Morphology and constitution of the compound layer formed on nitrided Fe–4wt.%V alloy

The microstructure of the compound (“white”) layer formed on the surface of Fe–4wt.%V alloy, by nitriding in a gas mixture of ammonia and hydrogen at 580 °C, has been investigated by employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The compound layer is dominantly composed of γ^|-Fe₄N nitride. Quantitative analysis of the composition data demonstrated that V is present in the compound layer as VN precipitates, i.e. V is not taken up significantly in (Fe, V) nitrides. A mechanism for compound-layer formation has been proposed.

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The synthesis and characterisation of grafted random styrene butadiene for biomedical applications

The work undertaken investigates the spectral, thermal and surface characteristics of a random styrene butadiene rubber (SBR) with monomeric graft(s) of acrylic acid (AA), N-vinyl-2-pyrrolidinone (NVP) or N-isopropylacrylamide (NIPAAm) synthesised using UV polymerisation. The grafted materials were characterised by differential scanning calorimetry (DSC), modulated differential scanning calorimetry (MDSC), attenuated total reflectance infrared Fourier transform spectrometry (ATR-FTIR) and atomic force microscopy (AFM). Thermograph analysis has shown an endothermic transition occurring at ~75 °C for all random SB-g-NVP copolymers, whereas the T _g value for random SB copolymer was found at 60 °C, thus suggesting that a chemical reaction between styrene and NVP had occurred. Similar thermal profiles to that of random SB-g-NVP copolymers were evident when random SB was UV polymerised with AA. When NIPAAm was grafted onto random SB, a notable exothermic transition was evident in all samples tested using DSC. It was established using MDSC that this exothermic transition was caused by the breakdown of crosslinks as a result of UV polymerisation.

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Melt synthesis of oxide phosphors with K2NiF4 structures: CaLa1? x Eu x GaO4

In order to synthesize compounds of various Perovskite-related structures, we have utilized a novel “melt synthesis technique” for phosphors rather than the conventional solid state reaction techniques. The solid state reactions require multi-step processes of heating/cooling with intermediate grindings to make homogeneous samples. However, for the melt synthesis, it is possible to make a homogeneous sample in a single step within a short period of time (1–60 s) due to the liquid phase reaction in the molten samples, which were melted by strong light radiation in an imaging furnace. In this study, we have prepared a red-phosphor CaLaGaO₄:Eu³⁺ which has a perovskite—related layered K₂NiF₄ structure. Well-crystallized CaLa_1−x Eu _x GaO₄ samples with the K₂NiF₄ structure have been obtained up to x = 0.25, but there was the formation of an olivine phase when x = 0.5–1.0. The red emission at 618 nm increased with the increasing value of x up to x = 0.25.

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Optical switching of a photochromic bis-phenylazo compound in PMMA films

We present our results on a newly synthesized bis-phenylazo derivative, namely bisperfluoroalkylsulfonylamino- arylazomethylene-triphenyl-phosphorane (BAM-TPP). Thin films of BAM-TPP in polymethylmethacrylate (PMMA) matrix were prepared. The films (thickness, d < 60 μm) were exposed to UV-vis light with variable intensity in order to stimulate the photochromic reaction of BAM-TPP. The resulting absorption changes of the BAM-TPP/PMMA films were investigated by spectrophotometry. The absorption spectra reveal that BAM-TPP molecules in PMMA undergo photoisomerization with resulting decrease of absorbance in the range 500–700 nm. Finally, the time response of film transmittance at 514 nm under increasing CW light intensity was recorded, showing that the reverse photochromic process brings the absorbance back to its pristine value. The obtained films thus proved to be suitable for optical switching applications.

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Compositional dependence of some physical properties of ZnO–PbO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Six glasses of the chemical composition 10ZnO–xPbO–(90−x) P₂O₅ were prepared. With an increase in PbO content a non-monotonous step like increase in the density, in the glass transition temperature, and in the refractive index was observed. From the Raman and IR spectra studied the evidence is given for the phosphate network depolymerization as PbO content increases. Increase in PbO content leads also to an increase in refractive index (n) up to n = 1.74, for x = 55, and to an increase in the glass transition temperature (T _g) from T _g = 270 °C (x = 30) to T _g = 360 °C (x = 55).

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Amorphous and nano crystalline phase formation in Ni<Subscript>2</Subscript>MnGa ferromagnetic shape memory alloy synthesized by melt spinning

Melt spinning technique was used to synthesize Ni₂MnGa ferromagnetic shape memory alloy ribbons. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis of the ribbon synthesized at lower wheel speed (20 m/s) reveal the formation of very fine clusters of austenitic phase of Ni₂MnGa. However at higher wheel speed (30 m/s) the formation of martensite and nanoparticles of Ni₂MnGa with a size range of 10–20 nm in the amorphous matrix is observed. Also an amorphous phase was observed at higher wheel speed in some areas of the ribbon. Annealing (1000 °C, 1 h) of the ribbon synthesized at higher wheel speed resulted in martensite and γ (gamma) phases. Amorphous phase, Ni₂MnGa nanoparticles, and the martensite phase are analyzed in detail.

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Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρ_c) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρ_c of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10⁻⁵ Ω−cm ² is measured for the Ag metal contacts formed on the n⁺ silicon surface. This value is much lower than the ρ_c data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρ_c of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

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Solid-particle erosion behavior of high-performance thermoplastic polymers

Solid-particle erosion tests were carried out to study the effect of matrix material, impact angle, and impact velocity on the erosion behavior of seven types of thermoplastic neat polymers (i.e., polyetherimide, polyetheretherketone, polyetherketone, polyphenylene sulfide, polyethersulfone, polysulfone, and ultrahigh molecular weight polyethylene). Steady-state erosion rates of these polymers have been evaluated at different impact angles (15–90°) and impact velocities (25–66 m/s). Silica sand of particle size 200 ± 50 μm was used as the erodent. These polymers have exhibited maximum erosion rate (E _max) at 30° impact angle indicating ductile erosion behavior. Some of these polymers have shown an incubation behavior at lower impact velocities for an impact angle of 90°. Correlations among steady-state erosion rate and mechanical properties and glass transition temperature (T _g) were established. Morphology of eroded surfaces was examined using scanning electron microscopy and possible wear mechanisms were discussed.

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Development of nano-macroporous soda-lime phosphofluorosilicate bioactive glass and glass-ceramics

We have extended the usefulness of bioactive glass-ceramics for the repair and reconstruction of hard tissues by introducing F ions that are known to be beneficial, especially in dentistry. Nano-macro multimodal porosity in soda-lime phosphofluorosilicate bulk samples was introduced by the recently developed melt-quench-heat-etch method. The choice of starting glass composition is based on 48SiO₂–2.7P₂O₅–xCaF₂–yCaO–zNa₂O where x = 0, 1, 4, 8, 10, 12, and (y + z) = 49.3 − x (mol%). The effect of thermal and chemical treatment on the microstructure of samples is characterized by SEM, XRD and EDX. We find the formation of many crystalline phases, but mainly sodium calcium silicate, calcium phosphate, fluorapatite and calcium silicate. The bioactivity of soda-lime phosphofluorosilicate glass-ceramics is assessed by monitoring the formation of hydroxyl apatite (HA) layer: fluorapatite phase accelerates the rate of HA layer formation; the initial composition and multi-modal porosity are other key parameters that impact the formation of HA. The present porous glass-ceramics should be superior candidates for use in dental bone regeneration.

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Microstructures and microwave dielectric properties of low-temperature sintered Ca2Zn4Ti15O36 ceramics

Low-temperature sintered Ca₂Zn₄Ti₁₅O₃₆ microwave dielectric ceramic was prepared by conventional solid state reaction method. The influences from V₂O₅ addition on the sintering behavior, crystalline phases, microstructures and microwave dielectric properties were investigated. The crystalline phases and microstructures of Ca₂Zn₄Ti₁₅O₃₆ ceramic with V₂O₅ addition were investigated by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). V₂O₅ addition lowered the sintering temperature of Ca₂Zn₄Ti₁₅O₃₆ ceramics from 1140 °C to 930 °C. Ca₂Zn₄Ti₁₅O₃₆ ceramic with 5wt% V₂O₅ addition could be densified well at 930 °C, and showed good microwave dielectric properties of ε_r ~ 46, Q × f ~ 13400 GHz, and temperature coefficient of resonant frequency (τ_f) ~ 164 ppm/°C.

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Nitrogen absorption by Fe-1.04at.%Cr alloy: uptake of excess nitrogen

By dedicated pre-nitriding (at 580 °C in an ammonia/hydrogen gas atmosphere) and de-nitriding (at 470 °C in a hydrogen gas atmosphere) experiments, performed on Fe-1.04at.%Cr alloy, it could be demonstrated that the uptake of “excess” nitrogen by the nitrided ferritic matrix is not due to the presence of iron in chromium-nitride precipitates, as it was suggested previously. The determination of nitrogen-absorption isotherms for these pre-nitrided and de-nitrided Fe-1.04at.%Cr alloy specimens revealed that the total amount of excess nitrogen in the alloy is composed of two parts: (a) nitrogen adsorbed at the precipitate/matrix interface, and (b) nitrogen dissolved interstitially in the ferrite matrix strained by the misfit between (coherent) the CrN precipitates and the matrix.

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SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si₃N₄) deposited over the n⁺-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si₃N₄ layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n⁺-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n⁺-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρ _c (in Ω-cm²) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρ _c  = 2.53 × 10⁻⁵ Ω-cm² is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρ _c is two orders of magnitude lower than the typical value of ρ _c  = 3 × 10⁻³ Ω-cm² reported previously for the Ag contacts of the solar cell. Such low value of ρ _c for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρ _c versus impurity doping level (N _s ) shows that measured value of the ρ _c follows a linear relationship with the N _s as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n⁺-Si surface of the silicon solar cell.

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A quenching apparatus for the gaseous products of the solar thermal dissociation of ZnO

Rapid cooling for avoiding the recombination of Zn vapor and O₂ derived from the solar thermal dissociation of ZnO is investigated using a thermogravimeter coupled to a quenching apparatus. The ZnO sample, which is placed in a cavity receiver and directly exposed to concentrated solar irradiation, underwent dissociation in the temperature range 1,820–2,050 K at a rate monitored by on-line thermogravimetry. The product gases were quenched by water-cooled surfaces and by injection of cold Ar at cooling rates from 20,000 to 120,000 K/s, suppressing the formation of ZnO in the gas phase and at the walls. Zinc content of the collected particles downstream varied in the range 40–94% for Ar/Zn(g) dilutions of 170 to 1,500.

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Measurement and modelling of heat transfer in paper coating structures

Thermal conductivity of paper coatings is increasingly important in the performance of many printing processes, including traditional heatset web offset and thermal papers, as well as digital processes, such as electrophotography. This work studies the extension of a Modified Lumped Parameter Model, previously used successfully to describe talc coatings, to model the thermal conductivity–coating structure relationships of calcium carbonate (gcc: 60 wt% < 2 μm) coatings. A series of compact tablets were used to provide experimental values of thermal diffusivity and conductivity. The samples studied covered a range of latex binder addition levels, namely 0–25 parts, based on 100 parts pigment, of 0.2 μm styrene acrylate latex. Combining the observed thermal properties with knowledge of the pore structure changes induced by the latex addition, it is possible to establish initial correlation with the model, in which the connectivity of the structure is increased at low latex dose illustrating the initial increase in effective thermal conductivity. The practically hard sphere properties of the latex used, combined with the broad size distribution of the gcc, produce a disruptive packing effect as the dose level increases, such that the conductivity reflects a competition between the increasing connectivity provided by the latex versus the increasing relative pore size in the network structure. It is recognized that the fixed pigment volume in the model unit cell diverts from a true representation of the residual porosity. At the highest latex dose levels, the intrinsically less conducting properties of the latex begin to dominate.

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Growth of massive cementite layers; thermodynamic parameters and kinetics

Massive, pure cementite layers were grown on ferrite substrates by nitrocarburising in a dedicated NH₃/H₂/CO/N₂ containing gas atmosphere at temperatures in the range of 783–843 K. From the parabolic layer-growth constants, an “apparent” activation energy for cementite-layer growth of 109 ± 12 kJ/mol was obtained. This “apparent” activation energy can be subdivided into a positive contribution due to the activation energy for (tracer) diffusion of carbon in cementite and a negative contribution due to the temperature dependence of the difference of the carbon activity in cementite at the surface and at the interface cementite/ferrite.

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Microstructure and stresses in a keatite solid–solution glass–ceramic

The microstructure of a translucent keatite solid–solution glass–ceramic (keatite s.s.) of the LAS-system (Li₂O–Al₂O₃–SiO₂) has been analyzed with SEM, AFM, XRF, XRD, and TEM. The glass–ceramic consists mainly of keatite s.s. with minor secondary phases such as zirconium titanate, gahnite and probably rutile. Furthermore the resistance to temperature differences (RTD) of this glass–ceramic was investigated. It is shown that, in spite of the relatively high coefficient of thermal expansion (CTE) of about 1 × 10⁻⁶ K⁻¹, an improved RTD can be achieved by special ceramization treatment. With this, compressive stresses in the first 100 μm to 150 μm are induced. These stresses can presumably be contributed to a difference in CTE between the surface-near zone and the bulk. Said CTE difference is caused by chemical gradients of CTE-relevant elements, such as Zn, K, and supposedly additional alkali elements such as Li. These stresses are useful to increase the strength and application range of glass–ceramics based on keatite s.s.

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Spallation of two thermal barrier coating systems: experimental study of adhesion and energetic approach to lifetime during cyclic oxidation

To understand the degradation of two thermal barrier coating (TBC) systems, we determined the adhesion energy between the bondcoat and the topcoat and its evolution during cyclic oxidation at 1,100 °C, by means of a modified 4-point bending test. An yttria stabilized zirconia (YSZ) ceramic topcoat was deposited by electron beam physical vapour deposition (EBPVD) on a Ni-based superalloy with either an intermediate β-(Ni,Pt)Al bondcoat or a newly developed Zr-doped β-NiAl bondcoat. Although a similar evolution of the adhesion energy during cyclic oxidation has been recorded for both systems, observations of the fracture surfaces combined with a microstructure study revealed different degradation mechanisms. An energetic model of spallation is applied to predict their lifetime. According to this approach, the TBC failure is induced by the accumulation of strain energy in the ceramic layers and resisted by the interfacial fracture toughness. The predicted lifetime is consistent with experiments for both systems.

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Self-assembled nanowires on semiconductor surfaces

A number of different families of nanowires which self-assemble on semiconductor surfaces have been identified in recent years. They are particularly interesting from the standpoint of nanoelectronics, which seeks non-lithographic ways of creating interconnects at the nm scale (though possibly for carrying signal rather than current), as well as from the standpoint of traditional materials science and surface science. We survey these families and consider their physical and electronic structure, as well as their formation and reactivity. Particular attention is paid to rare earth nanowires and the Bi nanoline, both of which self-assemble on Si(001).Further information within the topic of this review article, including an up-to-date list of relevant publications, can be found on our Website. The address is:

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