Synthesis of nanocrystalline titanium carbide by spark erosion
Interest in nanocrystaJline ceramic powders is due to their attractive processing and mechanical properties. The disadvantages of conventional ceramics, such as large internal flaws, negligible ductility, hightemperature processing, can possibly be eliminated or minimized by starting with finegrained powders. A previous study on the sinterability of nanocrystalline ceramic powders showed that high densities can be achieved at lower temperatures, both with and without the’ application of pressure [ 11. In addition, it is remarkable that superplastic behavior has been reported in nanocrystalline ceramics at room temperature [2]. One of the novel methods for nrenaring nanocrvstalline ceramic powders is reactive spark erosion [3-51. An important feature of this method is the-capability to synthesize new materials by reacting the eroded particles with the dielectric liguid. This technique utilizes two metal or conducting electrodes sparked in a suitable dielectric liquid that reacts with the pa&les to produce the desired ceramic powders. The high temperature provided by the spark results in vaporization of the electrodes and the surrounding dielectric liquid; the reaction products formed between the various vapor species are quenched in the liquid and produce spherical particles. The aim of the present note is to report on the titanium carbide particle formation in a carbonic dielectric liquid (pentane) with titanium electrodes and charges. The nanocrystalline titanium carbide was produced in a wide range of particle size distribution and in the form of single phase spheres.
Ceramics ductile at low temperature
It is generally recognized that ceramics frequently offer advantages over metals in their use, such as chemical resistivity, hardness, wear resistance, high melting temperature, low density and low price. However, the use of ceramics has so far been hampered by their brittleness at low temperature. Here we report observations that could open a way to remove these limitations. We observed that conventionally brittle ceramics became ductile permitting large (~100%) plastic deformations at low temperature if a polycrystalline ceramic was generated with a crystal size of a few nm. The ductility seems to originate from the diffusional flow of atoms along the intercrystalline interfaces.
Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion
The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with intrinsic separation of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, reacts with the solid oxygen carrier. In this work, copper(II) aluminate (CuAl2O4) was assessed as a potential oxygen carrier using methane as fuel. The carrier particles were produced by freeze–granulation and calcined at 1050 °C for a duration of 6 h. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized-bed reactor in the temperature range of 900–950 °C during 45 alternating redox cycles. The oxygen carrier exhibited reproducible and stable reactivity behavior in this temperature range. Neither agglomeration nor defluidization was noticed in any of the cycles carried out at 900–925 °C. However, after reactivity tests at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu–Al–O system during the redox cycle was carried out as a function of duration of reduction and oxygen concentration during the oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O, and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for oxidation into CuO and CuAl2O4. Despite this kinetic limitation, complete conversion of methane with reproducible reactivity of the oxygen carrier is achieved. Thus, CuAl2O4 could be a potential oxygen carrier for chemical-looping combustion.