摘要:

Considerable progress has been made toward the goal of diamond synthesis by chemical vapor deposition (CVD). This progress consists of improved methods for synthesis and understanding how diamond is formed. The field has rapidly expanded with industrial consortia, international joint ventures, specialized journals, novel methods for synthesis and processing, and the introduction of new products. Despite this expansion, many issues remain unresolved, generating considerable debate within the research community. Among these debates is the question of how diamond is formed. Both thermodynamics and kinetics are frequently debated at the many world‐wide meetings on this technology. The resolution of these issues awaits further progress and, with improved understanding, may have implications for the synthesis of other ceramic materials. The diamond research community includes not only ceramists and other materials scientists, but specialists in subjects that range from chemistry and chemical engineering to solid‐state physics and electrical engineering. Crystals are grown using methods that range from the use of high‐power plasmas and the combustion of acetylene in oxygen to the thermal decomposition of fluorocarbons and various hydrocarbons in the presence of fluorine. Although some evidence exists for diamond heteroepitaxy, the goal of large‐area heteroepitaxial diamond has proved elusive.Thick (>100 μm), free‐standing, polycrystalline diamond layers are being grown, and their properties rival those of natural crystals. Methods have been developed for the cutting, polishing, and brazing of diamond, and products are being tested in the marketplace. Engineering of the diamond–substrate interface for acceptable adhesion and reliability has progressed, although much work remains to be done. The central issues for commercialization are less the question of whether diamond can be grown in sufficient amounts or with sufficiently attractive properties, but rather whether the fabrication methods can be made sufficiently cost‐effective for the mark

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04411.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Polyacrylic acid (PAA) has been chosen to stabilize BaTiO3powder in aqueous solution. Zeta potential studies show that the Ba‐rich particle surface is positively charged in the pH range from 2 to 11 without PAA. Adsorption of PAA decreases the zeta potential in acidic solution and changes the surface to a negative charge in basic solution. Adsorption of PAA onto BaTiO3surfaces is found to be related to the pH‐dependent PAA conformation in solution. The amount of PAA adsorbed at pH 1.5 is one order of magnitude more than at pH 10.5, due to a more compact polymer conformation. Colloid stability of aqueous BaTiO3suspension is related to the dissociation of PAA as a function of pH. BaTiO3suspensions can be stabilized at pH values higher than 7, where more than 99% of carboxylic groups are ionized. At pH 10.5, suspension stability is related to the PAA coverage on BaTiO3particle surface. Stabilization can be achieved only when conditions of both PAA ionization and BaTiO3surface coverage are satisfied, suggesting an electrosteric stabilization mechanism. Green density of 62% can be achieved by slip casting at a pH above 10, compared to the best density of 58.6% by isostatic pressing at 242

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04412.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

We describe a new laboratory‐scale aerosol process for making submicrometer AIN powder by reaction of gas phase Al(C2H5)3and NH3. The process can produce spherical, near‐stoichiometric AIN powder with mean particle diameter ranging from 0.07 to 0.20 μm and polydispersity index between 0.2 and 0.5. The inlet TEA concentration is found to have the greatest effect on particle size, with the furnace temperature and carrier gas flow rate having lesser influence. Our results appear to favor a LaMer‐type mechanism (particle nucleation, followed by condensation growth) over fusion‐coalescence to explain particl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04413.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The lattice oxygen content in sintered polycrystalline aluminum nitride substrates was measured via secondary ion mass spectrometry (SIMS). This was achieved by quantitative analysis of spatial and depth‐resolved ion images of polished specimens using the solid‐state standard addition method. The thermal conductivity of the polycrystalline material, measured with the laser flash technique, was found to be strongly correlated with the oxygen content in the AIN grains. This dependence is similar to that observed in single‐crystal studies and is consistent with the phonon scattering model of AIN thermal conductivity. Scanning electron microscopy and SIMS images of a variety of other species (C, F, Cl, Y, Si, and Ca) were also obtained. In general, impurities were localized within second‐phase regions although calcium was also found to be distributed uniformly along AIN grain boundaries. Other impurity constituents are also di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04414.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The phase diagram of the system Sr–Cu–O has been determined at 1173 K. Several compositions in the ternary system were quenched after equilibration, and the phases present were identified by optical microscopy, X‐ray diffraction, and electron proble microanalysis. Four ternary compounds, SrCu2O2, Sr2CuO3, SrCuO2+δ, and Sr14Cu24O41, have been identified. Values for the nonstoichiometric parameter δ in SrCuO2+δcorresponding to the different three‐phase fields have been determined using the hydrogen reduction technique. The thermodynamic properties of all the ternary oxides were determined using solidstate cells incorporating either oxide or fluoride electrolyte. The results from both types of cells were internally consistent. A discrepancy in the recent literature regarding thermodynamic properties of Sr2CuO3has been resolved. The compound Sr14Cu24O41has been identified in some of the earlier investigations as Sr3Cu5O8+z˙Using a novel variation of the galvanic cell technique, in which the emf of a cell incorporating a fluoride electrolyte is measured as a function of oxygen potential of the gas phase in equilibrium with the condensed phase electrode, the formula Sr14Cu24O41has been confirmed. The compound does not have significant nonstoichiometry. The phase relations in the system Sr–Cu–O have been evaluated as a function of temperature and oxygen partial pressure from the the

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04415.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of pH, time, valence, and radius of the activator cation on the reaction products and microstructure of ground granulated iron blast‐furnace slag were studied by thermogravimetry and derivative thermogravimetry, X‐ray diffractometry, scanning electron microscopy, and energy dispersive X‐ray microanalysis. Blast‐furnace slag was activated by alkali‐metal hydroxides of Li, Na, and K (12.34 pH 14.71) and alkaline‐earth hydroxides of Ca, Sr, and Ba(12.47 pH 13.53) using a water/slag ratio of 0.4 and curing for 1 day to 26 months. Reactivity of the slag was more strongly dependent on pH than on time. The reaction products were mainly varieties of C─S─H, (C,M)4AH13, and minor amounts of Ca(OH)2and C2ASH8(strätlingite). The nature of C─S─H was dependent on pH. A 1.2‐nm peak appeared in X‐ray diffractograms only when the activation pH was ⌣14.7. Water was present in the C─S─H in a way similar to water in tobermorite and synthetic C─S─H. Leached Ca from unreacted blast‐furnace slag was present around the glass particles as an amorphous layer which crystallized into Ca(OH)2with time; Mg behaved similarly. The effect of the charge or ionic radius of th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04416.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

In the first two papers of this series, a model, cosistent with observations, was proposed for the densification of a ceramic powder matrix in the presence of dense inclusions. One aspect of the model concerned the breakup of grain bridges linking polycrystalline clusters. This breakup phenomenon results in the formation of large cracklike voids. The mechanism that produces this breakup phenomenon, which we calleddesintering, is the subject of this third paper, where we (1) present further experimental results concerning the breakup of polycrystalline bridges that undergo grain growth and (2) calculate the free energy of two evolving bridge configurations. The instability of grain bridges is shown to be a function of the grain size to bridge length ratio. That is, when the ratio exceeds a critical value, the bridge will break up, e.g., desinter, to reduce the free energy of the system. This instability condition can be independently achieved either by grain coarsening at constant bridge length or by extending the bridge length under an applied tensile stress at constant grain size.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04417.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The microstructures of sapphire single crystals explosively shocked at pressures of 5, 12, and 23 GPa, with the basal plane perpendicular to the shock wave direction, have been studied using transmission electron microscopy. There was no evidence of microplastic deformation, dislocations or twins, for crystals shocked at 5 GPa. Perfect dislocations with 1010 and ⅓1120 Burgers vectors on the basal planes were observed at the higher pressures. The gliding of 1010 dislocations on the basal plane established the activation of {0001}1010 slip, which has not been experimentally verified previously. In addition to dislocation generation and motion, a significant number of basal twins were observe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04418.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Two sintered SiC materials and a siliconized SiC materials were exposed to two gas compositions in the N2–H2–CO system at 1000°–1350°C, 133 Pa and 98.7 kPa for 8–1000 h. The two gases were a nitrogen‐based gas which contained 98.2% N2, 1.2% H2, and 0.6% CO; and an endothermic gas which contained 37.8% N2, 41.1% H2, and 21.1% CO by volume. Thermodynamics of reactions were modeled using the program SOLGASMIX. Kinetic studies were based on weight change measurements obtained from vacuum TGA and long‐term tube–furnace experiments. In the case of a siliconized SiC, reactions were complex, owing to its two phase composition and to the presence of impurities. Under many conditions, silicon was rapidly etched from the surface of the siliconized SiC during a short initial period due to the formation of SiO and carbon or SiO and β‐SiC. In the case of sintered mateials (and the siliconized material during the later stages of corrosion), etching of SiC was observed under many of the conditions studied, due to the apparent formation of SiO, HCN, and/or carbon. Corrosion in an endothermic gas was typically more severe than in a nitrogen‐based gas,

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04419.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Three SiC materials were exposed to two gas compositions in the N2–H2–CO system at 1200° and 1300°C, 133 Pa and 98.7 kPa for 50–1000 h. The durability was evaluated by fracturing bend bars at room temperature and C‐rings at room temperature and 1300°C in argon. Fractography was used to correlate changes in strength with changes in flaw populations. In all three materials, surface pitting was the primary cause of degradation. Under some conditions it was found that flaw healing and new flaw formation occurred simultaneously. C‐rings cut from tubes did not lose strength, although pitting occurred; however, machined bend bars lost strength under identical conditions. In the case of bend bars, following an initial loss of strength (∼10%–50%) after 50–150 h, there was typically no further loss observed with increased exposure time. Due to its higherPco, degradation in an endothermic gas was generally more severe than that in a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04420.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY