The relations between the shear modulus, the bulk modulus and Young's modulus for porous isotropic ceramic materials

A relation between the shear modulus and Young's modulus of isotropic porous ceramics has been derived based on the Mori–Tanaka mean-field approach. The applicability of the relation has been evaluated using the experimental values available in the literature for the shear modulus, the bulk modulus and Young's modulus of porous ceramics prepared using various processing techniques and powder sizes. It is also shown that the ratio of the shear to Young's modulus of porous ceramics can be approximated by a constant value of 0.391.     

Novel low-cost approach for removal of surface contamination before texturization of commercial monocrystalline silicon solar cells

This paper reports a novel approach on the surface treatment of monocrystalline silicon solar cells using an inorganic chemical, sodium hypochlorite (NaOCl) that has some remarkable properties. The treatment of contaminated crystalline silicon wafer with hot NaOCl helps the removal of organic contaminants due to its oxidizing properties. The objective of this paper is to establish the effectiveness of this treatment using hot NaOCl solution before the saw damage removal step of the conventional NaOH texturing approach. A comparative study of surface morphology and FTIR analyses of textured monocrystalline silicon surfaces with and without NaOCl pre-treatment is also reported. The process could result in a significant low cost approach viable for cleaning silicon wafers on a mass production scale.     

Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication

Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO₃–HF–CH₃COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO₃–H₂O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of 14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.     

Recent developments in the volatile corrosion inhibitor (VCI) coatings for metal: a review

Corrosion is a crucial worldwide problem that strongly affects metals. Out of the several ways to prevent corrosion, volatile corrosion inhibitors (VCIs) are predominantly used as a method of temporary protection. These compounds have the ability to vaporize and condense on the surface of the ferrous or nonferrous material and make the substrate less susceptible to corrosion but work only in a confined space. VCI compounds form a monomolecular layer over the metal surface, thereby preventing the electrochemical reactions on the metal surface which lead to corrosion. This review article outlines the use of volatile corrosion inhibitors (VCIs) as a temporary corrosion protection technique, their working mechanisms and the various compounds used as VCI. It also anchors on the latest research works which have been carried out on VCI coatings along with a glimpse of the works that were carried out in the past. The various factors affecting the volatile corrosion inhibitors along with the merits of VCI coatings are discussed in this paper. Formulations for both strippable and permanent VCI coatings are also reported with the various testing methods. Lastly, an overview of the recent developments of VCI coatings along with the various green VCI is given.     

Optimization of Pocket Geometry for Friction Reduction in Piston–Liner Contacts

It has recently been shown that rectangular surface pockets are effective in reducing friction in a piston–liner type contact, providing that they are oriented with their long axis transverse to the sliding direction so that entrained features fit completely within the contact area (Vl?descu, et al., Tribology International, 82, 28–42, 2015; Vl?descu, et al., Tribology International, 115, 140–153, 2017). The aim of the current study was to identify the optimal geometric parameters of theses rectangular features. To do this, a friction rig that simulated a piston–liner contact under highly controlled conditions was used to test a series of textured specimens with pockets of different depth, breadth and density. Each of these geometric parameters was varied and tested independently, while keeping the other two constant. Experimental conditions were set in order to place the contact in different lubrication regimes.

Results were analyzed to determine a set of criteria for the optimum pocket geometry; however, this was shown to change depending on the test conditions and should therefore be adjusted depending on the position along the stroke. Specifically, at low speed when the contact is operating under boundary lubrication, pockets should be deep, wide, and densely spaced. This confirms recent findings, which suggested that, in this regime, pocket volume is often a more critical parameter than depth, width, or spacing individually. Conversely, under mixed lubrication toward the transition to the full film regime, pockets should be narrow and sparsely spaced. These results also explain the difficulties encountered in several previous studies that attempted to define a single optimum pocket geometry.

Finally, the impact of pocket position relative to reversal was assessed for various lubrication conditions. This revealed how pockets should be placed close to, but not directly at, top and bottom dead center to provide a beneficial squeeze film, which is present at reversal.     

Multiwall carbon nanotubes grown by thermocatalytic carbonization of polyacrylonitrile

A method is reported for the growth of multiwall carbon nanotubes (MWCNTs) using polyacrylonitrile as a solid carbon source and nanosized SiO₂ particles as catalyst. The nanotubes were grown either on a Si substrate or as a freestanding film at temperatures as low as 800 °C. The smallest measured inner diameter of the resultant MWCNTs is about 0.6 nm and therefore this method provides a new direction to prepare MWCNTs with very small inner diameter from solid carbon source.     

Galvanic porous silicon composites for high-velocity nanoenergetics

Porous silicon (PS) films ～65-95 μm thick composed of pores with diameters less than 3 nm were fabricated using a galvanic etching approach that does not require an external power supply. A highly reactive, nanoenergetic composite was then created by impregnating the nanoscale pores with the strong oxidizer, sodium perchlorate (NaClO(4)). The combustion propagation velocity of the energetic composite was measured using microfabricated diagnostic devices in conjunction with high-speed optical imaging up to 930000 frames per second. Combustion velocities averaging 3050 m/s were observed for PS films with specific surface areas of ～840 m(2)/g and porosities of 65-67%.