Si在纯Fe及低硅钢中扩散行为

采用磁控溅射方法,分别在纯Fe以及低硅钢基片上沉积富Si膜,并对其进行真空扩散热处理.通过能谱分析及X射线衍射研究了Si在纯Fe与低硅钢基体中的扩散特征,运用DICTRA软件建立了扩散模型.研究发现Si在纯Fe基体中扩散时发生γ-Fe(Si)→α-Fe(Si)相转变,扩散速率受控于相界面的迁移.当沿截面Si含量梯度不足以驱动相界面正向迁移时,延长扩散时间会发生相界面回迁现象,最终趋于单一相内均匀化扩散过程.Si在低硅钢基体中的扩散符合Fick扩散第二定律.      

Novel instrumentation for measurement of diffusion coefficient by capillary zone electrophoresis and its application to aqua lanthanide ions

The novel, modified instrumentation of a capillary zone electrophoresis (CZE) system and theory are proposed for the measurement of diffusion coefficient. The solute zone in capillary tubing is forced back and forth repeatedly by the aid of negative and positive applied high voltage, respectively, and the peak widths are recorded for the estimation of diffusion coefficient. The diffusion coefficients of Ce(III) and Cu(II) in aqueous solution were determined, and the former hydration number of Ce(III) was estimated by comparison with both diffusion coefficients.     

Enhancing the Homogenized Diffusion of Chromium in Alloy Steel Ingot by Forging

The effects of forging on the diffusion of chromium (Cr) in alloy steel ingot are studied by diffusion model with finite element method (FEM) and verified by the experimental observations. Calculated results show that forging can enhance the diffusion of Cr in alloy steel ingot during the homogenization treatment, which is attributed to the geometry change and the increased diffusion coefficient caused by recrystallization. The geometry change caused by the forging is a factor to increase the diffusivity (from the high concentration region to the low one) of Cr. But, the diffusion coefficient of Cr in the deformed sample is obviously enhanced, the diffusivity is two orders of magnitude higher than that caused by geometry change. The enlarged diffusion coefficient is originated from the recrystallization, which plays a major role in enhancing the diffusion during the homogenization treatment. The experimental results are confirmed with the calculated predicts, the calculated deviation is less than 7%. It is shown that the effect of recrystallization is a new mechanism of enhancing the diffusion by forging .The theoretical explanations of the diffusion mechanism enhanced by forging are also proposed.     

Diffusion-induced stresses and plastic deformation

Thin-sheet (<0.2 mm thick in the direction of diffusion) Au/Ag couples undergo plastic deformation bending during interdiffusion at 750 and 850°; the Ag-rich side forms the concave surface of a deformed couple. This bending is caused primarily by diffusion-induced stress and can be prevented by either mechanical constraint during diffusion or mass constraint of the couple (thicker in the diffusion direction). Because the atomic radii of gold and silver are essentially equal, the stresses cannot arise from lattice parameter gradients. Rather, they are produced by processes (such as dislocation climb) associated with the annihilation and generation of vacancies which accommodate mass displacements within the diffusion zone. These nonconservative processes produce volume changes and corresponding stresses in the diffusion zone which are opposed in the transverse direction (normal to diffusion flow) by the elastic mass constraint of the outlying (especially nondiffused) regions of the couple, thereby creating transverse bending stresses across the couple. A simplified stress analysis indicates that when mass constraint is sufficiently small, the couple undergoes bending primarily by low-stress creep, the transverse stress in the outer fiber of a bent couple being approximately 1 MPa (150 psi). A much smaller, secondary contributor to bending is mass which is displaced in a directional normal to the diffusion flux, but this bend-producing mass displacement accounts for <1 pct of the total mass displaced by the operation of vacancy sinks and sources in the diffusion zone. Therefore, even in the thinnest, bending couples, nearly all (>99 pct) of the mass displacement is parallel to diffusion and this causes substantial marker shifting in all the (bending and nonbending) couples. This strong preference for mass displacement to occur parallel (rather than normal) to diffusion is consistent with both minimal mass constraint and the operation of vacancy sinks and sources in regions of maximum (vacancy) chemical stress. Formerly Graduate Student, Departmentof Metallurgical Engineering, Ohio State University, Columbus, OH43210     

High resolution renal diffusion imaging using a modified steady-state free precession sequence

A modified steady-state free precession (SSFP) diffusion sequence is proposed for high resolution renal imaging. A pair of bipolar diffusion gradients was used to minimize the errors in measured apparent diffusion coefficient (ADC) caused by variations in T1, T2, and RF flip angle that have been observed with previously employed SSFP diffusion sequences. Motion sensitivity was reduced by the use of compensated gradients, frame-by-frame averaging, and a repetition time of 22 ms, which for a single-acquisition 128 x 128 image requires only 3 s. High resolution was achieved by signal averaging. The modified sequence was applied to in vivo diffusion measurements. In six normal rat kidneys the ADCs (mean +/- SD; x 10(-3) mm2/s) of the cortex, outer medulla, and inner medulla were 2.28 +/- 0.05, 2.38 +/- 0.10, and 2.95 +/- 0.05, respectively. The technique requires relatively large gradients to achieve adequate diffusion weighting.     

A 129Xe NMR study on an ionomeric polymer blend system

The morphology of an ionomeric polymer blend consisting of an amino-silicone copolymer and zinc neutralized sulfonated polystyrene (ZnSPS) has been studied using proton spin diffusion and small angle X-ray scattering (SAXS). The extent of reaction between the two components in the blend was monitored by 13C CP MAS spectroscopy. All three types of experiment point to domain sizes in the nanometer range. 129Xe NMR was used to study exchange by translational diffusion between domains. A single xenon resonance was detected in temperatures ranging from 25 degrees C to -90 degrees C, and the chemical shift followed a weighted average of the isolated polymer shifts consistent with the small domain sizes. Pulse field gradient 129Xe NMR was used to determine the effective diffusion constants in the amino silicone starting material and the blend. The diffusion constant of xenon in poly(styrene) is known, allowing for comparison of the predictions of effective diffusion constants in the blend based on the values in the constituents of the blend. Simple two-site exchange equations incorrectly predict that diffusion in the blend would be dominated by the constituent with slow diffusion. The blend diffusion constant is close to the value of the amino silicone or the constituent with fast diffusion which is correctly predicted for a rapid exchange solution of the diffusion equation.     

Creep cavity growth under interaction between lattice diffusion and grain-boundary diffusion

A model and a method of numerical analysis of the cavity growth in grain boundaries in metals due to lattice diffusion are proposed. The growth behavior simulated is compared to that due to grain-boundary (GB) diffusion. The simulation method is extended to analyze the growth under the interaction between lattice diffusion and GB diffusion. The growth rate calculated under the interaction is approximately equal to the linear sum of those due to the pure lattice diffusion and the pure GB diffusion.     

Anisotropy of water diffusion in the myocardium of the rat

Pulsed field gradient nuclear magnetic resonance methods combined with nuclear magnetic resonance imaging were used to determine the water diffusion anisotropy in perfused rat hearts at 37 degrees C. It was found that the observed diffusion coefficient D(app) (apparent diffusion coefficient) depends on the orientation of the applied gradient g. When g is parallel to the epicardial surface, the observed diffusivity is D(app) parallel = 1.8 +/- 0.4 x 10(-9) m2.s-1, whereas when g is perpendicular to it, diffusivity is D(app) perpendicular = 2.5 +/- 0.5 x 10(-9) m2.s-1. To better characterize this directional dependence, images of the second-order diffusion tensor D of the myocardium were obtained. These data demonstrate several essential features of cardiac myoarchitecture, including the helicity of fiber orientation with respect to the ventricular axis and the variation of fiber pitch angle with transmural depth. Diffusion anisotropy may be quantified in a coordinate-independent manner by the eigenvalues of the diffusion tensor. In the myocardial midwall, these eigenvalues were E1 = 3.29 +/- 0.57, E2 = 2.01 +/- 0.42, and E3 = 0.77 +/- 0.58 x 10(-9) m2.s-1 (mean +/- SD). These data suggest that myocardial water diffusion is essentially unrestricted parallel to the myofibers. They further show that failure to measure the complete diffusion tensor may lead to substantial underestimates of diffusion anisotropy in the myocardium.     

Measurement of Solute Diffusion Behavior in Fractured Waste Glass Media

Determination of aqueous diffusion coefficients of solutes through fractured media is essential for understanding and modeling contaminant transport at many hazardous waste disposal sites. Development of experimental methods and measurements for the characterization of diffusion in fractured glass media is necessary for the design and performance assessment of glassified radionuclear waste disposal facilities. We report on the use of time-lag diffusion experimental method to assess the diffusion behavior of three different solutes (Cs, Sr, and pentafluoro benzoic acid) in fractured, immobilized low activity waste (ILAW) glass forms. A fractured media time-lag diffusion experimental apparatus, which allows the measurement of diffusion coefficients, has been designed and built for this purpose. Use of time-lag diffusion method, a considerably easier experimental method than the other available methods, was not previously demonstrated for measuring diffusion in fractured waste glass media. Hydraulic conductivity, porosity, and diffusion coefficients of a solute were experimentally measured in fractured glass blocks using this method for the first time. Results agree with the range of properties reported for similar rock media earlier, indicating that the time-lag experimental method can effectively characterize the diffusion coefficients of fractured ILAW glass media. Data presented are rare and useful for the design of vitrified glass disposal facilities.     

Diffusion tensor MR imaging of the human brain

PURPOSE: To assess intrinsic properties of water diffusion in normal human brain by using quantitative parameters derived from the diffusion tensor, D, which are insensitive to patient orientation. MATERIALS AND METHODS: Maps of the principal diffusivities of D, of Trace(D), and of diffusion anisotropy indices were calculated in eight healthy adults from 31 multisection, interleaved echo-planar diffusion-weighted images acquired in about 25 minutes. RESULTS: No statistically significant differences in Trace(D) (approximately 2,100 x 10(-6) mm2/sec) were found within normal brain parenchyma, except in the cortex, where Trace(D) was higher. Diffusion anisotropy varied widely among different white matter regions, reflecting differences in fiber-tract architecture. In the corpus callosum and pyramidal tracts, the ratio of parallel to perpendicular diffusivities was approximately threefold higher than previously reported, and diffusion appeared cylindrically symmetric. However, in other white matter regions, particularly in the centrum semiovale, diffusion anisotropy was low, and cylindrical symmetry was not observed. Maps of parameters derived from D were also used to segment tissues based on their diffusion properties. CONCLUSION: A quantitative characterization of water diffusion in anisotropic, heterogeneously oriented tissues is clinically feasible. This should improve the neuroradiologic assessment of a variety of gray and white matter disorders.     

Formation of intermetallics in a porous powder titanium-nickel diffusion couple

A method of fabricating porous powder Ti-Ni diffusion couples was developed, and the structure of the diffusion zone in these after isothermal annealing at 1073, 1173, and 1273 K studied. Microstructural analysis and microhardness measurements revealed that the width of the diffusion zone and the thickness of the intermetallic layers increased with increasing temperature. Mutual diffusion coefficients were calculated, and an explanation proposed for the more rapid intermetallic layer growth in porous powder than in solid diffusion couples above 1173 K. This was related to the presence of rapid diffusion paths (surfaces, grain boundaries) which persisted at high temperatures in the porous powder diffusion couples, while those in solid couples (point and line defects) were eliminated when the temperature was raised.Samarsk State Aerospace University. Translated from Poroshkovaya Metallurgiya, Nos. 5/6, pp. 62–70, May–June, 1995.     

Innovation Diffusion Modeling in the Construction Industry

The innovation diffusion modeling approach proposes that the diffusion of an innovation in an industry is mainly driven by internal and external influence factors. But the innovation diffusion models used in previous research studies conducted in the construction management literature are excessively restrictive. This research uses a more flexible model called the nonuniform influence (NUI) model to study the diffusion of a technological innovation, namely, computer aided design (CAD) technology in the Turkish architectural design practice; and an administrative innovation, namely, ISO 9000 certification in the Turkish precast concrete industry. The research findings point out that the NUI model performs significantly better than previously used models and that internal rather than external influence plays a predominant role in the diffusion of both types of innovation. The research findings provide additional insights into the diffusion of CAD technology and ISO 9000 certification which could not be captured by the previously used diffusion models. They show that internal influence on the diffusion of CAD technology increases with passage of time and that internal influence on the diffusion ISO 9000 certification decreases with passage of time. The research has academic importance because it addresses the limitations of previous innovation diffusion research and provides quantitative insights into the diffusion of innovations; knowledge that is lacking in the construction management literature. The research is of practical importance because it provides useful observations that architecture/engineering/construction firms can use to understand the presence of imitative behavior in the innovation diffusion process and the role of innovation characteristics in imitative behavior.     

Specimen size effects in DIGM,DIR and RID

Specimen size effect in diffusion induced grain boundary migration (DIGM), diffusion induced recrystallization (DIR) and recrystallization induced diffusion (RID) was studied. The Fisher model of grain boundary diffusion was used, provided the total diffusive impurity flux is reflected from the lower side of film. The limits of grain-boundary migration rates, for which this model is valid, were estimated. The threshold character of DIGM, DIR and RID was shown. Manifestations of specimen size effect of DIGM, DIR and RID below some critical thickness were discussed.     

High-rate carburizing in a glow- discharge methane plasma

Carburizing at high temperature (1040°C) in a glow-discharge methane plasma signifi-cantly reduces carburizing time. Sufficient carbon to produce a 1.0 mm case on conven-tional carburizing steels can be introduced in 10 min at methane pressures in the range of 1.3 to 2.7 kPa (10 to 20 Torr). To reduce the carbon concentration at the surface to an acceptable level (007E1.0 wt pct) the plasma carburizing stage is followed by a short (007E30 min) diffusion step. To aid in optimizing the division of time between the carburizing and diffusion stages, and in separating the effects of plasma carburizing and high-temperature diffusion, a computer program was written to model the diffusion equation for appropriate initial and boundary conditions. Both model and experimental results show that a diffusion time/carburizing time ratio of at least 3: 1 is required. The diffusion model indicates that the exceptionally high carburizing rates observed arise from the rapid infusion of carbon into the surface from the plasma, during the carburizing stage and not from anomalously high diffusion rates. Exceptionally uniform cases on surfaces of irregular geometry are achieved, and significant amounts of natural gas are saved owing to both the reduced car-burizing time and the low pressures employed.     

The clinical application of diffusion weighted magnetic resonance imaging to acute cerebrovascular disorders]

Diffusion is a measure of motion freedom and is a sensitive parameter to characterize the tissue at the microscopic level. The methods of measuring in vivo diffusion by magnetic resonance imaging (MRI) have been based mainly on the addition of two motion-probing gradients (MPG) to the spin echo sequence to produce signal attenuation for the spins moving at random. The resultant MR images reflect the intravoxel incoherent motions (IVIM), which contain both water molecule diffusion and perfusion in the capillary network, and can be quantified by an apparent diffusion coefficient (ADC). Diffusion weighted MRI, acquired from IVIM MR imaging by the addition of the very strong MPG predicate water diffusion and anisotropy. High signal or reduced ADC can be observed in case of the slower diffusion. The anisotropy depends upon the orientation of the subjects and the gradients. Greater signal attenuation (faster diffusion) can be observed when the relative orientation of white matter tracts to the MPG is parallel as compared to that obtained with a perpendicular alignment. This anisotropy may preclude the detection or delineation of an ischemic lesion. Diffusion tensor trace has been designated to eliminate this anisotropy effect. In ischemic animal models, low signal (fast diffusion) and high signal (slow diffusion) have been noted in the vasogenic edema and cytotoxic edema, respectively. High signal appears only in case of cerebral blood flow below 15-20 ml/100 g per minute, a value identical to the threshold of tissue at high energetic metabolism and ion homeostasis. ADC value decreases following the cerebral vessel occlusion, or remains unchanged when collateral circulation develops. It has been speculated that reduction in ADC reflects the water shift from extracellular space to intracellular space due to the membrane permeability and/or intracellular osmolality increase. These results suggest that diffusion weighted MRI correlates well with the cell metabolism, and cytotoxic edema plays an important role in the acute cerebral stroke. In clinical setting of acute cerebral ischemia, diffusion weighted MRI may detect superacute infarction by showing high signal (slower ADC) over the 6 hours following the insult, whereas conventional MRI generally fails to do so. In chronic liquefied cerebral infarction, increased ADC, or attenuated signal are the most frequent findings, suggestive of an elevated diffusion. Therefore, diffusion weighted MRI improves early diagnosis of stroke and help differentiate acute from chronic stroke. One disadvantage of diffusion weighted MRI is motion artifact, which may be reduced by the introduction of a navigator echo to correct for the phase shift caused by the first imaging echo, or by the utility of ultrafast imaging technique, such as echo planar. Another shortcomings is the susceptibility artifact incorporating the diffusion weighted MRI. The eddy current may also result from the strong gradients, producing shiftlike artifact. Such artifacts can be compensated for by appropriate shaping of the current pulses sent into the gradient coils, or by use of shielded gradients. As with rapid progresses in perfusion imaging of ischemia penumbra, misery perfusion and luxury perfusion, new insight into the diffusion weighted MRI will be significant.     

Experimental Investigation of the Ce-Mg-Mn Isothermal Section at 723 K (450 °C) via Diffusion Couples Technique

The isothermal section of the Ce-Mg-Mn phase diagram at 723 K (450 °C) was established experimentally by means of diffusion couples and key alloys. The phase relationships in the complete composition range were determined based on six solid–solid diffusion couples and twelve annealed key alloys. No ternary compounds were found in the Ce-Mg-Mn system at 723 K (450 °C). X-ray diffraction and energy-dispersive X-ray spectroscopy spot analyses were used for phase identification. EDS line-scans, across the diffusion layers, were performed to determine the binary and ternary homogeneity ranges. Mn was observed in the diffusion couples and key alloys microstructures as either a solute element in the Ce-Mg compounds or as a pure element, because it has no tendency to form intermetallic compounds with either Ce or Mg. The fast at. interdiffusion of Ce and Mg produces several binary compounds (Ce _x Mg _y ) during the diffusion process. Thus, the diffusion layers formed in the ternary diffusion couples were similar to those in the Ce-Mg binary diffusion couples, except that the ternary diffusion couples contain layers of Ce-Mg compounds that dissolve certain amount of Mn. Also, the ternary diffusion couples showed layers containing islands of pure Mn distributed in most diffusion zones. As a result, the phase boundary lines were pointing toward Mn-rich corner, which supports the tendency of Mn to be in equilibrium with all the phases in the system.     

Diffusion coefficients in the lateral intercellular spaces of Madin-Darby canine kidney cell epithelium determined with caged compounds

The diffusion coefficients of two caged fluorescent dyes were measured in free solution and in the lateral intercellular spaces (LIS) of cultured Madin-Darby canine kidney (MDCK) cells after photoactivation by illumination with a continuous or pulsed UV laser. Both quantitative video imaging and a new photometric method were utilized to determine the rates of diffusion of the caged fluorescent dyes: 8-((4,5-dimethoxy-2-nitrobenzyl)oxy)pyrene-1,3,6-trisulfonic acid (DMNB-HPTS) and (4,5-dimethoxy-2-nitrobenzyl) fluorescein dextran (10,000 MW) (DMNB-caged fluorescein dextran). The diffusion coefficients at 37 degrees C in free solution were 3.3 x 10(-6) cm2/s (HPTS) and 0.98 x 10(-6) cm2/s (10,000 MW dextran). Diffusion of HPTS within nominally linear stretches of the LIS of MDCK cells grown on glass coverslips was indistinguishable from that in free solution, whereas dextran showed a 1.6 +/- 0.5-fold reduction in diffusivity. Measurements of HPTS diffusion within the LIS of multicellular regions also exhibited a diffusivity comparable to the free solution value. The restriction to diffusion of the dextran within the LIS may be due to molecular hindrance.     

Ion transport studies in calcium alginate gels by magnetic resonance microscopy

Polyelectrolyte biopolymers such as calcium alginate are becoming increasingly important for the recovery of heavy metals from aqueous solutions. To understand the mechanism of ion transport in these biopolymer systems, the transport of copper ions into calcium alginate gels was investigated using proton nuclear magnetic resonance (NMR) microscopy. Copper ion transport was imaged using an inversion recovery technique which utilizes the paramagnetic effect of copper on water proton relaxation times. Diffusion experiments were performed in a diffusion cell designed to approximate a semi-infinite slab geometry at temperatures between 278 and 313 K using copper reservoir concentrations between 10 and 60 mM. The diffusion coefficient of copper in these gels was calculated from the NMR data to fit a combined diffusion-reaction model involving a diffusion term (D) and a kinetic binding term (k). At 23 degrees C, the diffusion coefficients in 1, 2, and 3% (w/v) gels were 3.1 x 10(10), 2.0 x 10(10), and 1.4 x 10(10) m2/s, respectively. The activation energy for diffusion in the 2% (w/v) gel was 28 kJ/mol.     

氢扩散阳极在电解锌中的研究

在湿法炼锌电解过程中,用氢扩散阳极代替传统的阳极,可以达到节能的目的.使用铂催化剂氢扩散阳极作阳极,在电解液中通入H2进行电解试验,节能可达50%左右.同时,介绍了氢扩散阳极的结构、节能原理以及在研制和试验时存在的一些问题.最后,对于氢扩散阳极在湿法冶金上的应用进行了展望.     

More on the speed and accuracy of positive and negative responses.

The account given by the diffusion model (Ratcliff, 1985) of the relation between positive and negative responses has been criticized by Proctor, who argues that the diffusion model provides no theory of criterion setting and that it violates psychophysical principles. In reply, I argue that psychophysical principles are not violated and that a quantitative theory of criterion placement is currently outside the scope of the diffusion model and many other models of memory. I point out that the strength of the diffusion model lies in its ability to account for aspects of the data other than just mean reaction time (RT), and in its generality across experimental paradigms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)