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991.
Nanocomposites containing Fe3O4 and alpha-Fe, respectively, in a SiO2 gel were prepared by subjecting a suitably chosen gel with iron ions to a reduction treatment at 923 K, followed by wet oxidation at the same temperature for 1 hour. The particle sizes of the two phases were estimated to have values in the range of 18 to 25 nm. The dc conductivity of the composites was found to arise due to a variable range hopping mechanism with a density of states calculated as approximately 10(18) eV(-1) cc(-1). The nanoparticles of alpha-Fe are believed to contribute to the latter. The ac conductivity variation as a function of frequency and temperature could be explained because of an overlapping small polaron tunneling mechanism in the Fe3O4 nanoparticles. The density of states estimated in the latter case was approximately 10(18) eV(-1) cc(-1). From the dielectric modulus spectra of the nanocomposites, a Kohlrausch-Williams-Watts (KWW) exponent of approximately 0.30 was extracted. This indicated the presence of a wide distribution of relaxation times in the system. 相似文献
992.
P. Prabhakar Rao S. K. Ghosh Peter Koshy 《Journal of Materials Science: Materials in Electronics》2001,12(12):729-732
(M=Sm or Y) compounds were synthesized by the conventional powder compaction and high temperature solid state reaction technique. The calcined compounds were pelletized and sintered in the range 1275–1325 °C for 4 h. The dielectric and ferroelectric properties of the sintered pellets were measured and further characterised by X-ray diffraction and scanning electron microscopy (SEM). Sm and Y substituted compounds were found to have tetragonal structure and shown dielectric constants of 145 and 49 respectively at 1 MHz. The dielectric loss increases with frequency. Samarium compound had undergone a diffuse phase transition in the temperature range 195–210 °C whereas the same with Yttrium had shown the transition at around 25 °C. 相似文献
993.
Co : Al2O3nanocermets are synthesized by co-reducing Co2+-cations dispersed in a mesoporous AlO(OH) · H2O matrix (amorphous) in a pure H2atmosphere at 850–1150 K. The dispersed Co2+-cations in pores co-reduce to separated Co-particles of controlled size, as small as 50 nm, encapsulated in thin Al2O3films, which are formed in-situ by molecular decomposition of the matrix, 2AlO(OH) · H2O Al2O3+ (2 + 1)H2O. The Al2O3film which coats Co-particles has an amorphous structure. This is possible only if it is extremely thin limited to a thickness of t< 2r
c, with r
c 1.9 nm the critical size of its nucleation and growth into a stable crystallite. The thin Al2O3surface film supports the formation and existence of Co-particles in a modified FCC or BCC crystal structure. As a result, unusually, large crystallites of an average 100 nm diameter could be observed. Normally, such large particles of pure Co-metal exist in an HCP structure which undergoes a reversible martensitic transformation to FCC structure at 695 K. The results are analyzed and discussed in terms of microstructure, x-ray diffraction and XPS studies of nanocermets prepared under selected conditions. 相似文献
994.
Pairs of electrodes with nanometer separation (nanogap) are achieved through an electromigration-induced break-junction (EIBJ) technique at room temperature. Lithographically defined gold (Au) wires are formed by e-beam evaporation over oxide-coated silicon substrates silanized with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and then subjected to electromigration at room temperature to create a nanometer scale gap between the two newly formed Au electrodes. The MPTMS is an efficient adhesive monolayer between SiO/sub 2/ and Au. Although the Au wires are initially 2 /spl mu/m wide, gaps with length /spl sim/1 nm and width /spl sim/5 nm are observed after breaking and imaging through a field effect scanning electron microscope. This technique eliminates the presence of any residual metal interlink in the adhesion layer (chromium or titanium for Au deposition over SiO/sub 2/) after breaking the gold wire, and it is much easier to implement than the commonly used low-temperature EIBJ technique which needs to be executed at 4.2 K. Metal-molecule-metal structures with symmetrical metal-molecule contacts at both ends of the molecule are fabricated by forming a self-assembled monolayer of -dithiol molecules between the EIBJ-created Au electrodes with nanometer separation. Electrical conduction through single molecules of 1,4-Benzenedimethanethiol (XYL) is tested using the Au/XYL/Au structure with chemisorbed gold-sulfur coupling at both contacts. 相似文献
995.
Abdulhaqq A. Hamid S. C. Jain P. K. Ghosh Subrata Ray 《Metallurgical and Materials Transactions A》2006,37(2):469-480
In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by
dispersion of externally added titanium dioxide (TiO2) particles in molten aluminum at different processing temperatures. Alumina particles (Al2O3) form in situ through chemical reaction of TiO2 particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten
aluminum and results in the formation of intermetallic phase Ti(Al1−x
,Fe
x
)3 during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic
phase Ti(Al1−x
,Fe
x
)3, (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with
increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness
is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount
of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses
of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile
stress more than the yield stress. In the cast in-situ composite containing 3.31 ± 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated
yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly
higher rate than that predicted by the shear-lag model. 相似文献
996.
H. S. Tripathi S. K. Das B. Mukherjee A. Ghosh G. Banerjee 《Ceramics International》2001,27(8):833-837
A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO2 (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO2 on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO2 slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO2. ZrO2 containing samples always show better resistance to thermal shock than the ZrO2 free samples. Scanning electron micrography shows that ZrO2 occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering. 相似文献
997.
Failure analysis of high temperature studs 总被引:1,自引:0,他引:1
Sandip Ghosh Chowdhury Pravesh Kumar Swapan K. Das D. K. Bhattacharya N. Parida 《Engineering Failure Analysis》2001,8(6):521-528
Studs in the interceptor valve of a 110 MW unit failed after a service life of 148,700 h. The studs were operated under a steam pressure of 35 kg/cm2 and a temperature of 535°C. The studs were fractured at one end of the threaded end. Various techniques were employed to analyse the failure of the studs. It has been concluded that the failure of the studs was due to reverse temper embrittlement. The failure was delayed due to the presence of Mo and V. To reduce the tendency to this kind of failure, the following steps were recommended: (a) reduce the phosphorus content in the steel to a low level or (b) reduce the grain size to about 10 μm. 相似文献
998.
B. M. Arora Sandip Ghosh Shouvik Datta Shailendra Kumar 《Materials Science in Semiconductor Processing》2001,4(6):796
Two spectroscopic techniques, modulated reflectance spectroscopy and surface photovoltage spectroscopy, are being used increasingly to probe the electronic structure of low-dimensional semiconductors. We have found improved versions of these techniques: soft contact electroreflectance and soft contact surface photovoltage which offer operational advantages as well as extend the range of these spectroscopies. We also provide analytic formulation for extracting the transition parameters from the measured surface photovoltage spectrum of a quantum structure. 相似文献
999.
Fabrication of in situ TiC reinforced aluminum matrix composites 总被引:2,自引:0,他引:2
In the present work, the room and elevated temperature mechanical behavior of Al/TiC, high-strength Al-Si/TiC and the elevated temperature-resistant Al-Fe(-V-Si)/TiC composites has been evaluated. The microstructural characteristics of ingot metallurgy (IM) or rapid solidification (RS) Al-Si/TiC and Al-Fe(-V-Si)/TiC composites could be thought of as a combination of the related alloy matrix microstructures and the IM or RS Al/TiC composites. The IM Al/TiC and the Al-Si/TiC composites show superior strength and ductility to the relevant aluminum based composites.The RS Al/TiC and the Al-Fe-V-Si/TiC exhibit high Young's moduli and substantial improvements in room and elevated temperature tensile properties compared to those of rapidly solidified alloys and conventional composites.The Young's modulus values of RS Al/TiC and Al-Fe-V-Si/TiC composites are well within Hashin-Shtrikman limits in keeping with the strong interfacial bonding. In the micromechanics approach, the principal strengthening mechanisms for the present dispersed particle-hardened RS in situ Al-TiC composites would include Orowan strengthening, grain-size and substructure strengthening, and solid-solution strengthening. The RS technique was used in the present work to maximize strength and ductility for a particular volume fraction, and influence the degree of flexibility available to meet these requirements: a fine, uniform particle size distribution; a high interfacial strength; control of particle shape; and a ductile matrix. 相似文献
1000.
S. Ghosh A.B. Chattopadhyay S. Paul 《International Journal of Machine Tools and Manufacture》2008,48(11):1242-1253
Grinding is a multi-point cutting operation. The specific energy or the energy expended for unit material removal in grinding is very high, typically one or two orders higher than the machining specific energy. Such high specific energy required in grinding can be attributed to the irregular and random geometry of the abrasive grits, which induce a lot of rubbing and ploughing actions along with the chip formation by shearing process. Also the effective angle in grinding is highly negative which is again responsible for such high-specific energy requirement in grinding. In grinding, a number of notable phenomena occur during the chip formation process, which actually consumes a significant percentage of energy. Such main energy consumers in grinding are:
- • Chip formation due to shearing
- • Primary rubbing
- • Secondary rubbing
- • Ploughing
- • Wear flat rubbing
- • Friction between the loaded chip and workpiece
- • Friction between bond and workpiece, etc.