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941.
The release of ibuprofen from mixtures of polyethylene glycol (PEG) with polyvinyl acetate (PVAc) has been studied in vitro and complemented by x-ray diffraction measurements, differential scanning calorimetry (DSC), and melting point determinations via hot-stage microscopy (HSM). Results indicate that ibuprofen release can be affected markedly by alteration of the PVAc concentration. The molecular weight of the PEG and the pH of the dissolution medium are also shown to affect the release profile. Visual observation during the drug release process revealed a complex behavior which included emission of liquidlike droplets, formation of a crust around the releasing mass, and/or production of flakes of solid material. This behavior appeared to have a disadvantageous effect on the reproducibility of drug release. Construction of a phase diagram from results of thermal analysis using DSC and HSM indicated the formation of an eutectic mixture with a composition of 35% ibuprofen and 65% PEG 1500 and a melting point of 36°C. The complex behavior of the drug-releasing mass is discussed in terms of this phase diagram. Only the release data for systems containing 4% w/w or more of PVAc could be linearized by plotting against the square root of time whereas data for all of the systems studied could be linearized by first-order plots. 相似文献
942.
The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation 总被引:2,自引:0,他引:2
D. Emadi J. E. Gruzleski J. M. Toguri 《Metallurgical and Materials Transactions B》1993,24(6):1055-1063
Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an
increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop
method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity
occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid
by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension
and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer
period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores
is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or
higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity
that is associated with modification. 相似文献
943.
Recent work concerning the interstitial modification of permanent magnet alloys based on rare-earth iron intermetallic compounds is reviewed. The crystal structure of the new materials is discussed, then the thermodynamic and statistical aspects of the low-temperature interstitial modification process are considered. Finally, reaction kinetics and thermal stability of the new compounds are discussed and future prospects assessed. 相似文献
944.
This paper presents a new computerized procedure for dealing with the design of horizontal ground heat exchangers (HGHE). The computer program is based on the transient model of coupled nonlinear partial differential equations governing heat and mass flow in soils. The model is two-dimensional and delineates the operation of ground heat storage with the HGHE and such phenomena as freezing/thawing and drying/rewetting of soil moisture. Comprehensive climatological data, such as ambient temperature, solar radiation, wind velocity, rainfall, snowfall, snow characterstics, and water vapour pressure is used to simulate conditions at the ground surface over any required length of time. The package can be applied to any geographical location by changing climatic and soil data input. The designer has the possibility of selecting any of 12 types of soils from sand to clay, 12 commercial heat pumps, nine different configurations of the HGHE, 16 plastic pipes for ground coils, and 13 ground coil fluids. The program, however, does not calculate the length of the HGHE but it evaluates the thermodynamic performance of a ground heat pump system and provides comprehensive data on thermal and hydraulic conditions in ground heat storage. The length of the ground heat exchanger is obtained from a line source theory model or from site dimensions and pipe spacing. Computed results for ground heat exchanger operation correlate fairly well with experimental data. Simulation of temperature and moisture content in the ground for natural conditions (no heat extraction/deposition) showed a fair agreement with field data. The entire computer program is user-friendly, interactive, menu-driven, and written in FORTRAN 77. 相似文献
945.
A BiCMOS dynamic carry lookahead circuit that is free from race problems is presented. A 16 b full-adder test circuit, which has been designed based on a 2 μm BiCMOS technology, shows a more than five times improvement in speed as compared to the CMOS Manchester carry lookahead (MCLA) circuit. The speed advantage of the BiCMOS dynamic carry lookahead circuit is even greater in a 32- or 64-b adder 相似文献
946.
R. H. Howell P. A. Sterne M. J. Fluss J. H. Kaiser K. Kitazawa H. Kojima 《Journal of Superconductivity and Novel Magnetism》1994,7(1):169-173
High-statistics (>4 × 108 counts), room-temperature measurements of the electron-positron momentum density of La2?x Sr x CuO4 have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane. 相似文献
947.
W. W. Charlesworth J. J. Cox D. C. Anderson 《International journal for numerical methods in engineering》1994,37(18):3093-3115
Domain composition, a recently described method for formulating continuum field problems, removes certain restrictions on the construction of finite element models such that it is possible to solve a finite element problem without using a global compatible mesh. The domain composition method couples or otherwise constrains meshes in local regions to obtain a solution equivalent to that produced by conventional finite element methods. In particular, the domain composition method enables finite element models to be formulated with overlapping elements. Several advantages come from this, including an ability to automatically generate a finite element model from a solid geometric model, an ability to use a variety of element types in a single finite element model and an ability to exactly match element boundaries to the local geometry. This paper shows in detail a finite element formulation of Poisson's equation using domain composition and presents certain key algorithms that incorporate the domain composition method into well-established finite element procedures. 相似文献
948.
JE Myers T Mieczkowski J Perel D Abbondanza TB Cooper JJ Mann 《Canadian Metallurgical Quarterly》1994,35(2):112-120
Serotonergic responsivity was assessed in 20 psychiatric patients by the prolactin response to a fenfluramine challenge test. During the fenfluramine challenge 6 of 20 patients (30%) spontaneously reported psychopathologic reactions that included: increased anxiety/agitation, psychotic symptoms, illusions, mood elevation, and anergia. The time of peak behavioral symptoms (2.5 +/- 0.8 hrs) corresponded closely to the time of peak increase in prolactin levels (3.0 +/- 1.1 hr). Abnormal behavioral responders had statistically significant greater increases in prolactin 1 to 4 hr after fenfluramine when compared to normal responders. Patients who developed an abnormal psychopathologic response to fenfluramine were characterized by higher levels of anxiety and agitation at the time of admission to the hospital but otherwise were not distinguishable on the basis of severity of other psychiatric symptoms. This study suggests that increased serotonergic transmission may trigger anxiety, psychosis, and mood elevation in specific vulnerable individuals, whereas other patients with similar psychiatric illnesses are not affected. 相似文献
949.
950.
Muller-Borer B.J. Erdman D.J. Buchanan J.W. 《IEEE transactions on bio-medical engineering》1994,41(5):445-454
Computer simulations were used to study the role of resistive couplings on flat-wave action potential propagation through a thin sheet of ventricular tissue. Unlike simulations using continuous or periodic structures, this unique electrical model includes random size cells with random spaced longitudinal and lateral connections to simulate the physiologic structure of the tissue. The resolution of the electrical model is ten microns, thus providing a simulated view at the subcellular level. Flat-wave longitudinal propagation was evaluated with an electrical circuit of over 140,000 circuit elements, modeling a 0.25 mm by 5.0 mm sheet of tissue. An electrical circuit of over 84,000 circuit elements, modeling a 0.5 mm by 1.5 mm sheet was used to study flat-wave transverse propagation. Under normal cellular coupling conditions, at the macrostructure level, electrical conduction through the simulated sheets appeared continuous and directional differences in conduction velocity, action potential amplitude and V˙max were observed. However, at the subcellular level (10 μm) unequal action potential delays were measured at the longitudinal and lateral gap junctions and irregular wave-shapes were observed in the propagating signal. Furthermore, when the modeled tissue was homogeneously uncoupled at the gap junctions conduction velocities decreased as the action potential delay between modeled cells increased. The variability in the measured action potential was most significant in areas with fewer lateral gap junctions, i.e., lateral gap junctions between fibers were separated by a distance of 100 μm or more 相似文献