systematically investigated and compared with conventionally fabricated alloys, and the initial composition and hot pressing temperature dependences of the rapidly solidified and hot pressed samples were quantitatively analyzed by measuring the thermoelectric properties such as Seebeck coefficient, electrical conduction, thermal conductivity and Hall coefficient.
Abstract: p-Type Bi2Te3–Sb2Te3 solid solutions were newly fabricated by rapid solidification and hot pressing, which is considered to be a mass production technique for this alloy. Structural homogeneity of the melt spun ribbon and plastic deformation of the hot consolidated body were systematically investigated and compared with conventionally fabricated alloys. Initial composition and hot pressing temperature dependences of the rapidly solidified and hot pressed samples were quantitatively analyzed by measuring the thermoelectric properties such as Seebeck coefficient, electrical conduction, thermal conductivity and Hall coefficient.

Journal ArticleDOI
TL;DR: In this paper, photoluminescence (PL) and optical absorption results on silicon nanocrystals in SiO2 matrices prepared by RF sputtering method are presented, where the samples have varying Si/SiO2 compositional fraction and are annealed at different temperature and duration.
Abstract: Raman, photoluminescence (PL) and optical absorption results on silicon nanocrystals in SiO2 matrices prepared by RF sputtering method are presented. The samples have varying Si/SiO2 compositional fraction and are annealed at different temperature and duration. The average size of the nanocrystals is determined by the lineshape analysis of the first-order Raman spectra. Raman results further indicate the existence of a threshold annealing temperature for the formation of nanocrystals and increase in the nanocrystal size with increasing temperature and duration. Size-dependent blue shift of the absorption edge is observed from absorption experiments and is supported by Raman results. Room temperature and temperature dependent PL results are analyzed with the help of a phenomenological model. The PL results in conjunction with the Raman and absorption results indicate the involvement of both the core states and the interfacial states in the luminescence process.

Journal ArticleDOI
TL;DR: In this article, different NASICON systems with general formula Am Bn P3O12 were prepared by melt quenching method, where A � /Na, and B� /Cu, Al, Fe, FeCd were characterized by X-ray diffraction and Fourier transform infrared spectroscopy.
Abstract: Different NASICON systems with general formula Am Bn P3O12 were prepared by melt quenching method, where A � /Na, and B� /Cu, Al, Fe, FeCd. The prepared compounds were characterized by X-ray diffraction, and Fourier transform infrared spectroscopy. The electrical conductivity measurements were made on the different NASICON materials as a function of frequency (20 Hz to 1 MHz) at different temperatures. The bulk conductivity was obtained from the impedance analysis. The dc conductivity activation energies are found to be in the range 0.40 � /1.53 eV. The conductivity and dielectric spectra show the power low feature. The framework of the Almond � /West conductivity formalism is applied to discuss the ac conductivity and determined the dc conductivity sdc, hopping frequency vp, and dimensionless frequency exponent n . The conductivity master curves for the conductivity spectra are obtained by scaling the frequency by (i) vp and (ii) sdcT , where T is temperature in Kelvin. Both types of scaling show same conductivity master curves. # 2002 Published by Elsevier Science B.V.

Journal ArticleDOI
TL;DR: In this article, the X-ray diffraction analysis confirmed the initial amorphous nature of as-deposited InS film and phase transition into crystalline In2S3 form upon annealing at 500°C.
Abstract: Indium sulfide thin films were grown by photochemical deposition technique from an aqueous solution by means of UV illumination. Both the as-grown and annealed films were studied by different analysis tools. The X-ray diffraction analysis confirmed the initial amorphous nature of as-deposited InS film and phase transition into crystalline In2S3 form upon annealing at 500 °C. The structural phase transition upon annealing has also been revealed by the Raman spectroscopic analysis. The compositional analysis by Auger electron spectroscopy indicated that the InS film contains oxygen as the impurity element. The bandgap energy of the as-deposited and annealed films was analyzed by means of optical transmission study.

Journal ArticleDOI
TL;DR: In this article, the synthesis and characterization of nanocrystalline Si prepared by the initial reaction of the metal silicide, Mg 2 Si, with either SiCl 4 or Br 2 and, subsequently, with LiAlH 4.
Abstract: We describe the synthesis and characterization of nanocrystalline Si prepared by the initial reaction of the metal silicide, Mg 2 Si, with either SiCl 4 or Br 2 and, subsequently, with LiAlH 4 . These reactions produce Si nanoparticles with surfaces that are covalently terminated with H. The resultant nanoparticles can be suspended in organic solvent and are characterized by Fourier transform infrared (FTIR), UV–vis absorption, and photoluminescence (PL) spectroscopy. This work provides the first direct evidence of hydrogen terminated Si nanoparticles synthesized in solution.

Journal ArticleDOI
TL;DR: The structural and optical properties of hydrogenated amorphous silicon carbon (a-Si 1− x C x :H) thin films, grown from argon diluted silane, ethylene, and hydrogen mixture by plasmaenhanced chemical vapor deposition (PE-CVD) technique, were studied.
Abstract: The structural and optical properties of hydrogenated amorphous silicon carbon (a-Si 1− x C x :H) thin films, grown from argon diluted silane, ethylene, and hydrogen mixture by plasma-enhanced chemical vapor deposition (PE-CVD) technique, were studied. Variable flow rates and other growth conditions were applied. A variety of techniques, including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, Raman scattering (RS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM), UV-VIS-NIR spectroscopy, and photoluminescence (PL) were used to characterize the grown materials. The results confirmed the multiphase structure of the grown a-Si 1− x C x :H thin films: Si–C network, carbon-like and silicon-like clusters coexisting. The room temperature (RT) PL shows a different result from the previous reports. It is suggested that both graphite-like phase and a-Si:H-like phase are light-emitting grains. The two types of grains and Si–C network are the origin of the PL in hydrogenated amorphous silicon carbide material.

Journal ArticleDOI
Wang Ruigang1, Pan Wei1, Jiang Mengning1, Chen Jian1, Luo Yongming1 
TL;DR: In this paper, a new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machINable Si3N4 ceramics.
Abstract: A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si3N4 ceramics. Silicon nitride/hexagonal boron nitride (h-BN) ceramic composites and functionally graded materials were fabricated by hot pressing at 1750 °C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si3N4/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. The texture of h-BN and β-Si3N4 was observed during hot pressing sintering. Physical and mechanical properties of Si3N4/h-BN with different content of h-BN were investigated, such as bulk density, Vickers's hardness, flexural strength, and elastic modulus. All of these properties are important for the design of the machinable Si3N4/h-BN FGM (Functionally Graded Materials).