Influence of a Weak Magnetic Field on Photoconductivity Spectrum of C60 Single Crystal

Comparative research of the excitation photoconductivity spectra (quantum light energy 2-5 eV) of C₆₀ single crystal in and out of magnetic field at the temperature T = 250-350 K has been carried. The spectral evolution at this temperature range is described. It is shown that the spectra changes abrupt at temperature T₁ ∼ 260 K and T₂ ∼ 315 K. An increase in the photoconductivity up to 15% was observed in the magnetic field (B = 0.4 T) within the photon energy range 2.5-4.5 eV. Local photoconductivity peak's appearances in the magnetic field have been proven that the charge transfer excitons take part in a photoconductivity.     

An analysis of crack extension in creeping solids

A theoretical analysis for the crack growth under creep conditions in ductile materials has been carried out based on the stress distribution in the ligament section and the plastic deformation around the crack zone. The two extreme cases namely: (a) large creep deformation over the entire ligament and; (b) negligible creep deformation even near the crack tip, are considered based on the model proposed. The resulting expressions are compared with the experimental data carried out on a 6061 Al alloy in the temperature range of 0.55–0.65 T_m.     

AC Conductivity and Dielectric Response of Polycrystalline and Amorphous C70

The AC conductivity and dielectric constant of polycrystalline and amorphous C₇₀ samples were measured in the 75-300 K temperature range and in the 100 Hz to 1 MHz frequency range. For polycrystalline samples, we observe effects caused by O₂ intercalation due to prolonged exposure to ambient air. The conductivity σ of these samples around 300 K depends on the measuring frequency ν as a σ ∼ νⁿ with n ≈ 0.88, implying a strong reduction of DC conductivity to less than 10^-12 S/cm. The dielectric constant of polycrystalline samples shows an anomaly at 285 K which is interpreted as due to the transition from its intermediate rhombohedral phase into its monoclinic low-temperature phase. In contrast with the polycrystalline samples, the amorphous C₇₀ samples prepared by sublimation do not contain interstitial 0₂, their conductivity at 300 K is of about 10^-6 S/cm, is independent of frequency, and is well described by the hopping mechanism (Davis-Mott T^1/4 law) in the 200-300 K range. All evidence of phase transitions disappears in the amorphous samples.     

Thermodynamic Properties and Hydrogen Accumulation Ability of Fullerene Hydride C60H36

Thermodynamic properties of fullerene hydride C₆₀H₃₆ in the ideal-gas and crystal states were studied by theoretical methods. Molecular structures and vibration frequencies were calculated for 9 isomers of C₆₀H₃₆ by the density functional theory (DFT) by use of a combination of the B3LYP functional with 6-31G* basis sets. Ideal-gas thermodynamic properties were calculated based on those parameters. Enthalpies of formation of C₆₀H₃₆ isomers in the ideal-gas state were derived from homodesmic reactions involving adamantane, cyclohexane, and C₆₀ fullerene. Using the standard methods of statistical mechanics, heat capacity and derived thermodynamic properties of crystalline C₆₀H₃₆ were calculated at 340-1000 K that extended the range of experimental measurements. With a crystal-gas heat capacity difference, the experimental value of sublimation enthalpy was extrapolated to room temperature as Δ_subH_m^o (298.15 K)=(193±10) kJ · mol^-1. Combining this value with the known experimental enthalpy of formation in the crystalline state, the ideal-gas enthalpy of formation of C₆₀H₃₆ at the synthesized sample isomer composition was obtained: Δ_f H_m^o (298.15 K)=(1206±28) kJ · mol^-1. Equilibrium constants and compositions were calculated for the reactions of hydrogenation of C₆₀ fullerene in different states. It was shown that C₆₀ can act as a hydrogen accumulator.     

Chemical vapour deposition of praseodymium oxide films on silicon: influence of temperature and oxygen pressure

Metal-organic chemical vapour deposition (MOCVD) of various phases in PrO_x system has been studied in relation with deposition temperature (450–750 °C) and oxygen partial pressure (0.027–100 Pa or 0.2–750 mTorr). Depositions were carried out by pulsed liquid injection MOCVD using Pr(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) precursor dissolved in toluene or monoglyme. By varying deposition temperature and oxygen partial pressure amorphous films or various crystalline PrO_x phases (Pr₂O₃, Pr₇O₁₂, Pr₆O₁₁) and their mixtures can be grown. The pure crystalline Pr₂O₃ phase grows only in a narrow range of partial oxygen pressure and temperature, while high oxygen pressure (40–100 Pa) always leads to the most stable Pr₆O₁₁ phase. The influence of annealing under vacuum at 750 °C on film phase composition was also studied. Near 90% step coverage conformity was achieved for PrO_x films on structured silicon substrates with aspect ratio 1:10. In air degradation of Pr₂O₃ films with transformation to Pr(OH)₃ was observed in contrast to Pr₆O₁₁ films.     

Surface Preparation of Aluminum Nitride for Metallization: Effect of Temperature on Surface Reactivity

The current aqueous cleaning step in the surface preparation of aluminum nitride (AlN) prior to metallization causes performance and reliability issues for the substrates used for microelectronic packaging due to surface reactions. These issues limit the use of AlN and its replacing of BeO, an environmentally hazardous material currently used. The aim of this investigation was to determine the effects of different solutions on the surface of AlN substrates under varying conditions at times up to 2419.2 ks (28 days). Concentration of the solutions, temperature, and immersion time were varied for the AlN samples in the solutions. Both elevated temperatures (50°C and 90°C) and low temperatures (5°C) were investigated.

Four general types of behavior were observed: minor changes in average surface roughness and microstructure, linear change in average surface roughness and pitted grains, nonlinear change in average surface roughness and product formation on AlN surface, and miscellaneous change in average surface roughness with surface product formation.

The surface roughening kinetics were very complex due to changes in both the reaction product morphology and reaction mechanism with temperature, solvent, and pH for a specific solvent. Minor changes in average surface roughness and microstructure were observed for HCl pH = 5, H₂ SO₄ pH = 5, NaOH pH = 8, NaOH pH = 10, NaOH pH = 12, deionized water and Alfred tap water at 5°C, HCl pH = 3 and oleic acid at 50°C and citric acid and oleic acid at 90°C. Linear changes in average surface roughness and pitted grains were observed for HCl pH = 2 and H₂SO₄ pH = 3 at 50°C and HCl pH = 2, H₂SO₄ pH = 3, and deionized water at 90°C. Non-linear change in average surface roughness and product formation on AlN surface was observed for HCl pH = 5, NaOH pH = 8 and Alfred tap water at 50°C and HCl pH = 5 and H₂SO₄ pH = 2 at 90°C. Miscellaneous changes in average surface roughness with surface product formation were observed for H₂SO₄ pH = 2, H₂SO₄ pH = 5, NaOH pH = 10, NaOH pH = 12, citric acid, Micro-90 and deionized water at 50°C and HCl pH = 3, H₂SO₄ pH = 5, NaOH pH = 8, NaOH pH = 10, NaOH pH = 12, Micro-90 and Alfred tap water at 90°C.     

Atmospheric plasma discharge chemical vapor deposition of SnOx thin films using various tin precursors

A series of 0.2–0.6 μm thick SnO_x films were deposited onto borosilicate and sodalime silica glass substrates by atmospheric plasma discharge chemical vapor deposition at 80 °C. SnO_x films deposited from monobutyltin trichloride contained a large percentage of SnCl₂:2H₂O, and therefore were partially soluble in water. SnO_x coatings deposited from tetrabutyltin were not soluble in water or organic solvents, had good adhesion even at growth rates as high as 2.3 nm/s, had high transparency of  90% and electrical resistivity of 10⁷ Ω cm. As-grown tin oxide coatings were amorphous with a small concentration of SnO₂, SnO and Sn crystalline phases as determined by grazing angle X-ray diffraction and X-ray photoelectron spectroscopy measurements. Upon annealing in air at 600 °C the resistivity of SnO_x films decreased to 5–7 Ω cm. Furthermore, optical and X-ray measurements indicated that SnO_x was converted into SnO₂ (cassiterite) with a direct band gap of 3.66 eV. Annealing of as-grown SnO_x films in vacuum at 340 °C led to formation of the p-type conductor SnO/SnO_x. The indirect band gap of SnO was calculated from the optical spectra to be 0.3 eV.     

Study of thermally and chemically unfolded conformations of bovine serum albumin by means of dynamic light scattering

The change of conformation of a bovine serum albumin (BSA) has been characterized using dynamic light scattering (DLS). Structural properties were investigated as a function of chemical denaturant concentrations and a temperature. In pure water, the protein keeps its native size at guanidine hydrochloride (GdmCl) concentrations below 1 M and behaves like an excluded volume chain above 4 M. A transition in structure and properties of the protein seems to occur around a GdmCl concentration of 1.7 M. Still, this protein forms an unfolded conformation in presence of 6 M of urea concentration and the value of the diffusive virial coefficient indicates that the interactions between the polypeptide chain and solvent are repulsive. The true value of the T_m = 43 °C, has obtained by linear extrapolation to 0 M GdmCl.     

Effect of temperature on the frequency dependency of crack propagation velocity in delayed failure under cyclic load

The crack propagation velocity and the incubation time in delayed failure under repeating load were examined at various frequency ƒ(0–800 cpm) and temperature T(5–80°C) using the pre-cracked specimen of JIS SNCM8 quenched and tempered. The crack propagation velocity under repeating load (da/dt)_R decreased with increase of frequency ƒ and revealed a minimum value (da/dt)_minat a certain ƒ. The activation energy obtained from the Arrhenius plot of (da/dt)_min vs 1/T was about 10,000 cal/mol, which was a little larger than 8100 cal/mol obtained from the relation between the crack propagation velocity under static load and 1/T. The reason for the existence of the minimum value in the crack propagation velocity was explained by assuming the interaction between hydrogen atoms invaded from crack tip and the cyclic change of the position with tri-axial tensile stress. The incubation time under repeating load did not seem to be controlled by the diffusion or concentration process of hydrogen atoms.     

Calculation of the ferrite volume in some dual phase steels

The relation between the γ/γ + boundary temperature, T, and the equivalent values of [Cr] and [Ni], as well as the variation of the ferrite volume, V_f, with the temperature in + γ dual-phase steels have been studied. With the aid of a computer, the regressive expressions derived from the experimental results are: T (°C) = T₃ + 21.2 [Cr] − 15.8 [Ni] + 223; V_f (%) = 0.715 exp [0.015(T − T_δ)] − exp[0.015(T_c − T_δ)] + 1.85 exp [0.0083(T − T_c]).     

Ambient temperature synthesis and magnetic properties of aluminum borate—Fe nanocomposites

Nanocomposites in which fine Fe particles are dispersed in a ceramic aluminum borate Al_xB_yO_z matrix were synthesized at ambient temperatures by simultaneous hydrolysis of aluminum butoxide and reduction of Fe²⁺ with sodium borohydride. The nanocomposites were characterized by X-ray powder diffraction, transmission electron microscopy, SQUID magnetometry, and Mössbauer spectroscopy. The as-prepared samples consist of crystalline Fe particles dispersed in an amorphous aluminum borate matrix, which on annealing in H₂ atmosphere at T ≥ 800 °C transforms to crystalline aluminum borate; the crystalline aluminum borate exhibits a needle-like morphology. The as-prepared samples all seem to consist of a mixture of ferromagnetic and superparamagnetic Fe particles. The superparamagnetic fraction vanishes completely on annealing in H₂ atmosphere at 900 °C. The magnetic results are supported by Mössbauer spectra recorded at room temperature.     

Epitaxial growth of AB type compounds under quasi-equilibrium conditions

Single-crystal films of CdS, CdSe and CdTe have been grown in vacuum on mica (fluophlogopite and muscovite) under isothermal conditions, i.e. with T_ev ≈ T_ep ≈ T_gr where T_ev and T_ep are the evaporation and epitaxial temperatures respectively and T_gr is the growth temperature. The synthesis was carried out in the temperature range 430°–800°C in the case of CdS, 300°–650°C for CdSe and 270°–550°C for CdTe. It is shown that the growth rate of single crystal layers (V_gr) depends exponentially on the growth temperature: V_gr (Å/sec) = D exp (−E/RT_gr) Perfect epitaxial CdS, CdSe and CdTe films have a wide range of electrophysical properties. Co-evaporation of CdS and sulphur and of CdSe and selenium allowed high-resistance films of cadmium sulphide and cadmium selenide of both n- and p-types to be obtained.     

The occurence of transcrystallization or row-nucleated cylindritic crystallization as a result of shearing in a glass-fiber-reinforced polypropylene

The formation and build-up of transcystalline-like superstructures produced by pulling a glass fiber (GF) from an isothermal crystallizing (T_c = 130−140°C) isotactic polypropylene (iPP) melt has been studied by thermo-optical methods. It was found that these morphological structures are caused by row-nucleation (self-nucleation), and that they should therefore be termed cylindrites instead of transcrystalline layers. The build-up of the cylindrites depends on whether the pulling and crystallization temperatures (T_pull and T_c, respectively) are above or below the upper threshold temperature of the possible β crystallization (T_β). The thermal stability and β nucleation ability of the primary row-nuclei at T_pull = T_c < T_β depend on their molecular orientation, caused by the shear stress field accommodated during pulling. In addition, the thermal stability of the row-nuclie is also affected by T_pull. It is suggested that the observed -β bifurcation at T_pull = T_c < T_β, which is the main argument for cylindritic crystallization, may be related to epitaxial growth.     

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature (T_FDBT) is about 130 K. When T > T_FDBT, the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T_FDBT, the FCG mechanism changes into microcleavage and the fatigue fracture toughness K_fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK_th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T_FDBT. By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ₋₁ and δk_th, at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T_FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.     

Effects of Injection Molding Parameters on the Production of Microstructures by Micropowder Injection Molding

Micropowder injection molding (μPIM) is a potential low-cost process for the mass production of metal or ceramic microstructures. In order to obtain good molded microstructures and to avoid molding defects, it is important to select suitable injection molding parameters. In this paper, the selection of injection molding conditions for the production of 316L stainless steel microstructures by μPIM is presented. Silicon mold inserts with 24 × 24 microcavities were injection molded on a conventional injection molding machine. The dimensions of each microcavity were Φ 100 μ m × depth 200 μm, giving an aspect ratio of 2. The distance between each microcavity was 200 μm. Five sets of experiments were conducted by varying one injection molding parameter at a time. The parameters included injection pressure, holding pressure, holding time, mold temperature, and melt temperature. Higher injection pressure and holding pressure were required during the injection molding process due to the small dimensions of the microcavities and the large number of microcavities (576 microcavities). High mold temperature was required for complete filling of the microcavities. Molded microstructures without visual defects were obtained using appropriate injection molding parameters. Catalytic debinding and sintering of the 316L stainless steel microstructures were successfully conducted.     

Synthesizing and Physical Properties of GdPr-123 High-Tc Superconductor

Ceramic Gd_1-xPr_xBa₂Cu₃O_7-y (GdPr-123) high-T_c superconductors have been synthesized by the standard solid state reaction method and characterized by XRD, SEM, TGA, and DT techniques in the range of x, 0.0 ≤ x ≤ 1.0 Samples have orthorhombic structure with Pmmm symmetry and there is a small percentage (less than 1%) of the Ba sites occupied by Pr ions in some Pr-rich samples. In these samples a small trace of BaCuO₂ and Pr BaO₃ secondary phases persist with the main peaks at 2θ = 29.3° and 28.9° respectively. Microstructure analysis indicates a uniform grain size distribution with a mean size of 5 μm. No significant change of grain size is shown throughout the range of x studied. The valence of Pr and Cu were determined by magnetization measurements in the temperature range 100 to 250°K. These data reveal a valence of 3.86+ for Pr in all samples independant of x. The similarity of the superconducting and insulating properties in this system to those of the oxygen deficient RBa₂Cu₃O₇ (R-123) (R: Y or rare earth) system implies that the mechanism of superconductivity in high-T_c. superconductors cannot be two-dimensional, even though the superconductive current occurs in two dimensions.     

The resistance of silicon carbide to static and impact local loading

The physical nature of the resistance of SiC crystals to static and local impact loading has been examined. Investigation of the temperature dependence of hardness for SiC crystals allows the determination of the characteristic deformation temperature (T* ≈ 1600 K), the parameter that characterizes the degree of covalence in interatomic bonds ( ≈ 6) and the temperature range in which a phase transition under pressure during indentation is possible (T < 800 K). Indentation technique gives possibility to construct stress-strain curves for brittle materials and to determine Hugoniot Elastic Limit. During dynamic penetration of a kinetic projectile into a SiC target the phase transition takes place.     

Combustion synthesis of advanced materials: Part I. Reaction parameters

An explanation of combustion (self propagating high temperature) synthesis (SHS) is given together with a historical perspective of the examination of such exothermic reactions. The application of thermochemical functions has been used to predict theoretically the maximum adiabatic temperature, T_ad. This, combined with a knowledge of the ignition temperature, T_ig, and the actual combustion temperature, T_c, has been used to determine the heat loss from the SHS reaction and the amount of heat needed to raise the adjacent, cold, reactant layer to the ignition temperature in order to maintain the self sustaining nature of the propagating mode of the reaction. The pertinent reaction parameters that control self propagating high temperature (combustion) synthesis reactions have been examined. These include: reactant particle size and shape; powder mixing and compaction; green density; reaction stoichiometry; impurities; volatiles and diluents; reaction environment; mode and technique of ignition; heating rate; and the effect of these parameters on the generation of heat, exothermicity and control of the SHS reaction.     

Study of Giant Magneto-Impedance Behavior in Co-Fe Based Amorphous/Nanocrystalline Materials

Giant magneto-impedance (GMI) behavior have been studied in melt spun Co_71-XFe_XCr₇Si₈B₁₄ (X = 0, 2, 3.2, 4, 6, 8, and 12 at%) alloys. The addition of Fe in the system changed the saturation magnetostriction constant from negative to positive values. The GMI property was measured with a driving current of 5 mA and 4 MHz frequency using a spectrum analyzer. The GMI ratio increased with the increase of the applied dc field and became maximum at a field H_k known as the anisotropy field of the materials. The maximum of GMI ratio (GMI_max) depended on the Fe content and was maximum (12%) for Fe = 4 at% for the alloys in the as-melt spun state. Substantial increase of GMI_max to 66% for Fe = 4 at% was observed after annealing the sample at 673 K.     

A comparison between the elastic strain energy and the plastic work at the crack tip

For ductile fractures it becomes progressively imperative to include the plastic components of the quantities defining each particular fracture criterion, in order to derive a relationship, which not only fits the experimental data but also satisfies some appropriate physical principle. In this paper the plastic components of stresses entering in the experessions for the specific plastic work were derived from the plastic singular solution described by the HRR stress field. In order to ascertain the accuracy of such approximate solutions for criteria based on energy balance, a comparison between the components of elastic strain energy density, and its total value, with the specific plastic work (W_p) was undertaken, in order to derive certain conclusions about the importance of the contribution of energy component and the specific plastic work. The analysis was based on the typical case of a transverse internal crack in plate under plane-stress conditions, which is submitted to a mode I deformation. While the dilatational (T_v) and the distortional (T_D) components of elastic SED were calculated for impending plasticity the specific plastic work (W_p) was evaluated by assuming ssy conditions and the theory of HRR field using the plastic singular solution for mode I. It was shown that the contributions of T_v, T_D, T_e (T_e = t_v + T_D) and W_P for different strain hardening exponents were varying with ductile materials, presenting a strong influence of W_p. The conclusion was that for highly strain-hardened materials the contribution of the elastic SED and particularly of T_v is important to the mode of fracture of the plate.