Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

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Review: liquid phase sintering

Bipolar plates for PEM fuel cells: A review

This article presents an overview of the up-to-date developments in boron nitride nanotubes (BNNTs), including theory, fabrication, structure, physical properties, chemical functionalization and applications. Soon after the discovery of carbon nanotubes, BNNTs were theoretically predicted, followed by their successful fabrication by arc-discharge in 1995. Subsequently, various methods were developed for the BNNT synthesis, although till now, the growth of highly pure single-walled BNNTs at large quantities remains a challenge. The physical property investigations reveal that BNNTs’ exhibit a stable wide band gap, superb mechanical strength, high thermal conductivity, ultra-violet light emission, etc. All these properties build up the solid basis for their future technological applications. Chemical modification is also a decent approach to adjust the BNNTs properties. In recent years the yield of multi-walled BNNTs has reached the grams level, that can allow their detailed chemical functionalization studies. So far, many kinds of functionalizations through different weak interactions and covalent bonding were developed. These treatments improved BNNT dispersions in solvents and extended their fields of applications. Moreover, some application-related studies on multi-walled BNNTs, such as composites fabrication, hydrogen storage, biocompatibility, and mechanical, and electrical breakdown tests have also been started in recent years.

Boron nitride nanotubes

Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

Gas diffusion layer for proton exchange membrane fuel cells—A review

Zinc phthalocyanine (ZnPc) is a promising candidate for solar-cell applications, because it is easily synthesized and is non-toxic to the environment. Recently, phthalocyanine (Pc) was considered by many researchers as the active part in all-organic solar cells, i.e. plastic solar cells. It is a self-assembling liquid crystal developed from a common deep-blue-green pigment. It exhibits a characteristic structural self-organization, which is reflected in an efficient energy migration in the form of extinction transport. In this paper we have report structural, surface morphological, optical and thermal properties of flash-evaporated zinc phthalocyanine thin films. The samples were prepared by using a vacuum coating unit on well-cleaned glass substrates under a pressure of 7×10-6 Torr. A constant rate of evaporation (1 A/s) was maintained throughout the evaporation of the ZnPc thin films. A rotary drive was employed to obtain uniform thickness during the evaporation. Thicknesses of the films were monitored by a quartz-crystal thickness monitor and were cross verified by the multiple-beam interferometry technique. The X-ray-diffraction pattern reveals the crystalline nature of the films deposited at higher substrate temperatures. Scanning electron microscope and scanning probe microscope nanoscope studies were carried out to determine the surface uniformity and homogeneity of the films for interfacing and application purposes. All the films were found to possess small crystallites less than 100 nm in size. The optical transmittance measurements were carried out using a spectrophotometer in the visible region (400–800 nm) and the films were found to be absorbing in nature. The band gap of the ZnPc thin films is 1.97 eV and the optical transition was found to be direct and allowed. The absorption coefficient, extinction coefficient and refractive index of the ZnPc films were evaluated and the results are discussed. Differential scanning calorimetry studies of ZnPc films were carried out and a phase transition from α to β was observed at 538 K.

Characterization of zinc phthalocyanine (ZnPc) for photovoltaic applications

Interfacial phenomena in wettability of TiC by Al–Mg alloys

Influence of the hydrophobic material content in the gas diffusion electrodes on the performance of a PEM fuel cell

The susceptibility to stress corrosion cracking (SCC) in a NACE solution saturated with H2S, of the X-52 and X-70 steels was studied using slow strain rate tests (SSRT) and electrochemical evaluations. SCC tests were performed in samples which include the longitudinal weld bead of the pipeline steels and were carried out in the NACE solution at both room temperature and 50 °C. After failure, the fracture surfaces were observed in a scanning electron microscope (SEM) and the chemical analysis were obtained using X-rays energy dispersive (EDXs) techniques. The specimens tested in air, exhibited a ductile type of failure, and whereas, those tested in the corrosive solution showed a brittle fracture. Specimens tested in the NACE solution saturated with H2S presented high susceptibility to SCC. Corrosion was found to be an important factor in the initiation of some cracks. In addition, the effect of the temperature on the corrosion attack was explored. The susceptibility to SCC was manifested as a decrease in the mechanical properties. Potentiodynamic polarization curves and hydrogen permeation measurements were made. The diffusion of atomic hydrogen was related to this fracture forms. The hydrogen permeation flux increased with the increasing of temperature.

Slow strain rate corrosion and fracture characteristics of X-52 and X-70 pipeline steels

Microstructural and mechanical properties characterizations of nanocrystalline Ni–Al+M alloys were carried out. M represents the addition of transition elements such as Fe, Ga and Mo. Mechanical alloying (MA) and densification by hot-pressing techniques were used to produce and consolidate the alloys. The mechanical alloyed powders have a microstructure consisting of nanometer size particles. The main effect of these elements on the microstructure of the NiAl alloy is the refinement of the grain size since this process gives rise to crystallite sizes in the nanometric range. Also, the three minor additional elements form a solid solution with the intermetallic structure of the NiAl. Additions of Mo in the range of 2–6 at.% cause second phase formation of Mo2C. Mechanical properties (hardness, yield stress and strain) were obtained as a function of composition and sintering temperature. The highest hardness value is obtained for the NiAl+2Ga+6Mo (at.%) alloy and always corresponded to the highest densifications (98%) for the sintered samples at 1200 °C during 30 min. The Ni–Al alloys with different additions of elements lead to higher deformations than those reported previously.

R. Pérez

Papers

Characterization of zinc phthalocyanine (ZnPc) for photovoltaic applications

Interfacial phenomena in wettability of TiC by Al–Mg alloys

Influence of the hydrophobic material content in the gas diffusion electrodes on the performance of a PEM fuel cell

Slow strain rate corrosion and fracture characteristics of X-52 and X-70 pipeline steels

Improvement of the mechanical properties in a nanocrystalline NiAl intermetallic alloy with Fe, Ga and Mo additions