Investigating the mechanisms of diamond polishing using Raman spectroscopy

Recent research has shown that a phase transformation of diamond to a different form of carbon is involved when diamonds are polished in the traditional fashion. The question as to how this phase transformation is activated and maintained to produce high wear rates is of great technological interest since it may radically change the way we view the processing of diamond. This paper describes the use of Raman spectroscopy to examine debris produced on the diamond polishing wheel, both during its preparation and during polishing. In addition, polished diamond surfaces were examined for the possible existence of non-diamond surface layers in an attempt to identify material removal mechanisms. Raman spectroscopy proves ideal for these analyses because its relatively high spatial resolution is well suited to the analysis of small wear features and debris particles, and because of the wealth of information it reveals about chemical structure. This level of structural information has been lacking in previous analyses of diamond polishing debris. In addition to the non-diamond carbon found in the wear debris, significant quantities of two iron oxides, magnetite (Fe₃O₄) and haematite (α-Fe₂O₃), were also found. An interesting observation was that a transformation from magnetite to haematite could be induced either by using high power laser excitation or by frictional heating during polishing. It is suggested that some of the Raman peaks previously attributed to lonsdaleite might better be explained by the presence of these oxides.     

RF-plasma assisted pulsed laser deposition of nitrogen-doped SrTiO3 thin films

Perovskite-type nitrogen substituted SrTiO₃ thin films were deposited with a one-step process by RF-plasma assisted pulsed laser deposition from a SrTiO₃ target using a N₂ plasma, while deposition with a NH₃ plasma yields films with almost no incorporated nitrogen. The deposited films exhibit a cubic perovskite-type crystal structure and reveal oriented growth on MgO(100) substrates. The unit cell parameters of the studied N-doped SrTiO₃ films range within 3.905<a<3.918 Å, which is slightly larger than for SrTiO₃ (a=3.905 Å). The nitrogen content in the deposited films varies from 0.2 to 0.7 atom%. The amount of incorporated nitrogen in the films decreases with increasing RF-power, while the N₂ flow rate does not have any pronounced influence on the N content. Nitrogen incorporation results in an increased optical absorption at 400–600 nm, which is associated with N(2p) energy states that have a higher energy level than the valence band in strontium titanate. The optical band gap energies in the studied N-doped SrTiO₃ films are at 3.2–3.3 eV, which is very similar to that of pure strontium titanate (～3.2 eV). Films deposited with NH₃ for the RF-plasma exhibit a lower degree of crystallinity and reveal almost no nitrogen incorporation into the crystal lattice.     

The defect chemistry of Ce(Pr, Zr)O2−δ

The influence of composition upon resultant mixed conductivity is analysed for the fluorite-type compositions Zr_0.1Ce_0.7Pr_0.2O_2−δ, and Ce_0.8Pr_0.2O_2−δ. Measurements of oxygen concentration cell e.m.f. combined with impedance spectroscopy at elevated temperatures reveal the materials to be predominantly ionic conductors in oxidising conditions and to show a decrease in both ionic and electronic conductivities with decreasing pO₂. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t₀=0.8 and 0.9, respectively, for the two compositions at 950 °C. The combination of this information with results of coulombic titration, facilitates analysis of the pO₂ dependence of total conductivity measured by a steady-state electrochemical technique between air and reducing conditions. In more oxidising conditions, depletion of total conductivity with decreasing pO₂ results predominantly from a decreasing ionic conductivity. This is one of first examples in the literature where a fluorite-type material is shown indisputably to exhibit such behaviour. These materials show significant levels of mixed conductivity in both reducing and oxidising conditions. In more reducing conditions, an increase in total conductivity is related to increased n-type conductivity upon the reduction of cerium.     

Testing heteroscedasticity by wavelets in a nonparametric regression model

In the nonparametric regression models, a homoscedastic structure is usually assumed. However, the homoscedasticity cannot be guaranteed a priori. Hence, testing the heteroscedasticity is needed. In this paper we propose a consistent nonparametric test for heteroscedasticity, based on wavelets. The empirical wavelet coefficients of the conditional variance in a regression model are defined first. Then they are shown to be asymptotically normal, based on which a test statistic for the heteroscedasticity is constructed by using Fan's wavelet thresholding idea. Simulations show that our test is superior to the traditional nonparametric test.     

Testing coeffcients of AR and bilinear time series models by a graphical approach

AR and bilinear time series models are expressed as time series chain graphical models, based on which, it is shown that the coefficients of AR and bilinear models are the conditional correlation coefficients conditioned on the other components of the time series. Then a graphically based procedure is proposed to test the significance of the coeffcients of AR and bilinear time series. Simulations show that our procedure performs well both in sizes and powers.     

Pulsed laser deposition and characterization of nitrogen-substituted SrTiO3 thin films

Nitrogen-substituted cubic perovskite-type SrTiO₃ thin films were deposited in a one-step process using pulsed reactive crossed beam laser ablation (PRCLA) and RF-plasma assisted pulsed laser deposition (RF-PLD). Both techniques yield preferentially oriented films on SrTiO₃(0 0 1), LaAlO₃(0 0 1) and MgO(0 0 1) substrates with the unit cell parameters within 0.390(5) < a < 0.394(9) nm. The nitrogen content is higher in films deposited by PRCLA (0.84-2.40 at.%) as compared to films deposited by RF-PLD with nitrogen plasma (0.10-0.66 at.%). PRCLA with an ammonia gas pulse leads to a higher nitrogen content compared to the films grown with a nitrogen gas pulse, while films deposited by RF-PLD with ammonia plasma reveal only minor nitrogen contents (<0.10 at.%). The amount of the incorporated nitrogen can be tuned by adjusting the deposition parameters. Films deposited by PRCLA have a lower roughness of 1-3 nm compared to 12-18 nm for the films grown by RF-PLD. PRCLA yields partially reduced films, which exhibit electronic conductivity, while films deposited by RF-PLD are insulating. There is also a pronounced influence of the substrate material on the resistivity of the films deposited by PRCLA: films grown on SrTiO₃ substrates exhibit a metallic-like behaviour, while the corresponding films grown on MgO and LaAlO₃ substrates reveal a metal-to-semiconductor/insulator transition. Nitrogen incorporation into the SrTiO₃ films results in an increased optical absorption at 370-500 nm which is associated with N(2p) localized states with the energy about 0.7 eV higher than the valence band energy in strontium titanate. The optical band gap energies in the studied N-substituted SrTiO₃ films are 3.35-3.40 eV.     

One-step preparation of N-doped strontium titanate films by pulsed laser deposition

Perovskite-type oxynitrides exhibit promising electrical and optical properties and can possibly be used in the future as functional materials for electrical, photo-, and electrochemical applications. Continuous heterovalent substitution of oxygen ions by nitrogen ions allows tuning of the desired optical and/or electronic properties to the application specifications. In the present work deposition of SrTiO₃:N films by pulsed reactive crossed beam laser ablation was studied in order to examine the influence of different deposition parameters on the film crystallinity and composition. The deposited films exhibit a perovskite-type crystal structure and reveals epitaxial growth on MgO(100) substrates. The unit cell parameters of the deposited SrTiO₃:N films range within , which is slightly larger than for polycrystalline SrTiO₃ (a=3.905). The studied films reveal an oxygen content in the range of (2.70-2.98)±0.15. The relative N content (vs. O) can be tuned within the range of 1.0–3.0% by adjusting the deposition parameters. The N:O concentration ratio increases with increasing laser fluence and target-to-substrate distances, while the substrate temperature has a more complex influence on the nitrogen concentration. In the range of 580–650 °C the [N]/[O] ratio increases while further heating results in a gradual decrease of the N content. PACS  81.15.Fg; 68.55.-a; 81.05.Zx     

Synthesis,Crystal Structure and Optical Properties of LaNbON2

LaNbON₂ was synthesized by thermal ammonolysis of crystalline LaNbO₄ prepared by a soft chemistry approach. LaNbON₂ crystallizes in a distorted GdFeO₃‐like perovskite structure with space group Pnma and a = 5.5720(2), b = 7.8656(4), c = 5.6066(3) Å and a random anionic arrangement as revealed by neutron diffraction. Elongated displacement ellipsoids found by neutron diffraction and a considerable degree of residual bond strain as revealed by the bond valence calculations suggest a local positional disorder of the niobium ions.The material exhibits good chemical stability at ambient conditions and a considerable thermal stability upon heating in synthetic air (up to T = 500 K) as well as a small optical band gap of ~1.7 eV.