Stability Of Amorphous Phase Research Articles

Formation and stability of metastable structures and amorphous phases in PU-V, PU-TA, and PU-YB systems with positive heats of mixing

The triode sputtering technique with a “split-target” arrangement was used to obtain metastable crystalline and amorphous phases in the Pu-V, Pu-Ta, and Pu-Yb systems. The proposed phase diagrams for these systems all exhibit liquid immiscibility. The heats of mixing are estimated to be highly positive, and the atomic radii of the component atoms differ by at least 10 pct. Extended amorphous and body-centered cubic (bcc) solid-solution regions were observed in the Pu-V and Pu-Ta systems. The corresponding lattice parameters appear to follow in each case an assumed Vegard’s Law extension. In the Pu-Yb system, no amorphous phase was obtained, but an extended face-centered cubic (fcc) solid-solution region (24 to 78 at. pct Yb) was observed with a large positive deviation of the lattice parameter (∼9 pct at 40 at. pct Yb) from a linear Vegard’s Law between the pure fcc components. The observed ranges of amorphous and metastable solid-solution phases have been interpreted in terms of predicated heats of formation for these phases using Miedema’s thermodynamic approximations that include chemical, elastic, and structural contributions. The effect of the high deposition rates on the formation of amorphous and metastable phases has also been considered. Thermal annealing of Pu-Ta amorphous alloys brings about a rapid diffusion of Pu to the free surface of the amorphous phase without crystallization of the remaining Ta-rich amorphous phase. Microhardness measurements indicate that amorphous Pu-V and Pu-Ta alloys are softer than the crystalline bcc solid-solution alloys in the same composition range. Several similarities in the formation of mixed phase regions (amorphous and solid solutions), microhardness, and resistance to decomposition on heating were noted between the Pu-Ta and Pu-V systems and the Cu-W system studied previously.

Aspects of Stability of Amorphous Grain Boundary Phase in Silicon Nitride Ceramics

Aspects of Stability of Amorphous Grain Boundary Phase in Silicon Nitride Ceramics

Stability and thermodynamic properties of supersaturated solid solution and amorphous phase formed by ball milling in the Zr- Al system

Zr100-xA1x (x ≤ 40) metastable alloys were synthesized by high- energy ball milling of elemental Zr and Al powders: supersaturated substitutional cph solid solution for x ≤ 15 and an amorphous phase for x ≥ 17.5. We performed a calorimetric study of the thermodynamics and kinetics of the metastable- to- equilibrium transformations of these phases. Their stability range (temperature/composition), as well as the apparent activation energies associated with the transformations, were determined. The transformation enthalpies were measured and used to determine the enthalpy of formation for these metastable phases. For both as- milled and relaxed amorphous phases, the measured enthalpy of crystallization is compared with those estimated for an undercooled liquid. Different amounts of retained entropy at the glass transition temperature were used to estimate the enthalpy loss upon undercooling due to the excess specific heat.

Phase diagram study of the formation and crystallization of Ge-metal amorphous alloy films

The Ge-Au and Ge-Ag alloy films were deposited in vacuum at room temperature and then systematically observed in the TEM. The maximum metallic concentrations in the alloy films,Cmax, which form the stable amorphous alloy phases of germanium with gold and silver, were obtained. The annealed crystallization temperatureTc, which falls with increasing metallic content in these films was also found. The structures of these films and their annealed specimens were also studied. There are various factors which influence the formation of amorphous alloy films deposited in vacuum for Ge-metal systems. A new formula forCmax has been derived. The annealed crystallization character has been explained by means of the variation of the free energy and the activation energy of crystallization. The activation energy of crystallization,Ea, can be obtained from the data values ofTc. For Ge-Au films,Ea (Au)=Eao/(−18.66CAu2+16.83CAu+1)±3.3 (kcal mol−1); for Ge-Ag films,Ea (Ag)=Eao/(−2.754CAg2+3.815CAg+ 1)±2.6 (kcal mol−1). In order to explain all these results, two kinds of phase diagram for the alloy films have been introduced. One is the three-dimensional relationship diagrams of phase formation in semiconductor-metallic alloy films; it was introduced to explain the influencing factors. The other is the three-dimensional phase diagrams of annealed semiconductor-metallic films systems. From this diagram all the phase transitions can be found.

Transient kinetics of nucleation and crystallization: Part II. Crystallization

Analytical expressions for the time-dependent crystallized volume fraction are derived from new results for the transient rate of nucleation reported in Part I. Conventional formulations that have been used in interpreting crystallization experimental data and for assessing the stability of amorphous phases are shown to be large time limits of the newly derived expressions. An approach for assessing the stability of an amorphous phase is proposed.

The fabrication of a novel composite GaAs/SiO2 nucleation layer on silicon for heteroepitaxial overgrowth by molecular beam epitaxy

We report on the fabrication of a composite GaAsSiO2 nucleation layer. The layer is formed by a deposition of a GaAs island layer by molecular beam epitaxy (MBE), followed by an oxidation step of the silicon regions surrounding the islands. In this way, small GaAs islands, for which the critical thickness for misfit dislocation generation is increased, are surrounded by a stable amorphous phase. Lateral overgrowth seeded by the individual GaAs islands might enhance the overall epilayer quality. We describe the fabrication and cleaning of such a composite GaAsSiOx nucleation layer that is compatible with the epitaxy process. Preliminary regrowth on a non-optimized composite surface resulted in GaAs-on-silicon quality equal to standard GaAs-on-silicon. Compared with GaAs epitaxy on porous silicon, another seeded growth technique, the composite surface technique has greater technological potential for the monolithic integration of GaAs and silicon electronics.

Electron irradiation induced crystalline-amorphous transition in a Co 75B 25 alloy

The relative stability of crystalline and amorphous phases in a Co 75B 25 alloy during 2 MeV electron irradiation was examined as a function of irradiation temperature by ultra-high voltage electron microscopy. At temperatures below 160 K, the crystalline phase is rendered amorphous during irradiation and the amorphous phase is always stable regardless of starting conditions, while at temperatures above 310 K, the amorphous phase undergoes the crystallization during irradiation and the crystalline phase is always stable relative to the amorphous phase. The crystallization temperature in the absence of irradiation was approximately 725 K. At temperatures between 160 K and 310 K, the alloy remains in the starting condition, i.e. either crystalline or amorphous, during irradiation.

Soft magnetic characteristics of Co/Zr thin-film layered structures (abstract)

We have extended the stable amorphous phase of the Co1−xZrx system to 7<x<52 by compositionally modulating very thin (five to ten atomic layers) layers of Co and Zr by rf diode sequential sputtering from the elemental targets onto rotating, water cooled substrates. This extension was attributed to a stable, two-phase, hetero-amorphous state similar to that previously reported for Co/B layered structures (Ref. 1). The measured compositional variation of Ms of our Co/Zr layered structures indicated that, as expected, the composition of the stable Co-rich clusters in the heterogeneous state lies near the stable Co-rich eutectic of the CoZr system. The composition variation of the coercivity of the as-deposited Co/Zr structures was remarkably low (<1 Oe) over the entire composition range. The uniaxial anisotropies of 15–30 Oe were reduced to ≊2 Oe by a combination of bias sputtering and rotating field annealing. Although they have reduced Ms, the permeabilities of the Zr-rich films are ≳1200, and constant (to 10%) to ≊50 MHz. They also have high resistivities (≊190 μΩ cm), potentially increased hardness and stability, and could be useful for magnetic recording head applications.

A possible similarity between lyotropic cubic phases and cholesteric blue phases

The common features in structures and properties of lyotropic cubic phases (LCP) and cholesteric blue phases (BP) are discussed. In both systems the cubic phases have nontrivial structures governed by frustrations. The frustration is connected with the double-twist in BPs and with the layer curvatures in LCPs. The structure of BP can be quantitatively described using the minimization of the Landau free energy with tensor order parameter. The order parameter of LCP is unknown till now; however, some minimization procedure can also be used for the solution of the phase problem. It is noted that in both systems the stable amorphous phases are observed (the fog phase or BPIII in BPs andL 3 phase in lyotropic crystals). The importance of the observation of «forbidden» reflections for the unequivocal identification of the space groups is emphasized.

Sputter-deposited WB x films

Films of WB x have been deposited on silicon and GaAs substrates from a compound target using an r.f. magnetron sputtering system to investigate the structural properties. A substrate bias has been applied to vary the deposition and film properties. Electrical resistivity, deposition rate and intrinsic film stress have been determined under various argon pressure and substrate bias conditions. As-deposited films are amorphous with resistivities of about 180ωμ cm. When annealed in a reducing atmosphere the films are found to be very stable. Crystallization starts at 840°C, as monitored by electrical resistance and X-ray diffraction patterns. The results are discussed on the basis of the binary phase diagram and empirical rules for the stability of amorphous phases, and the results are used to suggest that WB x films can be used as diffusion barriers.

Thermal stability of amorphous phases in vapour-deposited binary alloy thin films of palladium with vanadium, niobium and tantalum

Binary alloys of the systems V-Pd, Nb-Pd and Ta-Pd were vapour deposited and investigated by transmission electron microscopy. The atomic fraction,x, was varied in steps of 0.1 from one pure element to the other. The range over which an amorphous phase is observed is found to increase in width going from 3d to 5d alloying element in palladium: the compositions where amorphous phases are found are V1−xPd x (x = 0.5), Nb1−xPd x (x = 0.4 to 0.6) and Ta1−xPd x (x = 0.2 to 0.6). The composition range over which a crystalline phase is found correlates well with the single-phase solid solution region close to the melting temperature in the phase diagram. Crystallization of the V0.5Pd0.5 alloy takes place at 550 K. The amorphous Nb1−xPd x (x = 0.4, 0.5) films crystallize at 850 K, whereas the amorphous Ta-Pd films crystallize between 850 and 1050K, depending on the composition. For Nb1−xPd x (x = 0.4 to 0.5) and Ta1−txPd x (x = 0.2 to 0.6) primary crystallization takes place into an f c c phase. The second crystallization step leads to a phase with a complex structure. The result is a two-phase system. V0.5Pd0.5 and Nb0.4Pd0.6 crystallize polymorphically to an f c c solid solution. The crystallization temperatures for the compositions which display primary crystallization are higher than for the compositions which crystallize by a polymorphic reaction.

Structural studies of amorphous titanium diboride thin films by extended x-ray-absorption fine-structure and extended electron-energy-loss fine-structure techniques.

The local atomic structure of amorphous titanium diboride thin films, prepared by electron-beam vaporization (EBV) of the crystalline compound onto liquid-nitrogen-cooled substrates, was studied using extended x-ray-absorption fine-structure (EXAFS) and extended energy-loss fine-structure (EXELFS) techniques. From a comparison of the extended fine-structure spectra of the amorphous films with corresponding spectra of crystalline titanium diboride, accurate information was derived on the nature of the local structure, or short-range order, and on the coordination numbers, interatomic distances, and nanostructural atomic disorder in amorphous ${\mathrm{TiB}}_{2}$. A relaxation of the interatomic spacing and a reduction of coordination number for the nearest-neighbor atoms was inferred for the amorphous state. Local prismatic coordination with random 90\ifmmode^\circ\else\textdegree\fi{} rotations about prismatic planes is proposed as a likely atomic structure consistent with the data for the amorphous form. Finally, EXAFS and EXELFS were employed to examine in detail the structural changes induced in amorphous ${\mathrm{TiB}}_{2}$ by variations in the EBV deposition parameters, and to determine a set of optimized parameters for the EBV deposition of a ${\mathrm{TiB}}_{2}$ stable amorphous phase.

Applications of zachariasen's rules to different types of noncrystalline solids

Zachariasen's rules for glass formation were originally considered to be applicable to all glass forming systems. Later research indicated the application of Zachariasen's concepts was limited to only some common glass-forming oxide systems. Glass formation from the liquid was governed not only by structure but by kinetics of cooling as well. Non-crystalline solids, however, can be prepared also by methods other than cooling the liquid. For instance, vapor deposition, heat-treatment of gels and precipitation from solutions. The structures of such amorphous phases, their tendency to crystallize and their relation to one another and to melt-formed glasses are important questions. Zachariasen's rules have now been applied to many of these non-crystalline solids. It is shown that such a treatment can furnish useful information regarding the stability of such amorphous phases and the structural relationship between them.

Nonuniqueness of the state of amorphous pure iron.

Amorphous pure Fe has been studied with $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy by utilizing various Fe-based binary amorphous-alloy systems that have stable amorphous phases up to very high Fe concentrations and then extrapolating the properties toward those of amorphous pure Fe. The concentration dependence of the magnetic properties and the hyperfine interaction parameters of Fe--early transition metal (Ti, Zr, Hf, Nb, Ta, and Mo) and Fe-metalloid (B, Sb, Si, Ge) systems reveals different states of amorphous Fe which can be related to the theoretically proposed ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{2}$ states of fcc Fe.

The kinetic and nucleation limited stability of Amorphous U6Fe

Abstract The amorphous phase stability in U6Fe samples containing 2.7, 10.0, 20.0, 59.0, 87.0, 94.0, and 100% volume fraction amorphous phase has been examined. The crystallization temperature was observed to increase with an increasing volume fraction of amorphous phase. This observation is in direct conflict with the kinetic model of crystallization proposed for this alloy by Buschow and Beekmans. The observed transformation behavior can be explained using a crystal growth mechanism at low amorphous volume fractions and an interface energy/nucleation mechanism at larger volume fractions.

Polymorphism of amorphous pure iron

We investigate the state of amorphous pure Fe by utilizing various Fe-based amorphous alloy systems having stable amorphous phases up to very high Fe concentrations and extrapolating the properties toward those of amorphous pure Fe. Concentration dependence of hyperfine interaction parameters of Fe-early transition metal (Ti, Zr, Hf, Nb, Ta, and Mo) and Fe-metalloid (B, Sb, Si, and Ge) reveals the polymorphism of amorphous Fe.

Amorphous phase formation and stability in W-Ti-Si metallization materials

Amorphous phase formation and stability in W-Ti-Si metallization materials

Amorphous phase formation and stability in W-Ti-Si metallization materials

The range of composition over which co-sputtered alloys in the W-Ti-Si system are deposited in the amorphous state has been determined. Alloys are found to be crystalline only if they contain less than ∼ 10% Si and this is associated with the presence of terminal solid solutions in the respective binary phase diagrams. The stability of W-Si amorphous alloys was examined using differential scanning calorimetry and X-ray diffraction. The most stable glasses exist in the range 10 to 30% Si and about the WSi2 composition. There is some correlation between increasing stability and increasing activation energy for crystallization.

Compound materials for reversible, phase-change optical data storage

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.

Stability of amorphous and crystalline phases in an irradiation environment

The relative stability of crystalline and amorphous phases in metallic structures during ion irradiation is discussed in terms of the free energy changes and point defect mobility. The increase in free energy due to radiation induced defects is predicted to be larger for line compounds than compounds with wide solubility, which can explain the greater tendency toward amorphization for line compounds. Low defect mobility is also necessary to ensure a high total defect concentration. Nucleation of dislocation structure is an alternate way to relieve the high internal free energy and therefore may preclude subsequent amorphous transformation. Ion irradiation can also enhance the crystallization kinetics of the amorphous phase. A critical temperature region can be defined which separates the regions of crystalline stability from amorphous stability during irradiation.