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31.
The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m1/2 and 2480 kg/mm2, respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.  相似文献   
32.
A method to simultaneously synthesize and consolidate MoSi2 from powders of Mo and Si was investigated. Combustion synthesis was carried out under the combined effect of an electric field and mechanical pressure. Highly dense molybdenum silicide up to (99.2%) was produced from elemental powders in one step. Minor amounts of Mo5Si3 were present at the boundaries of MoSi2 grains in the interior of samples made from stoichiometric reactants. The addition of 2.5 mol% Si excess, however, resulted in Mo5Si3-free, dense MoSi2 products.  相似文献   
33.
Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.  相似文献   
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Dense 8YSZ was subjected to pulsed current activated sintering (PCAS) within 1 min of 8YSZ nanopowder preparation using a co-precipitation method. Sintering was accomplished by combining a pulsed current and mechanical pressure. Highly dense 8YSZ with a relative density of up to 99% was produced under simultaneous application of a pressure of 80 MPa and the pulsed current. The effects of the addition of Al2O3 on the sintering behavior, mechanical properties, and ionic conductivities of 8YSZ were investigated.  相似文献   
37.
Dense ultrafine NbSi2 was synthesized using the pulsed current activated combustion synthesis (PCACS) method within 2 min and in one step from mechanically activated powders of Nb and 2Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense NbSi2 with a relative density of up to 98 % was produced under simultaneous application of a 60 MPa pressure and a pulsed current. The average grain size and mechanical properties (hardness and fracture toughness) of the compound were investigated.  相似文献   
38.
This report describes studies of the relationships between the structures of organic monolayers and their molecular-scale frictional properties. Several distinct self-assembled monolayers (SAMs) were formed by the adsorption of a series of spiroalkanedithiols and a single structurally related normal alkanethiol. Measurements of hexadecane wettability, infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy revealed that the films possessed a wide range of interfacial structures and conformational orders. Atomic force microscopy was used to measure the frictional properties of the well-characterized SAMs on the molecular scale. Comparison of the frictional data with structural information derived from complementary analytical techniques revealed a high correlation between the conformational order of the films and the observed frictional response.  相似文献   
39.
Titanium carbide and composites and functionally graded materials (FGMs) of TiC– x Cu were synthesized by an electrothermal combustion (ETC) method. TiC synthesized by ETC showed small amounts of porosity relative to those synthesized by ignition using radiative heating. Composites and FGMs with higher copper content can be synthesized by ETC. In the FGM products a nearly linear change in composition in the graded region was observed in samples with 0 ≤ x (wt%) ≤ 75.  相似文献   
40.
Iron nanoparticles are becoming increasingly popular for the treatment of contaminated soil and groundwater; however, their mobility and reactivity in subsurface environments are significantly affected by their tendency to aggregate. Assessing their stability under environmental conditions is crucial for determining their environmental fate. A multi-method approach (including different size-measurement techniques and the DLVO theory) was used to thoroughly characterise the behaviour of iron oxide nanoparticles (Fe2O3NPs) under environmentally relevant conditions. Although recent studies have demonstrated the importance of using a multi-method approach when characterising nanoparticles, the majority of current studies continue to use a single-method approach.Under some soil conditions (i.e. pH 7, 10 mM NaCl and 2 mM CaCl2) and increasing particle concentration, Fe2O3NPs underwent extensive aggregation to form large aggregates (>1 μm). Coating the nanoparticles with dissolved organic matter (DOM) was investigated as an alternative “green” solution to overcoming the aggregation issue instead of using the more commonly proposed polyelectrolytes. At high concentrations, DOM effectively covered the surface of the Fe2O3NPs, thereby conferring negative surface charge on the particles across a wide range of pH values. This provided electrostatic stabilisation and considerably reduced the particle aggregation effect. DOM-coated Fe2O3NPs also proved to be more stable under high ionic strength conditions. The presence of CaCl2, however, even at low concentrations, induced the aggregation of DOM-coated Fe2O3NPs, mainly via charge neutralisation and bridging. This has significant implications in regards to the reactivity and fate of these materials in the environment.  相似文献   
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