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131.
High-Thermal-Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors 总被引:8,自引:0,他引:8
T. Barrett Jackson Anil V. Virkar Karren L. More Ralph B. Dinwiddie Jr. Raymond A. Cutler 《Journal of the American Ceramic Society》1997,80(6):1421-1435
Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al2 O3 ), as is clearly demonstrated in the aluminum nitride-yttria (AlN-Y2 O3 ) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln2 O3 : Al2 O3 , where Ln is a lanthanide element) ratios, with samaria (Sm2 O3 ) and lutetia (Lu2 O3 ) being the dopants that give the highest- and lowest-thermal-conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution-reprecipitation and bulk diffusion. Removal of grain-boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase-sintered AlN ceramics. 相似文献
132.
Angela S. Rocha Arnaldo C. Faro Jr. Laetitia Oliviero Jacob Van Gestel Franoise Maug 《Journal of Catalysis》2007,252(2):321-334
The activity of NiMoS catalysts supported on niobia, alumina, and niobia/alumina was compared for the thiophene hydrodesulfurization (HDS) and 2,6-dimethylaniline (2,6-DMA) hydrodenitrogenation (HDN) reactions. To evaluate the acidity of the supports and identify the nature of the sulfide sites, adsorption of 2,6-dimethylpyridine, pyridine, and CO was performed and followed by IR spectroscopy. This study has shown that with niobia as a support, the activity of NiMoS catalysts in thiophene HDS and in HDN of 2,6-DMA was no longer promoted by the synergy between Ni and Mo. The absence of synergy between molybdenum and nickel on niobia can be explained by the strong interaction of each metal with niobia at the expense of interaction with each other. Moreover, it has been shown that on a niobia/alumina support, the formation of the NiMoS phase can be directly linked to the presence of alumina not covered by niobia. However, niobia is an interesting support for the HDN of 2,6-DMA, because it favors the formation of xylene through direct ammonia elimination involving low H2 consumption. The activity for xylene formation on niobia is linked to the electron-deficient nature of the Mo sulfide site, as demonstrated by CO adsorption followed by IR. 相似文献
133.
J. Mustafa M. S. Ahmad Jr. A. Rauf S. M. Osman 《Journal of the American Oil Chemists' Society》1990,67(12):937-939
Branched-chain thioethers have been prepared from methyl 4-oxo-trans-2-hexadecenoate and 9,12-dioxo-trans-10-octadecenoic acid. The reagents involved in these preparations were mercaptoacetic and mercaptopropionic acids. The yields
of these thioethers are almost quantitative. 相似文献
134.
Phenolic acids and related compounds were separated by gas chromatography using three separate columns. One of these columns was coupled to a Fourier transform infrared spectrometer. The trimethylsilyl derivatives could be separated and identified by comparing the relative retention times of the three different columns. However, where there was overlap, the accompanying infrared data clearly distinguished between the questionable derivatives, thus enabling characterization of all derivatives.The use of trade, firm, or corporation names in this article is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture or the Forest Service of any product or service to the exclusion of others that may be suitable. 相似文献
135.
G. SCOTT GLAESEMANN JOHN E. RITTER Jr. KARL JAKUS 《Journal of the American Ceramic Society》1987,70(9):630-636
Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency. 相似文献
136.
Synthesis and Microstructure of Highly Oriented Lead Titanate Thin Films Prepared by a Sol-Gel Method 总被引:1,自引:0,他引:1
Chiahong Chen Daniel F. Ryder Jr. William A. Spurgeon 《Journal of the American Ceramic Society》1989,72(8):1495-1498
Thin films of PbTiO3 were deposited on fused silica, resistor-grade alumina, and single-crystal (100) MgO by a sol–gel processing method. Whereas the films deposited on silica and alumina substrates were randomly oriented and polycrystalline, highly {100} oriented PbTiO3 films were grown on the MgO single crystals. The perovskite-type structure was observed with films deposited on the single-crystal MgO and annealed at temperatures as low as 470°C, whereas a pyrochlore-type strcuture was observed with films on fused silica and alumina processed in a similar manner. All films heat-treated at temperatures in excess of 570°C showed significant formation of a second PbTi3 O7 phase. The films were characterized by electron microscopy and glancing-incidence-angle X-ray diffraction. 相似文献
137.
The Thermodynamic Properties of Silicon Oxynitride 总被引:1,自引:0,他引:1
A critical assessment of thermal and equilibrium data for silicon oxynitride (Si2 N2 O) is presented. Selected values for the heat of formation of Si2 N2 O from the elements and the absolute entropy of Si2 N2 O at 298.15 K are ΔH0 f,298 =−947.7 ± 5.4 kJ/mol and S0 298 = 45.35 ± 0.4 J/mol·K, respectively. A table of thermodynamic functions for Si2 N2 Ofrom 298 to 2500 K, which has been calculated from the analysis of the literature data, is also presented. 相似文献
138.
Shen TJ; Ho NT; Zou M; Sun DP; Cottam PF; Simplaceanu V; Tam MF; Bell DA Jr; Ho C 《Protein engineering, design & selection : PEDS》1997,10(9):1085-1097
A hemoglobin expression system in Escherichia coli is described. In order
to produce authentic human hemoglobin, we need to co-express both
methionine aminopeptidase and globin genes under the control of a strong
promoter. We have constructed three plasmids, pHE2, pHE4 and pHE7, for the
expression of human normal adult hemoglobin and a plasmid, pHE9, for the
expression of human fetal hemoglobin, in high yields. The globin genes can
be derived from either synthetic genes or human globin cDNAs. The extra
amino-terminal methionine residues of the expressed globins can be removed
by the co-expressed methionine aminopeptidase. The heme is inserted
correctly into the expressed alpha- globin from our expression plasmids. A
fraction (approximately 25%) of the heme is not inserted correctly into the
expressed beta- or gamma- globin. However, the incorrectly inserted hemes
can be converted into the correct conformation by carrying out a simple
oxidation-reduction process on the purified hemoglobin molecule. We have
investigated the functional properties of the expressed hemoglobins by
measuring their oxygen-binding properties and their structural features by
obtaining their 1H-NMR spectra. Our results show that authentic human
normal adult and fetal hemoglobins can be produced from our expression
plasmids in E. coli and in high yields. Our expression system allows us to
design and to produce any recombinant hemoglobins needed for our research
on the structure-function relationship in hemoglobin.
相似文献
139.
Thomas J. Brumm Charles R. Hurburgh Jr 《Journal of the American Oil Chemists' Society》1990,67(5):302-307
Interest in marketing soybeans on the basis of protein and oil content is increasing. Producers, breeders, handlers and buyers
of soybeans need a method of evaluating soybean lots of different composition. A model is presented that predicts, given soybean
composition and processing conditions, the yield of crude soybean oil and soybean meal from the processing of soybeans in
a solvent extraction plant. From these yields, an estimated processed value (EPV) was calculated. For one set of price conditions,
the EPV of typical soybeans had a range of $0.93 per bushel if premiums were paid for meal protein in excess of specifications
and a range of $0.53 per bushel if meal protein premiums were not paid. Trading rules established by the National Oilseed
Processors Association for domestic meal markets have a significant effect on the value and composition of soybean meal. 相似文献
140.
Joseph Eng Jr. Bernd Frühberger Jingguang G. Chen Brian E. Bent 《Catalysis Letters》1998,54(3):133-140
The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH2 group of isobutene decomposes upon heating to 150 K, producing either a /-bonded isobutenylidene [(CH3)2CCH] species or a 1,1-di-/-bonded isobutenyl [(CH3)2CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH3 or CH2 groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH3)2HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces. 相似文献