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981.
Boron nitride (BN) is a new polymer processing aid which not only eliminates surface melt fracture in the extrusion of molten polymers, but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values that depend on resin type and additive concentration. In this work, the influence of BN as a polymer processing additive is first examined in the extrusion blow molding of high‐density polyethylene (HDPE) resins in order to evaluate its usefulness and performance in operations other than continuous extrusion. The equipment used includes both a Battenfeld/Fisher 50‐mm extrusion blow molding machine and a parallel‐plate rheometer. Two types of HDPE, which are blended with boron nitride at various concentration levels, are tested accordingly. It is found that the degree of BN dispersion, characteristics of the HDPE resins, extrusion temperature, and induction time play an important role in eliminating melt fracture. Finally, the influence of combining BN with fluoroelastomer, as an enhanced and potentially better processing aid on the melt fracture of a third HDPE is examined. It is found that such a combination is a superior processing aid that allows extrusion blow molding at very high shear rates. 相似文献
982.
The placement of mine tailings at depth in the marine environment is reviewed. This review considers first the engineering aspects of a pipeline system which transports tailings slurry from a process plant to the coast and then to a location at a depth of typically 200 m on the continental slope. Next, the fluid dynamic behaviour of the tailings, once discharged from the end of the pipe, are discussed, and a numerical model is presented which simulates the fate of both the dissolved and solids component of the tailings slurry in the marine environment. This latter model can be used to estimate the environmental impact of a proposed deep sea tailings placement system. 相似文献
983.
Summary A spirobislactone, 3,3-spirobiphthalide (SBP), has been copolymerized with the diglycidyl ether of bisphenol A (DGEBA), and the properties of the products have been evaluated and related to the proportion (1–15%) of SBP monomer used. Polymers were characterized by infrared spectral analysis,13C NMR, thermal analysis, and mechanical testing. Polymers containing SBP have lower glass transition temperatures and higher decomposition temperatures than DGEBA polymers cured with phthalic anhydride (PA). Materials with higher SBP content have higher Talpha values. Mechanical properties of the SBP copolymers are similar to those of DGEBA-PA polymers. 相似文献
984.
This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low-voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed. 相似文献
985.
Microstructural Coarsening During Sintering of Boron Carbide 总被引:8,自引:0,他引:8
Stephen L. Dole Svante Prochazka Robert H. Doremus 《Journal of the American Ceramic Society》1989,72(6):958-966
The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density. 相似文献
986.
Current trends in spark ignition engines are toward multivalve engines capable of operating at relatively high speeds to maximize performance and fuel economy at acceptable emissions levels. Structural ceramics, with their low density and good high-temperature properties, appear to offer potential as an alternative valve material to the steels, superalloys, and titanium alloys currently in use. Progress to date on materials development, component design, component performance, and engine testing indicates that silicon nitride valves will operate satisfactorily. 相似文献
987.
A test system based around a thin‐walled extruded solid electrolyte tubular reactor has been developed, which enables the
fuel reforming catalysis and surface chemistry occurring within solid oxide fuel cells and the electrochemical performance
of the fuel cell to be studied under genuine operating conditions. It permits simultaneous monitoring of the catalytic chemistry
and the cell performance, allowing direct correlation between the fuel cell performance and the reforming characteristics
of the anode, as well as enabling the influence of drawing current on the catalysis and surface reaction pathways to be studied.
Temperature‐programmed reaction measurements can be carried out on anodes in an actual SOFC, and have been used to investigate
the reduction characteristics of different anode formulations, methane activation and methane steam reforming, and to evaluate
the nature and level of carbon deposition on the anode during reforming.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
988.
Marina Villegas Amador C. Caballero Carlos Moure Pedro Durán José F. Fernández Robert E. Newnham 《Journal of the American Ceramic Society》2000,83(1):141-46
Pb(Zn1/3 Nb2/3 )O3 -based ceramics have been prepared by two different processing methods: conventional (PZN-C) and reaction-sintering (PZN-RS). The conventionally prepared PZN-based ceramics densified at lower temperatures (950°C) than the reaction-sintered samples (1100°C), but the perovskite/pyrochlore ratio was always higher in PZN-RS. The presence of a substantial amount of pyrochlore phase in PZN-C ceramics caused a decrease in the electrical properties. The maximum dielectric constant values in PZN-C ceramics were 10%–15% lower than those of PZN-RS, despite a similar average grain size, 7 ± 0.2 μm. The temperature of the maximum of the dielectric constant ( T max ) was lower than that expected from the mixing rule because of the possible formation of Ba–Nb clusters. The higher chemical homogeneity in PZN-RS ceramics is the main reason for the higher dielectric constant, T max and electromechanical response, as well as for the lower difference between T max and the depolarization temperature ( T d ) and the lower diffusiveness parameter (δ). 相似文献
989.
Susan Schonfield Peter Frost Robert Cramb Chris Royle 《Journal of Automated Methods and Management in Chemistry》1989,11(1):22-27
The Nova Stat Profile 1 analyser, a combined sodium, potassium,
ionized calcium, haematocrit and conventional blood gas analyser,
was evaluated over a four month period. In addition to assessing
and demonstrating that the instrument met analytical requirements,
an appreciation of the use of ionised calcium (iCa) was made.
Prospective costs were characterised and practical problems of iCa
measurement addressed. 相似文献
990.
Robert A. Webster Peter Molnar Stephen E. Kahn 《Journal of Automated Methods and Management in Chemistry》1989,11(6):254-259
The negative interference caused by haemolysis in manual bilirubin
assays contrasts with the positive interference reported for some
automated methods utilizing the same basic chemistry. A
comparison was therefore made of the haemolysis interference
associated with several automated bilirubin methodologies: multilayer
film (Kodak Ektachem- total bilirubin (TBil), direct
bilirubin (DBil), conjugated bilirubin (Bc), unconjugated bilirubin
(Bu),; tableted reagents (Baxter Paramax- TBil, DBil);
continuous flow (Technicon SMAC — TBil). Thirty serum pools
were analysed (five concentrations of bilirubin, 2-229 μmol/l; six
concentrations of haemoglobin, 0.00002-0.052 mmol/1). All
methods, except one (Bc), exhibited both positive and negative
interference, depending upon the relative haemoglobin and bilirubin
concentrations. This interference, at any given haemoglobin
concentration, was neither constant nor proportional with increasing
bilirubin concentration. These complex patterns of interference
suggest that the best characterization of interference is obtained
when (1) both percentage-difference and absolute-difference
‘interferographs’ are plotted; and (2) the interference is determined
at multiple analyte concentrations. 相似文献