Formation of a crystallization courtyard in eutectic systems and crystal growth

The so-called crystallization courtyard is investigated that forms in processes of mass crystallization around the Ge and Si crystals and their solid solutions (Ge+Si) during cooling of hypereutectic alloys in the Ge-Al, Si-Al, and (Ge+Si)-Al eutectic systems. For the first time, data on the composition and microhardness of this crystallization courtyard are given and its role is shown as a stopper of cracking in an Al-(Ge,Si) system during rapid cooling after the heating system is turned off. For the first time, it is suggested that a crystallization courtyard forms in all hypereutectic systems (including every system in which the amount of the taken solvent does not correspond to the eutectic point).     

Protein crystallization in space

The microgravity environment of space is an ideal place to study the complicated protein crystallization process and to grow good-quality protein crystals. A series of crystal growth experiments of 10 different proteins was carried out in space on a Chinese re-entry satellite FSW-2 in August, 1992. The experiments were performed for about two weeks at a temperature of 18.5 +/- 0.5 degrees C using a tube-like crystallization apparatus made in the Shanghai Institute of Technical Physics, Academia Sinica. More than half of 48 samples from 6 proteins produced crystals, and the effects of microgravity on protein crystal growth were observed, especially for hen-egg white lysozyme and an acidic phospholipase A2 from the venom of Agkistrodon halys Pallas. Analyses of the crystallization of these two enzymes in this mission showed that the microgravity environment in space may be beneficial to improve size, external perfection, morphology, internal order, and nucleation of protein crystals. Some of these positive microgravity effects were also demonstrated by the growth of protein crystals in gelled solution with the above two enzymes. A structural analysis of the tetragonal lysozyme crystal grown in space is in progress.     

Sputtering fabrication of large scale ZnO nanobowl arrays using colloidal crystal templates

Ordered ZnO nanobowl arrays over cm2 areas were prepared by magnetron sputtering using the self-assembled polystyrene spheres arrays as templates. The process started with self-assembled sedimentation of there-dimensional (3D) hexagonal polystyrene sphere arrays. By depositing ZnO within the interstitials of 3D polystyrene colloidal crystal templates using magnetron sputtering, large-area ordered ZnO nanobowl sheets were prepared after removing the spheres by annealing. The whole nanobowl sheet could be lifted off, leaving accidentally observed inverse opal structures. The sizes of the nanobowls could be controlled by the size of the polystyrene spheres, the height of nanobowl sheets could be altered by changing sputtering parameters.     

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li₂O·2SiO₂ glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.     

Self-oscillations in processes of volumetric crystallization in nonkinetic regimes of crystal growth with consideration for fluctuations of the growth rate

Periodic processes of the formation of a new phase are investigated in a metastable medium for the case of crystallization from supersaturated solutions when phase inclusions grow with fluctuating rates depending on their sizes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 3, pp. 454–458, September, 1988.     

Effect of crystal growth on the effectiveness of fine purification through counterflow melt crystallization

A method is proposed for calculating the axial impurity profile in a crystallization column and assessing the effectiveness of the purification of a substance via counterflow melt crystallization. The method takes into account the combined effect of diffusion and crystallization processes. An approximate equation for the separation factor is derived. The results obtained for a few systems agree well with experiment.     

Protein crystal growth in microgravity using a liquid/liquid diffusion method

Crystallization of proteins by liquid liquid diffusion method was performed in microgravity using the MDA Minilab aboard the US Space Shuttle. Three proteins, namely lysozyme, trichosanthin, and a new lechin, were crystallized in the space experiment. In contrast to the results of space experiments with a tube-like vapor diffusion method, the crystallization conditions for growing better crystals in space are remarkably different from the conditions optimized on earth. This may be due to difficulties in ground optimization, which are caused by gravity-dependent phenomena, in particular the specific convective flow occurring with liquid liquid diffusion.     

Formation of microcrystalline silicon films using rapid crystal aluminum induced crystallization under low-temperature rapid thermal annealing

The process of obtaining thin film solar cells using the method of aluminum-induced crystallization under rapid thermal annealing (RTA) was investigated. 200-nm-thick amorphous Si (a-Si) film was deposited on a glass substrate using an ultra-high vacuum ion beam sputtering system. A 50-nm-thick crystal aluminum layer was then evaporated and deposited onto the a-Si film. In contrast to conventional furnace annealing, RTA can supply rapid thermal energy so that a-Si can be induced into microcrystalline-Si (μc-Si) in a short time at low temperatures. The crystal Al may promote the crystallization reaction because its surface energy is higher than 0.89 N/m, which is the minimum energy required to produce the (111) orientation. Free Si atoms are induced at the interface of the Al and Si sub-layers by the diffusion of Al along the grain boundaries. The Raman spectrum shows that the sample could be induced to crystallize at 350 °C. After the aluminum was etched, the maximum grain size was 4 μm. The carrier mobility was between 6.2 cm²/Vs and 18.8 cm²/Vs. The proposed method can be used to obtain μc-Si with reduced energy and time during the thermal annealing.     

Cavity formation and metallurgical changes induced by growth of oxide scale

Abstract

Even under conditions of protective oxidation, significant structural changes can develop in the metal substrate. One observation frequently reported is that of subsurface cavitation, but it is not likely that the same physical processes always apply. In some cases, vacancy injection by the removal of cations and subsequent Kirkendall void formation does seem to occur, but it is shown that such injection requires vacancies to have a low binding energy to an oxide/metal interface that has widely spaced sink sites. Other examples of subsurface cavities can be associated with particle dissociation, creep damage, and gas bubble formation. Some metallurgical changes are considered, which can be induced either by the selective removal of cations or by the injection of large amounts of oxygen. The basis of theoretical development is also discussed. Finally, consideration is given to the role of secondary reaction products, specifically carbon and hydrogen, which can cause precipitation when they enter the metal substrate.

MST/952     

Predictive control of crystal size distribution in protein crystallization

This work focuses on the modelling, simulation and control of a batch protein crystallization process that is used to produce the crystals of tetragonal hen egg-white (HEW) lysozyme. First, a model is presented that describes the formation of protein crystals via nucleation and growth. Existing experimental data are used to develop empirical models of the nucleation and growth mechanisms of the tetragonal HEW lysozyme crystal. The developed growth and nucleation rate expressions are used within a population balance model to simulate the batch crystallization process. Then, model reduction techniques are used to derive a reduced-order moments model for the purpose of controller design. Online measurements of the solute concentration and reactor temperature are assumed to be available, and a Luenberger-type observer is used to estimate the moments of the crystal size distribution based on the available measurements. A predictive controller, which uses the available state estimates, is designed to achieve the objective of maximizing the volume-averaged crystal size while respecting constraints on the manipulated input variables (which reflect physical limitations of control actuators) and on the process state variables (which reflect performance considerations). Simulation results demonstrate that the proposed predictive controller is able to increase the volume-averaged crystal size by 30% and 8.5% compared to constant temperature control (CTC) and constant supersaturation control (CSC) strategies, respectively, while reducing the number of fine crystals produced. Furthermore, a comparison of the crystal size distributions (CSDs) indicates that the product achieved by the proposed predictive control strategy has larger total volume and lower polydispersity compared to the CTC and CSC strategies. Finally, the robustness of the proposed method (achieved due to the presence of feedback) with respect to plant-model mismatch is demonstrated. The proposed method is demonstrated to successfully achieve the task of maximizing the volume-averaged crystal size in the presence of plant-model mismatch, and is found to be robust in comparison to open-loop optimal control strategies.     

Effects of oxygen on the growth of Ni induced lateral crystallization of amorphous silicon films

Effects of oxygen on the growth of metal (Ni) induced lateral crystallization (MILC) of amorphous silicon have been investigated. It is found that the oxygen in the annealing ambient did not degrade the MILC length or growth rate. The oxygen existence in Ni film does not degrade the MILC growth rate either. However, it retards the nucleation of poly-Si for about 4 h. This is because that NiO needed an incubation period to be reduced to nickel metal for the subsequent mediated crystallization of a-Si process.     

Secondary grain growth in ammonium nitrate crystallization

Crystallization and ageing of pure ammonium nitrate crystals was studied by scanning electron microscopy. Ageing the crystals beyond 1 h led to the growth of secondary grains along the grain boundaries of primary grains (140–200 m). After 6 h ageing secondary growth along grain boundaries was not observed; instead, distinct clusters with morphologies similar to the secondary grains were formed. The ancillary growth obeys approximately the parabolic relation L=(Kt)^1/n+1 where n=1 justifies grain growth in the pure crystals, and its formation is ascribed to the dissolution of dendrites, edges and corners. The high interfacial activity and interparticle voids can affect the storage and mechanical properties of the material.     

Stability of a crystal surface in fractional crystallization on a band

The problem on the morphological and thermal stability of a crystal surface in fractional crystallization on a band has been investigated theoretically. A model to describe the dynamics of the development of a striped perturbation of the crystal surface is suggested. It is used as a basis for analyzing the influence of the main factors on the stability, and a stability criterion has been derived. It has been established theoretically that the thermal instability must appear at small Reynolds numbers (Re < 10). Numerical experiments have shown that an increase in the length of the band, its inclination angle, as well as a decrease in the rate of melt feed and in the band motion velocity increase the thermal instability. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 106–113, January–February, 2009.     

CoCr-sputtering in large scale roll coating systems

CoCr-layers have been DC-magnetron sputtered onto Polyimide web in a 2 side sputter roll coater for 1.2 m web width. Magnetic as well as crystallographic properties have been investigated as function of the deposition parameters: layer thickness, coating drum temperature, background pressure and angle of incidence. First measurements of electro-magnetic properties also have been made. The influence of different protective layers on the lifetime of a CoCr floppy disk has been checked in first experiments.     

Structural coloring in large scale core-shell nanowires

We demonstrated two complementary size-dependent structural coloring mechanisms, interference and scattering, in indefinitely long core-shell nanowire arrays. The unusual nanostructures are comprised of an amorphous semiconducting core and a polymer shell layer with disparate refractive indices but with similar thermomechanical properties. Core-shell nanowires are mass produced from a macroscopic semiconductor rod by using a new top-to-bottom fabrication approach based on thermal size reduction. Nanostructures with diameters from 30 to 200 nm result in coloration that spans the whole visible spectrum via resonant Mie scattering. Nanoshell coloration based on thin film interference is proposed as a structural coloration mechanism which becomes dominant for nanowires having 700-1200 nm diameter. Controlled color generation in any part of visible and infrared spectral regions can be achieved by the simple scaling down procedure. Spectral color generation in mass-produced uniform core-shell nanowire arrays paves the way for applications such as spectral authentication at nanoscale, light-scattering ingredients in paints and cosmetics, large-area devices, and infrared shielding.     

Solvent induced crystallization in poly(aryl-ether-ether-ketone)

Sorptions of methylene chloride and methylene chloride/n-heptane liquid solutions in poly(aryl-ether-ether-ketone) (PEEK) are analysed and interpreted assuming multiple transport mechanisms. The presence of the highly interacting methylene chloride is responsible for anomalous sorption behaviours. PEEK films immersed in methylene chloride poor n-heptane solutions sorb by non-ideal Fickian diffusion, while limiting Case II and diffusion controlled swelling accompany equilibration in progressively richer methylene chloride solutions. The optical microscopy observation of cryogenically fractured samples conditioned in pure liquid methylene chloride at 5, 20 and 36 ° C for different times, revealed the presence of a sharp moving front. Differential scanning calorimetry (DSC) indicates that the glass transition temperature of the penetrated shells was always lower than the treatment temperature. The high levels of solvent swelling and plasticization were probably responsible for the crystallinity detected in DSC thermograms and wide angle X-ray scattering (WAXS) diffractograms of the solvent treated and initially amorphous samples. The differences of WAXS diffractograms of samples crystallized by immersion in methylene chloride at 5 and 36 ° C for various times and by thermal treatment are discussed. Increasing levels of crystallinity were induced in the samples equilibrated in progressively richer methylene chloride solutions.     

Stress induced crystallization of hydrogenated amorphous silicon

Hydrogenated amorphous silicon films were grown on to thermally oxidized silicon wafers by Radio Frequency magnetron sputtering, and SiN_x and Al₂O₃ capping layers were used to control the residual thermal stress. After annealing, a comparison of the silicon films with and without capping layers indicates that tensile stress induced by the capping layer enhances the crystallinity of the annealed amorphous silicon film. The stress is due to the mismatch between the coefficients of thermal expansion for the capping layer and amorphous silicon film. These results highlight the potential of thermal stress as a means to alter the crystallization in thin film architectures and suggest that even larger effects can be obtained with suitable choices of capping layer chemistry.     

Generation of large scale circulation in turbulent convection

In a horizontal layer of fluid heated below and cooled above, cellular convection with horizontal length scale comparable to the layer depth occurs in a certain range of Rayleigh numbers. As the Rayleigh number is increased, cellular flow disappears and is replaced by a random array of transient plumes. Upon further increase, these plumes drift in one direction near the bottom and in the opposite direction near the top of the layer with the axes of the plumes tilted in such a way that horizontal momentum is transported upwardsvia the Reynolds stress. With the onset of this large-scale horizontal flow, the largest scale of motion increases from a scale comparable to the layer depth to a scale comparable to the layer width. The conditions for the occurrence and the determination of the direction of this largescale circulation are described in this paper. A simple mathematical model is also described which, like the experiment, has the feature of spontaneous generation of a large-scale horizontal flow,u. A truncated Fourier representation of the streamfunction and temperature field is taken, which allows the possibility of a large-scale flow. The bifurcations of the resulting equations have been studied. The first bifurcation is from the conduction state to steady cellular convection. The second is from steady cellular convection to a steady tilted cell convection which has an Eulerian (but not Lagrangian) horizontal mean velocity,u. The third bifurcation is from steady tilted cells to an oscillatory flow, which may be described as consisting of transient, drifting, tilted plumes. The flow in this regime does have a net Lagrangian horizontal mean velocity. Embedded in this regime are also regions of hysteretic and other regions of chaotic flow. A qualitative similarity to the observed flows is noted.     

结晶温度对左旋聚乳酸的晶体改性和晶体形貌的影响

以左旋聚乳酸(PLLA)为原料,制备了无定型PLLA膜及无定型PLLA在不同温度下等温结晶的样品。利用红外光谱(FT-IR)、X射线衍射(XRD)和偏光显微镜(POM)分析了结晶温度的改变对PLLA样品的晶体改性及晶体形貌的影响。研究结果表明,结晶温度的改变对PLLA样品的晶体结构、晶型转变及晶粒形貌特征产生了较大的影响。当结晶温度低于100℃时,PLLA主要以无序的α’-晶型存在;随着结晶温度的增加,PLLA的晶体结构和晶型开始转变,当结晶温度超过120℃后,PLLA主要以α-晶型的形式存在;而当结晶温度在100～120℃范围内,则形成的是α’-与α-晶型PLLA的混合形式存在。另一方面,POM照片测试结果表明,随着结晶温度的升高,PLLA的晶体形貌发生了改变,晶粒尺寸随温度的增加而增加,结晶速度增加,晶粒数量减少,PLLA晶体由α’-向α-晶型发生了转变。