Coefficients of sliding friction of single crystals of high explosives under different rubbing conditions

The coefficients of sliding friction of single crystals of commonly used high explosives pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX) and beta-cyclotetramethylene tetranitramine (β-HMX) under several rubbing configurations and at a relative sliding speed of 0.22 mm s−1 were measured. The sliding configurations were (1) crystal–polished steel pairs, (2) like-crystal pairs and (3) unlike-crystal pairs. For every rubbing configuration the friction force showed oscillations, which are thought to be caused by the formation and shearing of the adhesive junctions formed at the surface of the rubbing crystals. This shearing of the adhesive junctions led to the formation of microscopic and sub-microscopic particles, which were confirmed by an environmental scanning electron microscope study. For every rubbing configuration and for relatively high normal loads pressing the rubbing crystals together, the coefficient of friction was generally in the range 0.2–0.25 and it has been concluded that the coefficient of friction is controlled by the adhesion with almost negligible contribution from the ploughing component. From a knowledge of the coefficient of friction and the uniaxial yield stress values of single crystals of RDX and β-HMX, the shear strength of these crystals were determined to be ~13.4 MPa and ~16.8 MPa, respectively.

Source:IOPscience

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The influence of crack face electrical boundary conditions on the nonlinear behavior of ferroelectric single crystal

The nonlinear electromechanical behavior of a cracked ferroelectric single crystal subjected to pure electrical loading is investigated by a three-dimensional phase field model for different crack face electrical boundary conditions. Phase field simulations show that crack face electrical boundary conditions have significant influence on the electrical and mechanical responses of the ferroelectric single crystal to an external electric field. The coercive field for the polarization switching of a single crystal with an electrically permeable crack is about 50% larger than that for a single crystal with an electrically impermeable crack. The remanent strain and the strain variation induced by polarization switching in a single crystal with a permeable crack are larger than those with an impermeable crack. The different macroscopic nonlinear behaviors are attributed to different polarization switching processes. It is found that domain switching takes place from the surface of a single crystal with a permeable crack, while it begins from the vicinity of the crack tip when the crack is impermeable. A ferroelectric single crystal with an impermeable crack exhibits strip 90° domain switching under a negative electric field, which is consistent with experimental observation.

Source:IOPscience

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On the optical properties of undoped and rare-earth-doped yttrium aluminium garnet single crystals

Optical absorption and photoluminescence (PL) measurements were performed on single crystals of undoped Y3Al5O12 (YAG) and a number of rare-earth-doped YAG to study the effect of dopant type and concentration, growth atmosphere, post-growth annealing and UV irradiation on the optical properties of YAG crystals. The presence of hydrogen in the growth atmosphere was found to be essential for enhancing the incorporation of Ce ions in the Ce3+ state in Ce-doped YAG (Ce : YAG). Annealing in air was shown to have no effect on the PL emission of Ce : YAG crystals. An absorption peak around 256 nm was observed in the undoped YAG and Ce : YAG crystals after air anneal at 1200 °C. Optical absorption and annealing experiments support the association of the 256 nm peak with Fe impurities and oxygen ions. UV irradiation modifies the valency of impurities and generates electronic defects leading to an increase in the optical density of YAG crystals. Optimizing the growth and annealing conditions is critical in order to develop Ce : YAG single crystals as efficient scintillators.

Source:IOPscience

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Float zone single crystals for testing rods, pulled under electron beam heating

The article is devoted to the description of mathematical modelling and attempts to grow silicon single crystals from a pedestal. The crystals are intended to be used for impurity composition testing in rods with a diameter of 300 mm grown with electron beam heating. The testing is being planned both by the method of FTIR spectroscopy and by functional testing of devices that might be manufactured using single crystals grown from pedestal. The article also describes the improvements of equipment, which were necessary for crystal growth attempts, and substantial difficulties that occurred in the process and hindered single crystal growth, allowing to obtain only a polycrystalline sample.

Source:IOPscience

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Light emission from organic single crystals operated by electrolyte doping

Light-emitting devices based on electrolytes, such as light-emitting electrochemical cells (LECs) and electric double-layer transistors (EDLTs), are solution-processable devices with a very simple structure. Therefore, it is necessary to apply this device structure into highly fluorescent organic materials for future printed applications. However, owing to compatibility problems between electrolytes and organic crystals, electrolyte-based single-crystal light-emitting devices have not yet been demonstrated. Here, we report on light-emitting devices based on organic single crystals and electrolytes. As the fluorescent materials, α,ω-bis(biphenylyl)terthiophene (BP3T) and 5,6,11,12-tetraphenylnaphthacene (rubrene) single crystals were selected. Using ionic liquids as electrolytes, we observed clear light emission from BP3T LECs and rubrene EDLTs.

Source:IOPscience

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