Abstract. This paper studies in experiment and theory the breakdown of argon, nitrogen, air and oxygen in a uniform dc electric field at different discharge gaps L, discharge tube radii R and cathode materials. At arbitrary geometric dimensions of the cylindrical discharge vessel and cathode materials the ratio of the breakdown electric field value to the gas pressure p at the minimum of the breakdown curves is shown to remain constant, (E dc /p) min ≈ constant. A modified breakdown law for the low-pressure dc dischargeThat is, the breakdown voltage U dc is shown to depend not only on the product pL, but also on the ratio L/R. A method is presented enabling one to predict a dc breakdown curve in the cylindrical discharge vessel possessing arbitrary L and R values from the measured data on the dc breakdown.
-The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves U dc ( pL ) shift toward the higher values of U dc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures. © 2000 MAIK "Nauka/Interperiodica".
As is known [1][2][3][4][5][6][7], the ignition curves of a glow discharge are described by the Paschen law U dc = f ( pL ); i.e., the breakdown voltage U dc is a function of the product of the gas pressure p and the interelectrode distance L . The Paschen law implies that the ignition curves U dc ( p ) measured for various distances L must coincide if they are drawn as the function U dc ( pL ). However, the measurements of the ignition curves of a glow discharge in neon [8] showed that, with equal values of the product pL , the breakdown voltage for a long discharge gap with planar electrodes is significantly higher than that for a short gap. More recent studies [9][10][11][12][13][14] confirmed this conclusion for some other gases (neon, argon, nitrogen, hydrogen, etc.). In spite of a great number of experimental and theoretical papers devoted to low-pressure gas breakdown in a dc electric field, a method for calculating the ignition curve at arbitrary values of the interelectrode distance L and the radius of the discharge chamber R is still lacking. This paper is devoted to the experimental study of a breakdown in nitrogen, air, and oxygen in a dc electric field in a discharge chamber with a variable interelectrode distance L . It is shown that, in the range of the ratio L / R under study, the ignition curves shift toward high values of the product pL and discharge voltage U dc as the gap length L increases. In this case, for any values of the gap length L , the ratio of the breakdown electric field to the gas pressure ( E dc / p ) min at the minimum of the ignition curve remains constant. A generalized scaling law for the low-pressure gas breakdownA method allowing one to calculate the ignition curve for a glow discharge in a cylindrical chamber with arbitrary dimensions from the known ignition curve for a narrow discharge gap (for L / R 0), i.e., from the usual Paschen curve, is described.We measured the ignition curves for a glow discharge in the range of dc voltages U dc ≤ 1000 V and pressures of p ≈ 10 -2 -10 torr. A discharge tube with an inner diameter of 63 mm was used. The interelectrode distance L was varied in the range 0.5-10 cm; consequently, the studies were conducted in the range L / R = 0.16-3.2. Planar parallel electrodes spanned the entire cross section of the discharge tube. Both the anode and the cathode were made from stainless steel. The breakdown voltage was measured accurate to ± 2 V. When determining the ignition voltage, the growth rate of the discharge voltage did not exceed 1 V/s. In all cases, our procedure for measuring the ignition curves was as follows. We fixed a certain distance L between the electrodes and then, for various gas pressures p , measured the breakdown voltage U dc . Below, we explain why only this way of measuring the ignition curves of a glow discharge is correct. Figure 1 shows the ignition curves measured by us in nitrogen for different distances L between the electrodes. It is seen from the figure that, as L increases, the ignition curves shift not only tow...
Hydroxyapatite Ca10(PO4)6(OH)2 (HAp) and calcium phosphate ceramic materials and coatings are widely used in medicine and dentistry because of their ability to enhance the tissue response to implant surfaces and promote bone ingrowth and osseoconduction processes. The deposition conditions have a great influence on the structure and biofunctionality of calcium phosphate coatings. Corrosion processes and poor adhesion to substrate material reduce the lifetime of implants with calcium phosphate coatings. The research has focused on the development of advanced methods to deposit double-layered ceramic oxide/calcium phosphate coatings by a hybrid technique of magnetron sputtering and thermal methods. The thermal method can promote the crystallization and the formation of HAp coatings on titanium alloy Ti6Al4V substrates at low temperature, based on the principle that the solubility of HAp in aqueous solutions decreases with increasing substrate temperature. By this method, hydroxyapatite directly coated the substrate without precipitation in the initial solution. Using a thermal substrate method, calcium phosphate coatings were prepared at substrate temperatures of 100-105 oC. The coated metallic implant surfaces with ceramic bond coats and calcium phosphate layers combine the excellent mechanical properties of metals with the chemical stability of ceramic materials. The corrosion test results show that the ceramic oxide (alumina) coatings and the double-layered alumina-calcium phosphate coatings improve the corrosion resistance compared with uncoated Ti6Al4V and single-layered Ti6Al4V/calcium phosphate substrates. In addition, the double-layered alumina/hydroxyapatite coatings demonstrate the best biocompatibility during in vitro tests.
The present paper addresses the problem of identification of microstructural, nanomechanical, and tribological properties of thin films of tantalum (Ta) and its compounds deposited on stainless steel substrates by direct current magnetron sputtering. The compositions of the obtained nanostructured films were determined by energy dispersive spectroscopy. Surface morphology was investigated using atomic force microscopy (AFM). The coatings were found to be homogeneous and have low roughness values (<10 nm). The values of microhardness and elastic modulus were obtained by means of nanoindentation. Elastic modulus values for all the coatings remained unchanged with different atomic percentage of tantalum in the films. The values of microhardness of the tantalum films were increased after incorporation of the oxygen and nitrogen atoms into the crystal lattice of the coatings. The coefficient of friction, CoF, was determined by the AFM method in the “sliding” and “plowing” modes. Deposition of the coatings on the substrates led to a decrease of CoF for the coating-substrate system compared to the substrates; thus, the final product utilizing such a coating will presumably have a longer service life. The tantalum nitride films were characterized by the smallest values of CoF and specific volumetric wear.
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