Stability Of Amorphous Phase Research Articles

Effect of solute contents on the glass forming ability and thermal stability of Al–Ni–Sr alloy

Abstract Glass forming ability (GFA) and structural characterization of amorphous Al–Ni–Sr alloy melt spun were investigated as a function of solute contents. The GFA was found to vary with the ratio of Ni to Sr. High resolution transmission electron microscopy (HREM) suggested that Al–Ni 5 –Sr 3 which showed the highest glass forming ability in the experimental conditions consisted of 2–5 nm precipitates embedded in the amorphous matrix. Thermal stability and devitrification behavior of amorphous phases were investigated using the continuous and isothermal heating technique on DCS, resulting that the crystallization occurs by the growth of pre-existent fcc-Al nuclei.

Formation of physically stable amorphous phase of ibuprofen by solid state milling with kaolin

Ibuprofen was milled in the solid state with kaolin (hydrated aluminium silicate) in different ratio to examine the extent of transformation from crystalline to amorphous state. The physical stability of the resultant drug was also investigated. X-ray powder diffractometry (XRD) and birefringence by Scanning Electron Microscopy (SEM) studies indicated almost complete amorphization of the drug on ball milling with kaolin at 1:2 ratio. Fourier transform infrared spectroscopy (FTIR) data showed a reduction in the absorbance of the free and the hydrogen-bonded acid carbonyl peak of carboxylic acid group accompanied by a corresponding increase in the absorbance of the carboxylate peak, indicating an acid–base reaction between the carboxylic acid containing ibuprofen and kaolin on milling. The extent of amorphization and reduction in the carbonyl peak and increase in carboxylate peak was a function of kaolin concentration in the milled powder. On storage of milled powder (at 40 °C and 75% RH for 10 weeks), XRD and birefringence of SEM study showed the absence of reversion to the crystalline state and FTIR data revealed continued reduction of carbonyl peak, whereas, ibuprofen converted from its crystalline acid form to amorphous salt form on milling with kaolin. Kaolin-bound state of ibuprofen was physically stable during storage. In-vitro dissolution studies revealed that percent release of ibuprofen from the kaolin co-milled powder is in the order: 1:2 > 1:1 > 1:0.5 > 1:0.1> milled alone ibuprofen > crystalline ibuprofen.

Molecular dynamics study of pressure effect on crystallization behaviour of amorphous CuNi alloy during isothermal annealing

The pressure effect on crystallization behaviour of amorphous CuNi alloys during isothermal annealing was studied by using molecular dynamic simulation method. Interaction between atoms in the model alloy system was calculated by Sutton–Chen type of embedded atom method. The structural phase transformation during isothermal annealing under different pressure values was analyzed based on the variations of the cohesive energy, unit cell volume and radial distribution functions (RDF). The obtained results show that the isothermal annealing of amorphous CuNi alloy causes a phase transformation from a metastable phase to a stable crystal phase and the increase of pressure during isothermal annealing increases the thermal stability of amorphous phase.

Relative stability of amorphous phase versus solid solution in the Ni–Ti system revealed bymolecular dynamics simulations

We employ molecular dynamics simulations withn-body potentials to calculate the energies of solid solutions and metallic glasses in the Ni–Tisystem to reveal relative stability of two phases against the change of solute concentration. ForNi1−xTix alloys,the calculations show that the energies of the solid solutions are lower than the amorphous phases whenx≤0.24 and viceversa for x>0.24, in accord with thermodynamics analysis and experiments. To reveal the underlyingphysics responsible for this, the evolution of structures in the solid solutions and theamorphous phases are studied via the coordination number (CN) and common neighbouranalyses. The results show that, with an increase in Ti concentration, the total averagecoordination number is close to 12 for all solid solutions owing to the face-centred cubic(fcc) crystalline structure remaining, while it changes from less than 12 to greater than 12for the amorphous phases, suggesting that more bonds or more fractions of large CNs(>12) exist in the amorphous phases with a high Ti content. The relative stability of theamorphous phases versus the solid solutions revealed by the order of energy is thuscorrelated with the structural change of the amorphous phases.

Nanoscale phase separation in amorphous immiscible copper-niobium alloy thin films

While amorphous phases have been reported in immiscible alloy systems, there is still some controversy regarding the reason for the stabilization of these unusual amorphous phases. In this letter, using three-dimensional atom probe tomography, direct evidence of nanoscale phase separation within the amorphous phase forming in immiscible Cu–Nb alloy thin films has been presented. This evidence clearly indicates that the nanoscale phase separation is responsible for the stabilization of the amorphous phase in such immiscible systems since it substantially reduces the free energy of the undercooled liquid (or amorphous) phase, below that of the competing supersaturated crystalline phases.

Phase Stability of Crystalline and Amorphous Phases and Formation of Nanostructure in Zr-Pd and Zr-Pt Alloys Under Electron Irradiation

AbstractElectron irradiation induced phase transition behavior of Zr-Pd and Zr-Pt alloys was investigated focusing on phase selection in crystallization by thermal annealing and electron irradiation. Nano quasi-crystalline (QC) phase was formed by thermal crystallization in Zr66.7Pd33.3 and Zr80Pt20 alloys, while nano two-type f.c.c. super-saturated solid solutions were formed by irradiation induced crystallization at 298K. In Zr66.7Pt33.3 alloy, polycrystalline Zr5Pt3 and Zr9Pt11 phases transformed to two-type f.c.c. nano-crystalline phase through an amorphous state by crystal-to-amorphous-to-crystal (C-A-C) transition during electron irradiation. Nano-composite phase composed of f.c.c. super-saturated solid solution and residual amorphous phase was stable rather than an amorphous single phase, thermal equilibrium crystalline phase and quasi-crystalline phase under 2.0MV electron irradiation at 298K, resulting in the formation of nano-composite structure by irradiation induced amorphization and crystallization.

Controlled release of precipitating agents through solvothermal destabilization of microemulsions: one-pot synthesis of monoclinic zirconia nanostructures

We report a versatile one-step multiphase approach to control size, crystallinity and assembly at the nanoscale. The method takes advantage of the controlled release of precipitating agents achieved through the solvothermal destabilization of reverse microemulsions. This method permits the preparation of nanoparticles (as an example we prepare zirconia materials) self-assembled in open (palmate leaf-like) and compact nanostructures that could be easily processed using existing solution routes. The mechanisms of formation of these nanostructures are presented on the basis of the structural information and surface charge data provided by various techniques. The morphology of the nanostructures is found to be the combined result of several growth features, among which self-assembly driven by electrostatic forces plays a critical role. In addition, we have found that more open nanostructures lead to monoclinic zirconia samples with a higher degree of crystallinity, which supports the idea that surface energy must play a critical role in the stabilization of tetragonal polymorphs or amorphous zirconia phases.

Phase Stability of an Amorphous Phase Against Electron Irradiation Induced Crystallization in Fe-Based Metallic Glasses

Electron irradiation induced nano-crystallization of melt-spun amorphous phase in Fe-Zr-B ternary alloys was investigated focusing on the difference in phase stability of amorphous phase during thermal annealing and electron irradiation. Nano-composite structure composed of nanocrystalline α-Fe precipitates with b.c.c.-structure, metallic compounds and a residual amorphous matrix was formed under 2.0MeV electron irradiation at 298 K, while such nano-structure was hardly realized by thermal annealing. The phase stability of an amorphous phase against electron irradiation was discussed based on the relationship between thermal properties and critical onset total dose of electron irradiation induced crystallization at 298 K.

Development of rapidly solidified Al–Y–Ni-based alloys

The present study is concerned with the effect of alloying additions (e.g. Co, Nb, Pd, La and Y) to the glass forming ability (GFA) of Al–Y–Ni alloys. Rapidly solidified ribbons of the following systems have been prepared by melt-spinning process: Al 88Ni x/2 Pd x/2 Y 12− x ( x = 2, 5, 10), Al 88Ni 1Co 1Y 10− x La x ( x = 0, 5, 10), Al 88Ni 1Nb 1Y 10, and Al 86Ni 4− x Co x Y 10 ( x = 1, 2, 3). Characterisation of the melt spun alloys was carried out through a combination of X-ray diffractometry, differential scanning calorimetry, and transmission electron microscopy. GFA in Al 88Ni 1TM 1Y 10 (where TM = Co, Nb, Pd) increases in the following order: Nb < Co < Pd. To maintain a fully amorphous structure in Al 88–Ni–Pd–Y systems the optimum quantity of yttrium is 10 at.%. A complete substitution of Y with La, or aluminium with 2 at.% of (Co,Ni) decreases the glass forming ability in Al 88Ni 1Co 1Y 10 but increases thermal stability of the residual amorphous phase. Partial replacement of Y with La significantly improves the thermal stability of the amorphous phase in Al–Ni–Co–Y.

Charge‐transporting polymers and molecular glasses for optoelectronic applications

AbstractOrganic charge transport materials used in optoelectronic devices such as electrophotographic photoreceptors, light emitting diodes, solar cells, photorefractive materials can be devided into polymers, molecularly doped polymers and molecular glasses.Among charge‐transporting polymers hole‐transporting polymers are the most widely used and studied. An aromatic amino group is a common feature of many known charge‐transporting polymers. Almost all practical systems used in electrophotography belong to this category.Organic low‐molar‐mass compounds with stable solid amorphous phase above room temperature are called molecular glasses, amorphous molecular materials or low‐molar‐mass glasses. A real explosion of interest in organic electronically active low‐molar‐mass glass‐forming materials occurred after the discovery of high performance multilayer electroluminescent devices prepared from vacuum‐sublimed organic dye amorphous films.In this presentation the recent results of the work on the synthesis and properties of charge‐transporting polymers and molecular glasses performed in the author's laboratories are reviewed. The main attention is paid to the synthesis and studies of hole‐transporting polymers and molecular glasses. In the design and synthesis of these materials aromatic amino groups such as carbazolyl, tripehenylamino, phenothiazinyl and hydrazone moieties, have been used as the main building blocks. The properties of the charge‐transporting compounds synthesized are reported. Ionization potentials established by the electron photoemission in air technique range from 5.00 to 5.60 eV. Hole drift mobilities in the amorphous films of some hydrazones and aromatic amines approach 10−2 cm2/V sec at high electric fields as revealed by the time‐of‐flight technique. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Structural and electrical properties of sputtered indium–zinc oxide thin films

In this study, indium–zinc oxide (IZO) thin films have been prepared at a room temperature, 200 and 300 °C by radio frequency magnetron sputtering from a In2O3–12 wt.% ZnO sintered ceramic target, and their dependence of electrical and structural properties on the oxygen content in sputter gas, the substrate temperature and the post-heat treatment was investigated. X-ray diffraction measurements showed that amorphous IZO films were formed at room temperature (RT) regardless of oxygen content in sputter gas, and micro-crystalline and In2O3-oriented crystalline films were obtained at 200 and 300 °C, respectively. From the analysis on the electrical and the structural properties of annealed IZO films under Ar atmosphere at 200, 300, 400 and 500 °C, it was shown that oxygen content in sputter gas is a critical parameter that determines the local structure of amorphous IZO film, stability of amorphous phase as well as its eventual crystalline structure, which again decide the electrical properties of the IZO films. As-prepared amorphous IZO film deposited at RT gave specific resistivity as low as 4.48×10−4 Ω cm, and the highest mobility value amounting to 47 cm2/V s was obtained from amorphous IZO film which was deposited in 0.5% oxygen content in sputter gas and subsequently annealed at 400 °C in Ar atmosphere.

Thermodynamic modeling of glass formation in metallic glasses

In the study of metallic glasses, it is vitally important for us to understand glass formation and glass forming ability (GFA). This letter presents a thermodynamic model for evaluating glass formation in metallic glasses. An expression has been derived from considering both the stability of amorphous phases and the resistance to the formation of crystalline intermetallic phases. This equation is very useful for identifying alloy compositions with good GFA in binary and other alloy systems. The prediction of this equation has been verified by the experimental study of glass formation and glass forming ability in the Zr–Cu system.

Solid state amorphization and crystallization in Zr 66.7Pd 33.3 metallic glass

We observed quasicrystal-to-amorphous-to-crystal (Q–A–C) transition in Zr 66.7Pd 33.3 metallic glass. The unique disordering–ordering phase transition was induced by 2 MeV electron irradiation at 298 K. Electron irradiation can induce not only the solid-state amorphization but also crystallization of a glassy structure under the same irradiation conditions. The Q–A–C transition can be explained by change in the phase stability of crystal, quasicrystal and amorphous phases by electron irradiation induced atomic displacement.

Effects of nitrogen content on microstructure and oxidation behaviors of Ti–B–N nanocomposite thin films

Several different types of thin films, TiB0.65,TiB0.62N0.18,TiB0.61N1.04, and pure TiN, were deposited on Si(100) substrates at 500 °C by reactive unbalanced close-field dc-magnetron sputtering using two Ti and two TiB2 targets. The oxidation experiments of these films were carried out in air at fixed temperatures in a temperature regime of 600–1000 °C. As-deposited and oxidized films were characterized and analyzed using x-ray diffraction, plan-view and cross-sectional scanning electron microscopies, atomic force microscopy, and x-ray photoelectron spectroscopy (XPS). It was found that the microstructure and bonding configuration of Ti–B–N thin films were strongly dependent on nitrogen flow rate during deposition. Depending upon the amount of N addition, the films showed two- or three-phase nanocomposite structure. Nitrogen-free films were amorphous compound comprising of Ti and TiB2 (Ti–TiB2 compound). At 10 at. % N addition (TiB0.62N0.18), the films consisted of mainly TiN and TiB2 bondings with microstructures comprising of nanocrystalline (nc)-Ti(N) embedded in an amorphous (a)-TiB2 matrix. As the N concentration increased up to 38 at. % (TiB0.61N1.04), the films consisted of nc-TiN in a-(TiB2, BN) matrix. The oxidation experiment illustrated that the nanocomposite TiB0.61N1.04 thin films exhibited a much higher high-temperature oxidation resistance than TiN, TiB0.65, and TiB0.62N0.18 thin films. A two-stage oxidation process took place in these nanocomposite films in the whole temperature regime. A low oxidation rate accompanied with formation of small-grained Ti oxide occurred below 800 °C, while above 800 °C a rapid oxidation process accompanied with formation of large-grained Ti oxide with rough surface took place. It is believed that the two-stage oxidation process was related to oxidation resistance of nanocrystallites and thermal stability of amorphous matrix phase. By XPS, the oxides were determined to consist mainly of various types of Ti oxides in the oxidation temperatures of 600–1000 °C, such as TiO, TiNxOy, Ti2O3, and TiO2. It was also found that no elemental B was detected in oxide formed above 600 °C, which may be due to a low melting temperature of B2O3.

Control of a Nanocomposite Structure in Fe&lt;SUB&gt;86&lt;/SUB&gt;Nd&lt;SUB&gt;9&lt;/SUB&gt;B&lt;SUB&gt;5&lt;/SUB&gt; Alloy by Electron Irradiation

Change in nanocomposite structure in rapidly solidified Fe 86 Nd 9 B 5 alloy during electron irradiation was investigated. Nanocrystalline structure composed of α-Fe, Fe 3 B, Nd 2 Fe 14 B and Nd 2 Fe 23 B 3 crystalline phases was formed by rapid solidification. Electron irradiation can introduce amorphization of intermetallic compounds and crystallization of an amorphous phase, resulting in the formation of a novel nanocomposite structure in which α-Fe and Nd 2 Fe 14 B nanocrystals are embedded in the amorphous matrix. The mechanism of nanocomposite structure formation was discussed based on the phase stability of amorphous and crystalline phases under electron irradiation.

Novel Aromatic Amines for Photoconductor and Organic Electronic Applications

Aromatic amines are frequently used as hole conductors in copiers and laser printers. In the last decade there has been increasing interest to extend the use of aromatic amines to organic light emitting diodes (OLEDs) as well as to organic field-effect transistors (OFETs) developed for printable organic electronics. In this contribution star-shaped aromatic amines which form stable amorphous phases are introduced. Such materials are often called molecular glasses. We have used molecular glasses with triphenylamine, triphenylbenzene and fluorene units as blue emitters and hole blocking materials in OLEDs. The layer thickness of the hole transport and emission layer and the hole blocking layer were optimized in a combinatorial approach which leads to pure blue emitting OLEDs with color coordinates of x=0.15 / y=0.15, a brightness of 400 cd/m2 and an efficiency of 2 cd/A. We have also used molecular glasses with triphenylamine and carbazole units as organic semiconductors in OFETs and obtained environmentally stable transistors with mobilities up to 10&#x2212;4 cm2/Vs.

Internal clusters in crystalline phases related to Zr-based bulk amorphous alloys

The crystalline phases that form with the continuous heating of Zr-based bulk amorphous alloys were classified for the first time into three different groups based on the type of internal cluster, representing the locally ordered atoms within the unit cells of the crystalline phases: two-shell Bergman internal cluster in the face-centered cubic cF-Zr 2Ni phase and ZrTiNi (C14-type) Laves phase, truncated octahedron internal cluster in the tetragonal phases of tI-Zr 2Cu and tI-Zr 2Pd, and Gaskell internal cluster (trigonal prism capped by three half octahedra) in the hexagonal phases of hP-Zr 6Al 2Ni and hP-ZrBe 2. The phases with a similar internal cluster type were also found to have similar valence electron concentrations ( e/ a), even though some crystalline structures are different from each other. The density of states for the primary phases calculated using the density functional theory indicated that the high stability of certain amorphous phases could be attributed to a difference in the internal cluster structure between the amorphous phases and the primary phases.

Thermodynamic Evaluation of the Effects of Amorphism on Film Breakdown and Pitting Initiation

Maximum differences in the chemical free energy between a crystalline oxide and its fully amorphous counterpart at are estimated from melting point and enthalpy of fusion data for oxides in ten different metal systems (Be, Zn, Cu, Al, Cr, Fe, Ni, Zr, Sn, and Ti). The results are used to determine the relative thermodynamic stability of crystalline and amorphous phases in aqueous solutions in order to understand the possible influence of amorphism on precursor pitting sites in passive films. Variations in the degree of amorphism are treated by using different values, where , and examining the resulting effects on pH-dependent oxide solubility and equilibria. Estimated surface energies of high-angle grain boundaries are used to show that grain boundaries in crystalline oxide films may be treated as amorphous regions.

Electron irradiation induced phase transformation in Zr66.7Cu33.3 and Zr66.7Pd33.3 metallic glass

Abstract Crystallization and phase selection in Zr66.7Cu33.3 and Zr66.7Pd33.3 metallic glass during thermal annealing and electron irradiation were examined. During thermal annealing an equilibrium C11b–Zr2Cu phase directly precipitated in the amorphous phase of Zr66.7Cu33.3 metallic glass while a thermal equilibrium C11b–Zr2Pd phase formed after icosahedral quasi-crystalline phase precipitation in Zr66.7Pd33.3 metallic glass. The amorphous phase was not stable under electron irradiation and metastable crystalline phases with face-centered cubic-based structure formed in both kinds of metallic glass by electron irradiation induced crystallization. The unique phase selection in electron irradiation induced crystallization is due to a change in the phase stability of crystal, quasi-crystal and amorphous phase under electron irradiation.

Compositional Effect on the Stability of the Amorphous Phase in Al-La-Ni Alloys

Compositional Effect on the Stability of the Amorphous Phase in Al-La-Ni Alloys