Showing papers on "Amorphous solid published in 1986"

The microstructure of sputter-deposited coatings

Abstract: Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This pap...

Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics

Abstract: Amorphous powders of Ni32Ti68 and of Ni45Nb55 were synthesized by mechanical alloying (MA) starting from either a mixture of pure metal powders (in the appropriate molar ratio) or from powders of the crystalline intermetallics NiTi2 and Ni45Nb55, respectively. For both alloys, the peak temperature increase (above the average processing temperature) in the powder particles trapped between colliding balls is estimated at 38 K. Thus, the amorphization is attributed to a process other than the formation of local melts followed by the rapid solidification of these melts into the amorphous phase. The amorphization by MA starting from a mixture of pure crystalline powders is attributed to a solid state interdiffusion reaction, the kinetics of which is controlled by the excess point and lattice defects generated by plastic deformation. The amorphization by MA starting from powders of crystalline intermetallics is attributed to the accumulation of point and lattice defects which raise the free energy of the faulted intermetallic above that of the amorphous alloy.

Raman spectroscopy of SiO2 glass at high pressure.

Abstract: A new technique has been developed for optical studies of amorphous solids to very high pressures. Raman spectra of Si${\mathrm{O}}_{2}$ glass measured at 8 GPa indicate a significant reduction in the width of the Si-O-Si angle distribution, which has been associated with a number of anomalous properties of silica glass under ambient conditions. Between 8 and \ensuremath{\sim} 30 GPa irreversible changes in the Raman spectrum occur that are consistent with a shift in ring statistics in densified glass. The spectra suggest a breakdown in intermediate-range order at higher pressure.

Morphology and structure of highly elastic poly(vinyl alcohol) hydrogel prepared by repeated freezing-and-melting

Abstract: Morphology and structure of poly(vinyl alcohol) (PVA) hydrogel prepared by the “repeated freezing-and-melting” method have been investigated by X-ray diffraction, scanning electron microscopy, light-optical microscopy, and simple tension test. The PVA aqueous solution gelled highly by using this method to show rubber-like elasticity, reflecting the gel network in which the amorphous chains are physically cross-linked by the crystallites. The gel morphology was characterized by the porous structure, which was originated from the gelation of continuous PVA-rich solution phase segregated around copious ice crystal phases formed upon freezing. The high gelling ability involved in this method was closely related to the segregation mechanism.

Compound materials for reversible, phase‐change optical data storage

Abstract: Results of rapid (laser induced) and slow (heating stage induced) crystallization studies on Te1−xGex, 0≤x≤0.6, are reported. The time it takes to laser crystallize varies with x by more than four orders of magnitude. Films with stoichiometric compositions, Te and GeTe, can be crystallized using laser pulses of less than 100 ns duration. Unlike Te, which spontaneously crystallizes at room temperature, GeTe has a crystallization temperature of >150 °C. From these results we argue that, in general, compound materials allow realization of fast‐switching, reversible, phase‐change optical recording media. Furthermore, this fast‐switching capability, from the amorphous to the crystalline state and back, can be attained simultaneously with long term data (amorphous phase) stability, provided the melting temperature of the compound is sufficiently high.

Formation of ultra-fine amorphous alloy particles by reduction in aqueous solution

Abstract: Amorphous alloys are normally prepared as thin ribbons or films by the liquid quench technique or by vapour deposition. Recently we have shown1 that ultra-fine amorphous Fe–C alloy particles can be prepared by thermal decomposition of Fe(CO)5 in an organic liquid; that is, by a chemical reaction. Here we report the preparation of small alloy particles by reduction of metal ions by KBH4 in aqueous solutions. Mossbauer and X-ray diffraction studies show that the particles are amorphous. The amorphous phase is formed because the chemical reaction takes place below the glass transition temperature and because boron atoms are present in the particles. The method may be used for the large-scale production of ultra-fine amorphous alloy particles, which may have applications in ferrofluids, magnetic memory systems and catalysis.

Molecular-dynamics simulation of amorphous germanium.

Abstract: We have developed a set of two-body and three-body potentials for modeling the structure of solid phases of germanium. The potential is of the same functional form as that of Stillinger and Weber for silicon, but it has different values of the parameters. The potential gives an excellent structural representation of amorphous solid Ge as well as crystalline Ge and gives good results for several thermodynamic properties of the crystalline phase and the phonon dispersion relations of the crystal.

Thermal conductivity of amorphous solids

Abstract: The thermal conductivities of glassy solids, when scaled by material parameters, have a similar magnitude and temperature dependence, from the lowest temperatures yet measured, to the melt. Both empirical results and theoretical models are reviewed briefly. It is emphasized that the thermal conductivity of an amorphous solid is not understood in any temperature range.

Astrophysical implications of amorphous ice—a microporous solid

Abstract: Since the observation of the 3-µm band in interstellar infrared sources1–3, vapour-deposited amorphous ice, H2O(as), has been discussed as a major component of comets4–9, of satellites of the outer planets10,11 and of interstellar dust11–13. Some of the physical properties of H2O(as) important for these discussions appear to be contradictory: for example N2 adsorption isotherms at 77 K evaluated by the BET method24 gave surface areas of 241 m2 g−1 according to Ghormley14,15, but less than 12 m2 g−1 according to others16,17. We have reinvestigated adsorption of N2 on H2O(as) and find that it is a microporous solid. The differences between adsorption phenomena taking place in micropores and on the surface of intermediate pores or non-porous adsorbents should be of general importance for the various speculations of the role of H2O(as) in space. In particular, we discuss the influence of micro-pores on the H2 recombination rate on H2O(as) in interstellar dust and on adsorption of volatile gases in comets.

Microstructural evolution in reaction-bonded silicon carbide

Abstract: A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This epitaxial layer grows with a habit characteristic ofβ (cubic) SiC and then transforms toα-SiC in the high temperature region behind the reaction front. The formation of faceted grain boundaries is explained by this growth morphology. Furthermore, SiC:SiC grain boundaries, SiC:SiC epitaxial boundaries and SiC:Si interfaces have all been characterized by TEM techniques. The grain boundaries are of particular interest since they usually comprise a thin (∼1 nm) layer of amorphous SiC with occasional suicide and graphite inclusions. The general cleanliness of the vast majority of interfacial area is a result of the removal of the impurities insoluble in SiC by liquid silicon moving through the sample. The overall distribution of impurities is discussed. Other microstructural features have been characterized and texturing due to original particle alignment during fabrication of the green compact investigated. The control of the mechanical properties of reaction-bonded silicon carbide by these various microstructural features is discussed. A basis for an explanation of the interesting trace-impurity-controlled contrast seen in secondary electron images of these materials is also established.

Preparation of amorphous Ti1−xCux (0.10<x<=0.87) by mechanical alloying

Abstract: We show that amorphous alloy powders of Cu and Ti can be produced over a broad composition range by mechanical alloying of the elemental powders in a high-energy ball mill. The amorphous powders are compared with liquid-quenched amorphous alloys of the same compositions using the techniques of x-ray diffraction and thermal analysis of crystallization. The two types of samples are found to have similar characteristics. Chemical analysis shows that only trace amounts of impurities are introduced in the alloys by the milling process.

Electronic Properties of the Interface between Si and TiO2 Deposited at Very Low Temperatures

Abstract: Uniform TiO2 thin films with a large er (up to 86) were prepared at low temperatures (200-400°C) by CVD. The films deposited at 200°C were amorphous and those at high temperatures were polycrystalline structures of anatase. The electronic properties of a TiO2/Si interface were analyzed in detail using metal-insulator-semiconductor structures. The minimum interface state density in the bandgap was as low as 2×1011 cm-2 eV-1, showing the usefulness of the TiO2 films for the gate insulators of MIS diodes. An anomalous behavior of photo-induced current observed for the first time is also presented.

Defects in Amorphous Silicon: A New Perspective

Abstract: Defects in amorphous Si are universally viewed as due to undercoordinated atoms. The dominant EPR-active center is identified as threefold-coordinated Si (dangling bond), but evidence is inconclusive. I introduce a new viewpoint, motivated by results on defects in crystalline Si, and conclude that overcoordination defects are as likely. I propose that most EPR-active centers are fivefold-coordinated Si, with an electron in a state that I label "floating bond." This new analysis is favored by experiments and theory and leads to new predictions.

Solid-state reaction and structure in compositionally modulated zirconium-nickel and titanium-nickel films

Abstract: Structure and solid-state formation of amorphous alloys have been studied in sputter-deposited zirconium-nickel and titanium-nickel multilayered samples. The as-deposited samples in both systems had an amorphous structure at very small composition-modulation wavelengths (l5 atomic planes of each constituent per layer), and a textured polycrystalline structure at a composition modulation wavelength of 200 atomic planes of each constituent per layer. Intermediate composition-modulation wavelength samples showed differences between the two systems with titanium-nickel samples showing structural coherence in the growth direction. Anneals produced amorphous phase growth in zirconium-nickel multilayered samples for all composition-modulation wavelengths. Amorphous phase growth was also observed in the titanium-nickel sample with a composition-modulation wavelength of 200 atomic planes per layer. The intermediate composition-modulation wavelength titanium-nickel samples which showed structural coherence did not react to form an amorphous phase. This attributed to the lack of an incoherent or disordered interfacial layer to act as a nucleus for amorphous phase growth. The kinetics of the amorphous phase growth process extracted by monitoring resistivity during the anneals revealed that growth followed a square-root-of-time law indicative of diffusion-limited layer growth process. Arrhenius plots of the reaction rate gave activation energies between 1.3 and 1.7 eV. A simple predictive scheme based on the thermodynamic driving force and relative atomic volumes is proposed for indicating whether a solid-state reaction will produce an amorphous phase.

Magnetic properties of amorphous Tb-Fe thin films with an artificially layered structure

Abstract: An alternating terbium‐iron (Tb‐Fe) multilayer structure artificially made in amorphous Tb‐Fe thin films gives rise to excellent magnetic properties of large perpendicular uniaxial anisotropy, large saturation magnetization, and large coercivity over a wide range of Tb composition in the films. The films are superior to amorphous Tb‐Fe alloy thin films, especially when they are piled up with a monatomic layer of Tb and several atomic layers of Fe in an alternating fashion. Small‐angle x‐ray diffraction analysis confirmed the layering of monatomic layers of Tb and Fe, where the periodicity of the layers was found to be about 5.9 A. Direct evidence for an artificially layered structure was obtained by transmission electron microscopic and Auger electron spectroscopic observations. Together with magnetic measurements of hysteresis loops and torque curves, it has been concluded that the most important origin of the large magnetic uniaxial anisotropy can be attributed to the Tb‐Fe pairs aligned perpendicular t...

Polymer-polymer miscibility

Abstract: The theoretical and practical aspects of polymer-polymer miscibility in the solid amorphous state are reviewed. The polymers include homopolymers and both random and block copolymers. Although present theoretical treatments of polymer-polymer miscibility all contain the random mixing hypothesis and are thus not applicable to mixtures that involve specific interactions between the components, most of the observed singlephase polymer-polymer mixtures involve hydrogen-bonding or other specific interactions between the components. Even in the absence of specific interactions, the composition of a random copolymer can often be tailored to provide miscibility with a particular homopolyner. Many polymer-polymer mixtures have lower critical solution temperatures, and a small number of such mixtures have given indications of upper critical solution temperatures. The special phenomena that may be observed when other polymers are mixed with block copolymers are discussed.

Unbalanced magnetron ion-assisted deposition and property modification of thin films

Abstract: Unbalanced magnetron (UM‐gun) sputtering sources with the unique characteristic of a high deposition rate and concomitant high ion flux represent an exciting new development in ion‐assisted deposition of thin films. We have used a UM‐gun capable of producing ion current densities up to 5 mA cm−2 (ion flux 3×1016 cm−2 s−1) when operated at a power of 500 W to produce a variety of thin films of amorphous and crystalline materials by varying both the bombarding ion energy in the range 2–100 eV and the ion/atom arrival rate ratio in the range 0.4–10. The great flexibility and usefulness of UM‐guns is demonstrated with examples which include (a) hard diamondlike a‐C films prepared under very low ion energy (13–16 eV) bombardment which possess a metastable bonding configuration consisting of a mixture of tetrahedral and trigonal coordination that varies with ion energy, (b) hard and wear‐resistant TiN films whose electrical and optical properties change dramatically with ion bombardment, and (c) Ni/Cr alloy fil...

Threshold for superconductivity in ultrathin amorphous gallium films

Abstract: Systematic studies of the onset of superconductivity in ultrathin amorphous Ga films have revealed the existence of a threshold dependent only on the normal-state sheet resistance. Global superconductivity, or zero resistance, develops only when the normal-state sheet resistance falls below 6000 \ensuremath{\Omega}/\ensuremath{\square}. This result agrees with previous observations on crystalline Sn films and further supports the notion of a universal resistance threshold.

Preparation of highly photosensitive hydrogenated amorphous Si‐C alloys from a glow‐discharge plasma

Abstract: The preparation conditions for the deposition of hydrogenated amorphous silicon‐carbon alloys (a‐SiC:H) from a glow‐discharge plasma have been systematically scanned by changing the starting‐gas materials as well as the starting gas to hydrogen dilution ratio. A highly photosensitive alloy showing a photoconductivity to dark conductivity ratio of 107 at a band gap of 2.0 eV was prepared under optimized conditions.