THE NUCLEATION OF MICROCELLULAR FOAMS IN SEMI CRYSTALLINE THERMOPLASTICS*

The nucleation of microcellular foams in amorphous thermoplastics has been performed by supersaturation with gas at an elevated temperature. Pressure and temperature are then carefully reduced in the vicinity of the glass transition temperature of the material. The result is a foam structure with cells on the order of 10 microns. This material exhibits greatly increased impact strength, as well as thermal and electrical insulation properties as compared to conventional foams. A new process has been developed to produce microcellular foams in semi-crystalline polymers. The process operates in the vicinity of the polymer's melting point, as opposed to the glass transition point. This is due the low solubility of the gases in and the rigidity of the crystalline phase. Microcellular foams have been produced successfully in polypropylene. The effects of various additives have been investigated experimentally. The theory developed for the amorphous materials has been compared to these new experimental results and again qualitatively agrees.     

Fast Melting of Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.     

Amorphous silicon exhibits a glass transition

Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.     

Optical properties of silicon thin films related to LPCVD growth condition

In this paper we study the changes in the microstructural and optical properties of silicon thin films produced by the variation of the parameters (temperature and pressure) of the low-pressure chemical vapour deposition (LPCVD) process. Silicon thin films prepared by LPCVD on oxidized silicon substrates over a large range of process parameters (T_dep=500-615°C, p_dep=20-100 Pa) have been characterized by Raman spectroscopy, spectroscopic ellipsometry (SE), X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. The phase transition of as-deposited silicon from an amorphous to a crystalline phase via an intermediate mixed phase (few grains in amorphous silicon matrix) can be monitored by the changes in the optical properties and in the Raman spectra. LPCVD parameters, which control the deposition kinetics, are able to influence the optical properties, the structure and/or morphology of the as-deposited LPCVD silicon films. The SE and Raman results prove that it is possible to grow by LPCVD (from pure silane), a silicon film in a (poly)crystalline state at a temperature as low as 500°C.     

A study of the morphology and microstructure of LPCVD polysilicon

The morphology and microstructure of polysilicon films deposited by low-pressure chemical vapour deposition (LPCVD) have been investigated as a function of deposition conditions. The deposition temperature was varied from 540–640C. As-deposited polysilicon films had a rough surface with (110) textured columnar grain structure, while as-deposited amorphous films had a smooth surface. The polysilicon film deposited at the amorphous to polycrystalline transition temperature had an extra-rough, rugged surface with (311) texture. At the transition temperature, the grain structure tended to shift from the polycrystalline to the amorphous state with increasing deposition pressure and film thickness. It was found that nucleation of amorphous film duringin situ annealing at the transition temperature without breaking the vacuum began to occur from surface silicon atom migration, in contrast to a heterogeneous nucleation during film deposition.     

Influence of microstructure and hydrogen concentration on amorphous silicon crystallization

Hydrogenated amorphous silicon samples were deposited on glass substrates at different temperatures by high frequency plasma-enhanced chemical vapor deposition. In this way, samples with different hydrogen concentrations and structures were obtained. The transition from an amorphous to a crystalline material, induced by a four-step thermal annealing sequence, has been followed. Effusion of hydrogen from the films plays an important role in the nucleation and growth mechanisms of crystalline silicon grains. Measurements of hydrogen concentrations, Raman scattering, X-ray diffraction and UV reflectance showed that an enhanced crystallization was obtained on samples deposited at lower substrate temperatures. A correlation between these measurements allows to analyze the evolution of structural properties of the samples. The presence of voids in the material, related to disorder in the amorphous matrix, results in a better quality of the resulting nanocrystalline silicon thin films.     

Hot-wire photonics: materials, science, and technology

The prospect of an integrated photonic technology has fueled an effort to understand the optical properties and to gauge the photonic engineering potential of hydrogenated amorphous silicon-based materials. Of particular interest for photonic engineering is the tunable range of the refractive index in amorphous silicon and the fast and slow light induced optical changes. The advance of photonic-engineered amorphous silicon technology requires an investigation into the relationships among fabrication processes, material properties, and the interrelations among the various optically important parameters. Here, the experimental investigation into H-implant refractive engineered amorphous silicon materials is detailed. Interestingly, the H-implant can interact with the amorphous structure to produce compacting of the structure, which may indicate refractive index increase. In addition, the evolving prospects for an amorphous silicon-based photonic technology will be up-dated. Waveguide-based light valve structures for the further scientific investigation of light induced refractive index change in amorphous silicon and technological applications are described.     

电子束扫描铝硅合金熔凝特征

为研究电子束扫描铝硅合金表面的熔化及凝固特征,基于价电子理论计算了合金内各相的价电子结构;利用金相显微镜、扫描电镜、显微硬度计等测试了试样微观组织与机械性能;分析了凝固后硅相形貌尺寸对材料硬度的影响;探讨了硅相中心产生裂纹的原因.研究表明:硅相及共晶体的结构破坏要比铝的结构破坏难;电子束表面熔凝处理后,试样分为熔化区、过渡区和基体三个区域;且经电子束扫描处理后,硅相得到了明显细化;熔化区内硅相的形貌尺寸小于8.5μm,该区的最高硬度是基体硬度的1.39倍;溶解后硅相周围的密度差是硅相中心产生裂纹的原因.     

Evolution of structural and optical properties of nanostructured silicon carbon films deposited by plasma enhanced chemical vapour deposition

Nanostructured silicon carbon films composed of silicon nanocrystallites embedded in hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition technique using silane and methane gas mixture highly diluted in hydrogen. The structural and optical properties of the films have been investigated by X-ray diffraction, Raman, Fourier transform infrared, ultra violet-visible-near infrared and photoluminescence spectroscopies while the composition of the films has been obtained from nuclear techniques. The study has demonstrated that the structure of the films evolves from microcrystalline to nanocrystalline phase with the increase in radio frequency (rf) power. Further, it is shown that with increasing the rf power the size of silicon nanocrystallites decreases while the optical gap increases and a blueshift of visible room temperature photoluminescence peak can be observed.     

Polycrystalline silicon films on glass grown by amorphous-liquid-crystalline transition at temperatures below 330 °C

An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330 °C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320 °C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy.     

Volume changes during melting and heating of silicon and germanium melts

The temperature dependence of the density of silicon and germanium in the neighborhood of the crystal-melt phase transition is investigated by an improved thermometric method. Changes in volume occurring during transition from the solid to the liquid state are estimated. It is shown that the density increases in the process of crystal-melt phase transition and, accordingly, the specific volume decreases in both silicon and germanium; an increase in pressure noticeably decreases the melting points of both investigated substances. A linear temperature dependence of density in the liquid phase is obtained. The strength characteristics of interatomic bonds are estimated such as the characteristic Debye temperatures and root-mean-square dynamic displacements of atoms from the equilibrium position in the short-range order structure of the melts of the investigated substances. It is shown that the melting process noticeably weakens the cohesive forces between particles and substantially changes the pattern of their oscillation spectrum.     

Interfacial transformations in the a-SiC/a-Si/6H-SiC structure caused by high-temperature (1500°C) annealing

We have studied the reactions that take place at interfaces in an a-SiC/a-Si/6H-SiC sandwich structure, which was obtained by the sequential deposition of amorphous silicon (a-Si) and amorphous silicon carbide (a-SiC) onto a 6H-SiC substrate by ion sputtering in vacuum and then annealed at 1500°C (i.e., above the melting point of silicon). It is shown that the annealing leads to complete îdissipationî of the silicon film in SiC, probably as a result of the dissolution of carbon in the silicon melt and the diffusion of silicon into SiC.     

In-vitro cellular behavior on amorphous carbon containing silicon

Amorphous carbon has been studied for its biological properties. Doping can alter the film structure to achieve certain desirable properties, like lowering of stress. The incorporation of a secondary element however also alters the film surface properties, which in turn, the biological response. We have investigated the response of fibroblast and osteoblast-like cells on amorphous carbon and amorphous carbon containing silicon. The amorphous carbon with the highest silicon concentration exhibits higher surface energy, with higher polar component. Both cell types adhered, spread and proliferated well on all films, especially on the one with the highest silicon content.     

Synthesis and properties of novel materials at high pressure and temperature—molecular dynamics simulation studies

Molecular dynamics simulations, in combination with lattice dynamics studies, based on semiempirical interatomic potentials, have been very useful in the study of properties of complex novel materials at high temperature and pressure. Various properties such as the equation of state, elastic and thermodynamic properties, phase transitions and melting have been studied. These studies help in understanding the synthesis of important new and novel materials, especially the amorphous materials, compounds with unusually coordinated atoms, (e.g. with five-coordinated silicon atoms), materials with controlled thermal expansion, etc. A few examples will be discussed from our recent studies.     

新型三明治结构铝诱导非晶硅薄膜低温晶化增强

本文对比探索了新型Al/α -Si/Al三明治结构与传统的Al/α -Si双层结构对铝诱导非晶硅薄膜低温晶化结果的影响,结果表明在相同的制备及退火条件下,前者更利于形成高度晶化的多晶硅薄 .膜.本研究工作利用拉曼光谱、掠入射X射线衍射、光学显微镜、方块电阻等表征方法探讨了薄膜厚度比和退火温度对金属诱导非晶硅薄膜晶化的程度、晶体结构以及电学性能的影响,最终制备出了电阻率仅为2.5 Ω·cm的多晶硅薄膜.     

氢稀释比与衬底温度对硅基薄膜相变及其光电性能影响的研究

采用热丝辅助微波电子回旋共振化学气相沉积法(HWAMWECR-CVD),通过改变衬底温度及氢稀释比制备了系列硅基薄膜,研究了衬底温度及氢稀释比对薄膜由非晶相转晶相相变及其光电性能的影响。研究结果表明,当采用低温制备硅基薄膜时,衬底温度和氢稀释比的提高都有利于非晶相向晶相的转变,但提高氢稀释比对相变的影响更为显著;晶化比越高并不代表薄膜光电性能越好,95%氢稀释比条件下制备的微晶硅薄膜具有优良的光电性能。     

Transport properties of μc-Si:H analyzed by means of numerical simulation

Microcrystalline silicon is a two-phase material. Its composition can be interpreted as grains of crystalline silicon imbedded in an amorphous silicon tissue, with a high concentration of danglind bonds in the transition regions. In this paper, results obtained by means of numerical simulations about the transport properties of a μc-Si:H p-i-n junction are reported. The role played by the boundary regions between the crystalline grains and the amorphous matrix is taken in account, and these regions are treated similarly to a heterojunction interface. The influence of the local electric field at the grains boundary transition regions on the internal electric configuration of the device is outlined under illumination and applied external bias.     

Effect of processing parameters on the microstructure and mechanical behavior of silica-calcium phosphate nanocomposite

Silica-calcium phosphate nanocomposite (SCPC) is a bioactive ceramic characterized by superior bone regenerative capacity and resorbability when compared to traditional bioactive ceramics. The aim of the present study is to evaluate the effect of processing parameters on the microstructure and mechanical properties of SCPC. Cylinders were prepared by pressing the ceramic powder at 200, 300 or 400 MPa and sintering at 900, 1000 or 1100°C for 3 h, respectively. XRD results indicate that the crystalline structure of the material is made of β-NaCaPO₄ and α-cristobalite solid solutions. The increase in sintering temperature results in an increase in the grain size and the formation of a melting phase that coats the grains. TEM analyses reveal that the melting phase is amorphous and rich in silicon. The mechanical properties of SCPC cylinders are dependent on the content of the melting phase and the microstructure of the material. The ranges of compressive strength and modulus of elasticity of the SCPC are 62–204 MPa and 6–14 GPa, respectively, which are comparable to those of cortical bone. The results suggest that the interaction between crystalline and amorphous phases modulated the mechanical behavior of SCPC. It is possible to engineer the mechanical properties of SCPC by controlling the processing parameters to synthesize various fixation devices for orthopedic and cranio-maxillofacial applications.     

Laser stimulated low-temperature crystallization of amorphous silicon

Laser annealing of amorphous silicon (a-Si) at different initial temperatures (77 and 300 K) has been studied. It is established that the laser-stimulated crystallization of silicon is possible at relatively low temperatures. A theoretical model is proposed, which explains this phenomenon by melting via the electron mechanism followed by recrystallization.     

Thermodynamic properties of amorphous silicon investigated by pulsed laser heating

Nanosecond (=347 nm, =25 ns) and picosecond (=532 nm, =20 ps) pulsed laser irradiation have been used to induce surface melting in ion implanted and annealed amorphous silicon layers. Time-resolved reflectivity technique was employed to detect the melting onset, from which the melting temperatures of the amorphous phases have been evaluated. Thermal properties of the relaxed amorphous have also been investigated, and in particular, the differences in the heat capacity and in the thermal conductivity of the relaxed amorphous with respect to the as-implanted one were determined. Using these results, the free energy diagram of both relaxed and unrelaxed amorphous silicon has been constructed.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.