Zuhair A. Munir

Activation Energy for the Sublimation of Gallium Nitride

Gallium nitride was found to sublime congruently from a torsion—effusion cell when the ratio of orifice area to sample area was about 1/30 and incongruently to yield nitrogen gas and liquid gallium when this ratio was about 1/100 or less. A mass‐spectrometer investigation revealed no measurable concentrations of gallium nitride vapor molecules. The heat of activation for the reaction 2GaN(s)=2Ga(1)+N2(g) was calculated to be 39 kcal at 1300°K from the temperature dependence of the effusion data.The rate of the reaction 2GaN(s)=2Ga(g)+N2(g) was measured by a torsion—Langmuir method. From the temperature dependence of sublimation the heat of activation for this reaction was calculated to be ΔH1300‡=218.6 kcal compared to 173 kcal for the equilibrium reaction, and the entropy of activation was calculated to be 74.3 cal/deg.

Mechanical properties of β-SiC fabricated by spark plasma sintering

The consolidation of SiC nanopowder synthesized by the mechanical alloying method was subsequently accomplished by spark plasma sintering of 1700 °C for 10 min under an applied pressure of 40 MPa. The SiC sintered compact with relative density of 98% consisted of nano-sized particles smaller than 100 nm. This phenomenon resulted in the ordering process of stacking disordered structure formed by mechanical alloying. In this work, the effect of grain size and relative density on the mechanical properties were studied. The mechanical properties of sintered compacts were evaluated and compared with the reference samples fabricated from the commercial SiC powder (β-SiC, 0.3 µm, IBIDEN Co., Gifu, Japan) with sintering additive (B–C mixture). The Vickers hardness and bending strength of those sintered compacts increased with the increment of the density. However, the mechanical properties were lower than those of reference samples in case of lower density, even though the mechanical property was close to that of reference sample in case of higher density. This phenomenon was considered for the difference of bond strength between grains because those sintered compacts were fabricated without any sintering additives, while those reference samples were fabricated by accelerating the grain bonding with a sintering additive of B–C mixture. In other words, those results indicated that the effect of sintering additive affected on mechanical properties directly.

The combustion synthesis of refractory nitrides

The self-sustaining combustion of niobium in a nitrogen atmosphere was investigated. The effect of initial porosity, nitrogen pressure, and the amount of added diluent on the combustion process and on the nature of the products of combustion was determined. Auxiliary investigations on the interaction between niobium and nitrogen were made under isothermal conditions. Results of X-ray analyses of these investigations are compared to those obtained under self-propagating reaction conditions.

Effect of pulsed DC current on neck growth between tungsten wires and tungsten plates during the initial stage of sintering by the spark plasma sintering method

The spark plasma sintering of tungsten wires to tungsten plates was investigated to assess the effect of the current. Neck growth between wires and plates was about 1.5 larger in the presence of current than when the current was absent. The neck growth was controlled by an evaporation–condensation mechanism. The mechanism, however, does not relate to the sintering of tungsten, but to the control of a surface oxide reduction, in agreement with previous investigations.

Investigation of ZnO-Based Polycrystalline Ceramic Scintillators for Use as

ZnO-based scintillators are particularly well suited for use as the associated particle detector in a deuterium-tritium (D-T) neutron generator. Application requirements include the exclusion of organic materials, outstanding timing resolution, and high radiation resistance. ZnO, ZnO:Ga, ZnO:In, ZnO:In,Li, and ZnO:Er,Li have demonstrated fast (sub-nanosecond) decay times with relatively low light yields. ZnO:Ga has been used in a powder form as the associated particle detector for a D-T neutron generator. Unfortunately, detectors using powders are difficult to assemble and the light yield from powders is less than satisfactory. Single-crystal ZnO of sufficient size has only recently become available. New applications for D-T neutron generators require better timing resolution and higher count rates than are currently available with associated particle detectors using YAP:Ce as the scintillator. Recent work suggests that ZnO-based scintillators can provide alpha-particle-excited light yields comparable to YAP:Ce scintillators. ZnO-based polycrystalline ceramic scintillators offer the advantages of high light yield, ease of fabrication, low cost, and robust mechanical properties. Precursor powders used in these studies include ZnO and ZnO:Ga powders synthesized using solution-phase, urea precipitation, and combustion synthesis techniques as well as ZnO powder from a commercial vendor. Precursor powders have been sintered using uniaxial hot pressing and spark plasma sintering techniques. Photoluminescence measurements have confirmed that, for most samples, the emissions from these sintered bodies consist primarily of slow, visible emissions rather than the desired sub-nanosecond near-band-edge emissions. Subsequent hydrogen treatments have shown significant improvements in the luminescence characteristics of some ceramic bodies, while other samples have shown no change in luminescence.

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The goal of consolidating powders to achieve high densities at lower temperatures and with a small grain size has motivated considerable efforts in the search for methods to activate the sintering process. Enhancement of the consolidation process has been attempted through various approaches including mechanical activation of the powders, the addition of sintering aids, and the use of electromagnetic fields. The latter approach has received considerable attention in recent years, largely due to the widespread use of devices utilizing current and pressure to consolidate powders. The Spark Plasma Sintering method (also known by other names) has seen a remarkable increase in its utilization over the past two decades. This was largely due to the many significant, and in some cases, unique accomplishments. In this chapter we will focus then on the role of the electric field in sintering with emphasis on recent observations, particularly those pertaining to the consolidation of nanostructured materials.