The influence of trace impurities on the mechanical characteristics of a superplastic 2 mol% yttria stabilized zirconia

In contrast to metallic alloys, the mechanical characteristics of superplastic ceramics are very sensitive to minor changes in levels of trace impurities. In the present study, the mechanical behavior of a 2 mol% yttria stabilized tetragonal zirconia was studied in tension and compression in two batches of material, with small variations in levels of trace impurities, to examine the influence of stress axis and impurity content on the deformation behavior. The mechanical properties of the material were characterized in terms of the expression: (epsilon)over dot proportional to sigma(n) where (epsilon)over dot is the strain rate, sigma is the stress and n is termed the stress exponent. The mechanical behavior of the ceramic was identical in tension and compression, for a material with a given level of impurity. The high purity specimens exhibited a transition from a stress exponent of similar to 3 to similar to 2 with an increase in stress, whereas the low purity material displayed only n similar to 2 behavior over the entire stress range studied. Detailed high resolution and analytical electron microscopy studies revealed that there was no amorphous phase at interfaces in both batches of material; however, segregation of Al at interfaces was detected only in the low purity material. The observed transition in stress exponents can be rationalized in terms of two sequential mechanisms: grain boundary sliding with n similar to 2 and interface reaction controlled grain boundary sliding with n similar to 3. The transition from n similar to 3 to similar to 2 occurred at lower stresses with an increase in the grain size and a decrease in the purity level.

Stability of pargasite during ultrahigh-temperature metamorphism:A consequence of titanium and REE partitioning

Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F (XF up to 0.56) content in pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 °C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust.

Synergy between tribo-oxidation and strain rate response on governing the dry sliding wear behavior of titanium.

The present work provides an insight into the dry sliding wear behavior of titanium based on synergy between tribo-oxidation and strain rate response. Pin-on-disc tribometer was used to characterize the friction and wear behavior of titanium pin in sliding contact with polycrystalline alumina disk under ambient and vacuum condition. The sliding speed was varied from 0.01 to 1.4 ms(-1), normal load was varied from 15.3 to 76 N and with a sliding distance of 1500 m. It was seen that dry sliding wear behavior of titanium was governed by combination of tribo-oxidation and strain rate response in near surface region of titanium. Strain rate response of titanium was recorded by conducting uni-axial compression tests at constant true strain rate of 100 s(-1) in the temperature range from 298 to 873 K. Coefficient of friction and wear rate were reduced with increased sliding speed from 0.01 to 1.0 ms(-1). This is attributed to the formation of in situ self lubricating oxide film (TiO) and reduction in the intensity of adiabatic shear band cracking in the near surface region. This trend was confirmed by performing series of dry sliding tests under vacuum condition of 2 x 10(-4) Torr. Characterization tools such as optical microscopy, scanning electron microscopy, and X-ray diffractometer provided evidence of such processes. These experimental findings can be applied to enhance the dry sliding wear behavior of titanium with proper choice of operating conditions such as sliding speed, normal load, and environment.

A simple circuit to aid direct measurement of capture cross sections of deep level impurities

An inexpensive and simple circuit to aid the direct measurement of majority carrier capture cross sections of impurity levels in the band gap of a semiconductor by the variable width filling pulse technique is presented. With proper synchronisation, during the period of application of the pulse, the device is disconnected from the capacitance meter to avoid distortion of the pulse and is reconnected again to the meter to record the emission transient. Modes of operation include manual triggering for long emission transients, repetitive triggering for isothermal and DLTS measurements and the DLTS mode which is to be used with signal analysers that already provide a synchronising pulse for disconnection.

Internal nitride formation during gas-phase thermal nitridation of titanium

Titanium nitride surface layers were prepared by gas-phase thermal nitridation of pure titanium in an ammonia atmosphere at 1373 K for different times. In addition to the surface nitride layer, nitride/hydride formation was observed in the bulk of the specimen. The cross-section of the specimen was characterized by various techniques such as optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, secondary ion mass spectrometry and nanomechanical testing, and the mechanism of formation of these phases is discussed.

Thermodynamic scaling theory for impurities in metals

A perturbative scaling theory for calculating static thermodynamic properties of arbitrary local impurity degrees of freedom interacting with the conduction electrons of a metal is presented. The basic features are developments of the ideas of Anderson and Wilson, but the precise formulation is new and is capable of taking into account band-edge effects which cannot be neglected in certain problems. Recursion relations are derived for arbitrary interaction Hamiltonians up to third order in perturbation theory. A generalized impurity Hamiltonian is defined and its scaling equations are derived up to third order. The strategy of using such perturbative scaling equations is delineated and the renormalization-group aspects are discussed. The method is illustrated by applying it to the single-impurity Kondo problem whose static properties are well understood.

Microstructural study of rapidly solidified titanium eutectoid alloysstar, open

Rapid solidification of Ti-7.3wt.%Cu (near-eutectoid composition), Ti-36.2wt.%Ni and Ti-34.3wt.% Ni-5.8wt.%Si alloys has been carried out by electron beam melting and splat quenching on a water-cooled rotating copper disc. The product obtained was in the form of thin ribbons 60–100 μm thick. Transmission electron microscopy studies of Ti---Cu alloy splats showed that the microstructure consisted of a mixture of martensite and a lamellar eutectoid product. The formation of the intermetallic compound Ti2Cu involved a diffusionless ω transformation and spinodal clustering. In the case of Ti---Ni alloy the as-quenched microstructure is complex, consisting of α, transformed β and intermetallic phases. This could have arisen possibly as a result of local variation in cooling rates. Rapid solidification of Ti---Ni---Si alloy resulted in partial quenching of an amorphous phase. The amorphous phase was seen to be extremely hard (a Vickers hardness of about 800 HV).

Electrical transport in layered crystalline semiconductors GeS doped with Ag, P impurities at high pressure

A study of the transport properties of layered crystalline semiconductors GeS (undoped and doped with Ag, P impurity) under quasihydrostatic pressure using Bridgman anvil system is made for the first time. Pressure-induced effects in undoped crystals reveal initial rise in resistivity followed by two broad peaks at higher pressures. Silver doping induces only minor changes in the behaviour except removing the second peak. Phosphorous impurity is found to have drastic effect on the transport properties. Temperature dependence of the resistivity exhibits two activation energies having opposite pressure coefficients. Results are discussed in the light of intrinsic features of the layered semiconductors.

Texture dependent stress corrosion failure of commercial titanium sheets in bromine-methanol solution

The stress corrosion cracking (SCC) characteristics of agr-titanium sheets in a bromine-methanol solution have been studied in the annealed and cold-rolled conditions using longitudinal and transverse specimens. The times to failure for annealed longitudinal specimens were longer than those for similarly tested transverse specimens. The cold-rolled specimens developed resistance to SCC, but failed by cleavage when notched, unlike the intergranular separation in annealed titanium. The apparent activation energy was found to be texture dependent and was in the range 30 to 51 kJ mol–1 for annealed titanium, and 15kJ mol–1 for cold-rolled titanium. The dependence of SCC behaviour on the texture is related to the changes in the crack initiation times. These are caused by changes in the passivation and repassivation characteristics of the particular thickness plane. The thickness planes are identified with the help of X-ray pole figures obtained on annealed and cold-rolled material. On the basis of the activation energy and the electrochemical measurements, the mechanism of SCC in annealed titanium is identified to be the one involving stress-aided anodic dissolution. On the other hand, the results on the cold-rolled titanium are in support of the hydrogen embrittlement mechanism consisting of hydride precipitation. The cleavage planes identified from the texture data match with the reported habit planes for hydride formation.

Approximate solution for the redistribution of impurities during second oxidation

The impurity profile for the second oxidation, used in MOST fabrication, has been obtained by Margalit et al. [1]. The disadvantage of this technique is that the accuracy of their solution is directly dependent on the computer time. In this article, an analytical solution is presented using the approximation of linearizing the second oxidation procedure.

Field Emission Properties of Carbon Nanotube Arrays with Defects and Impurities

It has been found experimentally that the results related to the collective field emission performance of carbon nanotube (CNT) arrays show variability. The emission performance depends on the electronic structure of CNTs (especially their tips). Due to limitations in the synthesis process, production of highly pure and defect free CNTs is very difficult. The presence of defects and impurities affects the electronic structure of CNTs. Therefore, it is essential to analyze the effect of defects on the electronic structure, and hence, the field emission current. In this paper, we develop a modeling approach for evaluating the effect of defects and impurities on the overall field emission performance of a CNT array. We employ a concept of effective stiffness degradation for segments of CNTs, which is due to structural defects. Then, we incorporate the vacancy defects and charge impurity effects in our Green's function based approach. Simulation results indicate decrease in average current due to the presence of such defects and impurities.

Implantation Induced Hardening of Nanocrystalline Titanium Thin Films

Formation of nanocrystalline TiN at low temperatures was demonstrated by combining Pulsed Laser Deposition (PLD) and ion implantation techniques. The Ti films of nominal thickness similar to 250 nm were deposited at a substrate temperature of 200 degrees C by ablating a high pure titanium target in UHV conditions using a nanosecond pulsed Nd:YAG laser operating at 1064 nm. These films were implanted with 100 keV N+ ions with fluence ranging from 1.0 x 10(16) ions/cm(2) to 1.0 x 10(17) ions/cm(2). The structural, compositional and morphological evolutions were tracked using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometry (SIMS) and Atomic Force Microscopy (AFM), respectively. TEM analysis revealed that the as-deposited titanium film is an fcc phase. With increasing ion fluence, its structure becomes amorphous phase before precipitation of nanocrystalline fcc TiN phase. Compositional depth profiles obtained from SIMS have shown the extent of nitrogen concentration gradient in the implantation zone. Both as-deposited and ion implanted films showed much higher hardness as compared to the bulk titanium. AFM studies revealed a gradual increase in surface roughness leading to surface patterning with increase in ion fluence.

Reactive Pulsed Laser Deposition of titanium nitride thin film: Optimization of process parameters using Secondary Ion Mass Spectrometry

Reactive Pulsed Laser Deposition is a single step process wherein the ablated elemental metal reacts with a low pressure ambient gas to form a compound. We report here a Secondary Ion Mass Spectrometry based analytical methodology to conduct minimum number of experiments to arrive at optimal process parameters to obtain high quality TiN thin film. Quality of these films was confirmed by electron microscopic analysis. This methodology can be extended for optimization of other process parameters and materials. (C) 2009 Elsevier B.V. All rights reserved.

Reversible double insertion of aryl isocyanates into the Ti-O bond of titanium(IV) isopropoxide

The insertion of phenyl isocyanate into titanium isopropoxide leads to the formation of a dimeric complex [Ti(O ' Pr)(2)(mu-O ' Pr){C6H5N(O ' Pr)CO}](2) (1) which has been structurally characterized. Reaction of titanium isopropoxide with two and more than 2 equiv. of phenyl isocyanate is complicated by competitive, reversible insertion between the titanium carbamate and titanium isopropoxide. The ligand formed by insertion of phenyl isocyanate into the titanium carbamate has been structurally characterized in its protonated form C6H5N{C(O ' Pr)O}C(O)N(H)C6H5 (3aH). Insertion into the carbamate is kinetically favored whereas insertion into isopropoxide gives the thermodynamically favored product. (c) 2004 Elsevier B.V. All rights reserved.

Localised modes due to impurities in III–V semiconductors

Local mode frequencies due to substitutional impurities in some III–V semiconductors are calculated using Green functions on the mass defect approximation and compared with experimental results.