Microstructured vacuum gauges and their future perspectives

New prototypes and concepts of microstructured vacuum gauges have been developed by using the fabrication technologies for micro electro mechanical systems (MEMS). The realization of such microstructured gauges requires sophisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS vacuum gauges is demonstrated by a few examples.Micro-Pirani gauges are based on the principle that the heat transfer between two surfaces is proportional to the number of molecules (and hence the pressure) transferring the heat, when the mean free path in the gas is larger than the distance between the surfaces. In contrast to conventional Pirani gauges with heated thin wires, in a micro-Pirani gauge the heat transfer takes place between an extremely thin heated membrane and the surrounding. The thin membrane (200-nm thick) is realized by deposition of siliconoxide/siliconnitride, photolithography and anisotropic etching of silicon wafers. Heating is performed by a meander-shaped aluminum thin film heater in the center of the membrane. This micro-Pirani gauge shows a high sensitivity in the pressure range between 10⁻⁴ and 100 mbar. By using a silicon “microbridge” with 10 μm small gap between heated membrane and surrounding, the pressure sensitivity of the chip is extended up to 1000 mbar.Similar concepts are presented and discussed with respect to the miniaturization of spinning rotor gauges. The new concept is based on the application of microfabricated disks (instead of spheres) and of electrostatic instead of magnetic driving forces. The extension of the sensitivity range for miniaturized spinning rotor gauges is also discussed.Finally, new perspectives for mechanical vacuum gauges are demonstrated. By application of micromechanical processes, very thin stress-compensated membranes can be fabricated which enable sensitive mechanical gauges even for pressures in the high vacuum range. First, experimental results with respect to these membranes are represented.     

Ionization vacuum gauge with a carbon nanotube field electron emitter combined with a shield electrode

The applicability of carbon nanotubes to an electron source for a Bayard-Alpert type vacuum gauge has been investigated. Three gauge configurations are designed to optimize the gauge performance. The optimized gauge, in which an additional shield electrode is fixed on a gate electrode, exhibits good measurement of linearity between ion current and system pressure from 10⁻⁷ to 10⁻² Pa. A gauge sensitivity of 0.05 Pa⁻¹ has been achieved under 100 μA emission current for nitrogen, comparable with 0.07 Pa⁻¹ of commercial ionization gauges.     

磁悬浮转子规传递系数的变化研究

磁悬浮转子规是一种精密的黏滞性真空规,被广泛用作参考标准,其核心部件是一个悬浮在真空中自由旋转的金属球(转子).对磁悬浮转子规传递系数的长期稳定性以及随气体和温度的变化关系进行了实验研究,得出的结论对于以磁悬浮转子规作为参考标准的校准装置,在进行不确定度评估时具有参考价值.     

Stability tests of ionization gauges using two-stage flow-dividing system

Stability tests of four ionization gauges (a BA gauge, an extractor gauge, an AT gauge, and a BA gauge with a heating electrode) were performed using a two-stage flow-dividing system from four viewpoints: (1) the fluctuation and drift of pressure readings, (2) the repeatability of pressure readings, (3) the change in sensitivity owing to prior conditions of use and (4) long-term stability. These tests were performed at pressures from 8 × 10⁻⁶ Pa to 8 × 10⁻⁴ Pa using N₂ gas under tightly controlled conditions.The fluctuation and drift of the four IGs were within 1% over 1 h. Their repeatability was also within 1% during eight cycles with an interval of 1 h between each cycle. Although changes in sensitivity of several percent owing to prior conditions of use were observed, the sensitivity was recovered to within 1% of its original values after operation in ultrahigh vacuum for one day. The result of a long-term stability test over a year showed that the sensitivity of the four IGs tended to decrease by 2.6-5.4% due to aging, depending on the gauge.     

A low-voltage Penning cell for vacuum measurement and vacuum technology

This paper presents new results on the performance of a special cold-cathode gauge. The use of an AC and DC voltage simultaneously as the power source makes it possible to ignite and sustain a discharge at a significantly lower voltage than normal over a wide pressure range (from 5 to 3×10⁻⁶ Pa). The Penning discharge voltage can be as low as 20 V at a pressure of 0.3 Pa and 300 V in high vacuum. For low values of pressure, the discharge current has a monotonously increasing dependence on the gas pressure. If a low RF voltage is applied to a high-voltage DC discharge, the discharge current increases in many times. These discharge properties can be used to improve the performance of devices that use the Penning and other magnetic discharges (total and partial pressure gauges, leak detectors, ion pumps, ion sources and hot plasma sources). A vacuum gauge of this type combines the best features of magnetic discharge and hot cathode ionization gauges.     

Comparisons to establish a force-balanced piston gauge and a spinning rotor gauge as the new measurement standards of MIKES

Centre for Metrology and Accreditation (MIKES) has worked towards expanding its calibration capabilities in the absolute pressure range downwards from 0.2 Pa and lowering measurement uncertainties in the range <10 kPa using a force-balanced piston gauge (FPG) and a spinning rotor gauge (SRG).MIKES was the first national pressure laboratory to purchase a novel type of piston manometer, FPG, developed by DH Instruments Inc., USA. The effective area of the FPG was at first determined at MIKES by a comparison with a conventional pressure balance. The result was confirmed in comparisons in the same pressure range with two other laboratories, Swedish National Testing and Research Institute (SP), Sweden and Institute for Metrology and Technology—Van Swinden Laboratorium (NMi-VSL), Netherlands. Next a direct comparison with the mercury column manometer of Physikalisch-Technische Bundesanstalt (PTB), Germany in the absolute pressure range from 1 to 15 kPa was carried out. Use of the FPG for calibration of capacitance diaphragm gauges (CDGs) is also presented.MIKES now uses two SRGs as reference standards, manufactured by MKS Instruments, Inc., USA. A comparison of SRGs in the range from 0.1 to 5 Pa between MIKES, SP and Vaisala Oyj accredited laboratory was performed in 2001-2002.     

Optimization in fabricating bismuth telluride thin films by ion beam sputtering deposition

N-type bismuth telluride (Bi₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputtering method. Various substrate temperatures were tried to obtain optimal thermoelectric performance. The influence of deposition temperature on microstructure, surface morphology and thermoelectric properties was investigated. X-ray diffraction shows that the films are rhombohedral with c-axis as the preferred crystal orientation when the deposition temperature is above 250 °C. All the films with single Bi₂Te₃ phase are obtained by comparing X-ray diffraction and Raman spectroscopy. Scanning electron microscopy result reveals that the average grain size of the film is larger than 500 nm when the deposition temperature is above 300 °C. Thermoelectric properties including Seebeck coefficient and electrical conductivities were measured at room temperature, respectively. It is found that Seebeck coefficients increase from − 28 μV k^− 1 to − 146 μV k^− 1 and the electrical conductivities increase from 1.87 × 10³ S cm^− 1 to 3.94 × 10³ S cm^− 1 when the deposition temperature rose to 250 °C and 300 °C, respectively. An optimal power factor of 6.45 × 10^− 3 Wm^− 1 K^− 2 is gained when the deposition temperature is 300 °C. The thermoelectric properties of bismuth telluride thin films have been found to be strongly enhanced by increasing the deposition temperature.     

Neural network modelling of cold-cathode gauge parameters

This article describes modelling of the operating characteristics of a cold-cathode ionisation gauge (CCG). The gauge characteristics were measured on a gauge comparison UHV calibration system with a test chamber, an extractor gauge, a spinning rotor gauge, and a gas manifold with a precise leak valve. Discharge intensity was measured vs. anode voltage at different pressures selected in the range from 1×10⁻⁹ to 1×10⁻⁵ mbar, and vs. pressure at different operating voltages ranging from 1.2 to 9 kV. In all cases the magnetic flux density was the same and amounted to about 0.13 T. The CCG exhibits an extremely low thermal outgassing rate and a low measurement limit. Therefore, it is suitable for pressure measurements in the ultrahigh vacuum range; however, it has a significant disadvantage. The discharge current vs. the pressure characteristic is non-linear and, in some cases, even discontinuous.The measured CCG characteristics were used as an input for the artificial neural network, which was used to generate a non-linear CCG input-output function used for linearisation purposes. It is generally known and strictly proven that neural networks are capable of learning and building any kind of real and non-polynomial input-output function. Furthermore, it was also mathematically proven that the single hidden neural layer system can learn any function. Other authors have reported that the learned function characteristics are not always continuous.In our experimental work, no mapping discontinuities in the formed model were detected. Despite the fact that learning of the input-output characteristics can be obtained by the neural networks with only one hidden layer, we have used the multilayer neural networks that exhibit a faster convergent and smoother learning process. The neural networks were trained to perform the transfer function between the input gauge parameters and the pressure. The neural networks are a suitable solution for CCG characteristics modelling and thus offer the possibility to overcome the disadvantages of the CCG.     

Outgassing from stainless steel and the effects of the gauges

The true outgassing from surfaces is often masked by the production of gases and pumping within the measuring gauge. Quadrupole mass analysers have been used to measure the outgassing from carefully prepared samples of stainless steel. By using an isolation pressure rise technique coupled to non-evaporable getter pumping, the problem of production of methane by the gauge is overcome and it has been possible to measure the outgassing of methane to very low levels. The specific outgassing rate of methane from 316 L stainless steel is found to be ?5×10⁻²² mbar l s⁻¹ cm⁻². The outgassing of the other gases has been measured using several gauges. The specific outgassing of hydrogen is found to be 4.5×10⁻¹⁵ mbar l s⁻¹ cm⁻² and the total of all other gasses amounted to no more than 10% of this figure. Although the measurements of the other gases are more ambiguous than those for methane, it is concluded that, in all probability, only hydrogen is outgassed from stainless steel.     

Contribution to carrier densities of the oxygen vacancy in a low-resistivity tin-doped indium oxide film by the hot-cathode plasma sputtering method

In order to clarify the contribution to carrier density by oxygen vacancies in tin-doped indium oxide (ITO) films prepared on glass substrates by the hot-cathode plasma sputtering method, we have investigated the effect of annealing on the electrical properties of an ITO film with a resistivity of 1.0 × 10^− 4 Ω cm. A drastic decrease in carrier density from 2.0 × 10²¹ to 0.88 × 10²¹ cm^− 3 was found with gradual increase in the Hall mobility from 29 to 35 cm² V^− 1 s^− 1 for repeated annealing cycles, when the ITO film was exposed for one hour to 400 °C oxygen gas at atmospheric pressure. The results indicate that the contribution of oxygen vacancies to carrier density was ca. 1.12 × 10²¹ cm^− 3 for the ITO film with an overall carrier density of 2.0 × 10²¹ cm^− 3.     

The influence of dynamic strain aging on resistance to strain reversal as assessed through the Bauschinger effect

The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361 °C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260 °C for an engineering strain rate of 10⁻⁴ s⁻¹. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260 °C and 309 °C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active.     

Forming behavior and workability of 6061/B4CP composite during hot deformation

Compression tests of 6061/B₄C_P composite have been performed in the compression temperature range from 300 °C to 500 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300 °C and 400 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹ and the other was situated at the temperature between 425 °C and 500 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹.The estimated apparent activation energies of these two domains, were 129 kJ/mol and 149 kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350 °C - 0.01 s⁻¹ and 500 °C - 0.01 s⁻¹. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite.     

The effect of deposition parameters on the properties of SrCu2O2 films fabricated by pulsed laser deposition

SrCu₂O₂ (SCO) thin films have been fabricated by pulsed laser deposition at oxygen partial pressures between 5 × 10^− 5-5 × 10^− 2 mbar and substrate temperatures from 300 °C to 500 °C. All films were single-phase SrCu₂O₂, p-type materials. Films deposited at a substrate temperature of 300 °C and oxygen pressure 5 × 10^− 4 mbar exhibited the highest transparency (∼ 80%), having conductivity 10^− 3 S/cm and carrier concentration around 10¹³ cm^− 3. Films deposited at oxygen partial pressure higher than 10^− 3 mbar exhibited higher conductivity and carrier concentration but lower transmittance. Depositions at substrate temperatures higher than 300 °C gave films of high crystallinity and transmittance even for films as thick as 800 nm. The energy gap of SrCu₂O₂ thin films was found to be around 3.3 eV.     

Thermo-mechanical characterisation of AA 6056-T4 and estimation of its material properties using Genetic Algorithm

This paper presents an experimental investigation of thermo-mechanical material properties of AA 6056-T4, which is used extensively in aeronautic applications. Monotonic tensile tests have been carried out on the dog-bone type specimens at temperatures ranging from room temperature (16 °C) to high temperature (450 °C) with two different strain rates; viz. high strain rate (∼0.002 s⁻¹) and low strain rate (∼0.0002 s⁻¹). Specimens were heated with the help of Joule heating system using Gleeble® 3500 machine at a temperature rate of 25 °C/s. Material properties which were investigated include the Young’s modulus, yield strength at 0.1% plastic strain and hardening modulus.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Research on the hot deformation behavior of Ti40 alloy using processing map

The deformation behavior of a Ti40 titanium alloy was investigated with compression tests at different temperatures and strain rates to evaluate the activation energy and to establish the constitutive equation, which reveals the dependence of the flow stress on strain, strain rate and deformation temperature. The tests were carried out in the temperature range between 900 and 1100 °C and at strain rates between 0.01 and 10 s⁻¹. Hot deformation activation energy of the Ti40 alloy was calculated to be about 372.96 kJ/mol. In order to demonstrate the workability of Ti40 alloy further, the processing maps at strain of 0.5 and 0.6 were generated respectively based on the dynamic materials model. It is found that the dynamic recrystallization of Ti40 alloy occurs at the temperatures of 1050-1100 °C and strain rates of 0.01-0.1 s⁻¹, with peak efficiency of power dissipation of 64% occurring at about 1050 °C and 0.01 s⁻¹, indicating that this domain is optimum processing window for hot working. Flow instability domains were noticed at higher stain rate (≥1 s⁻¹) and stain (≥0.6), which located at the upper part of the processing maps. The evidence of deformation in these domains has been identified by the microstructure observations of Ti40 titanium alloy.     

Extrusion properties of a Zr-based bulk metallic glass

The extrusion behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 metallic glasses in the supercooled liquid region was investigated. Good extrusion formability was observed under low strain rates at temperatures higher than 395 °C. The metallic glasses were fully extruded without crystallization and failure within the range of T = 395-415 °C under strain rates from 5 × 10^− 3 s^− 1 to 5 × 10^− 2 s^− 1, and the deformation behavior of the metallic glasses during the extrusion was found to be in a Newtonian viscous flow mode by a strain rate sensitivity of 1.0.     

Deposition of silicon nitride thin films by hot-wire CVD at 100 °C and 250 °C

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 ?/s) and at low substrate temperature. Films were deposited using NH₃/SiH₄ flow rate ratios between 1 and 70 and substrate temperatures of 100 °C and 250 °C. For NH₃/SiH₄ ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56-2.74 g/cm³) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 ?/s and < 0.5 ?/s were obtained for films deposited at 100 °C and 250 °C, respectively. Films deposited at both substrate temperatures showed electrical conductivity ~ 10^− 14 Ω^− 1 cm^− 1 and breakdown fields > 10 MV cm^− 1.     

Structural, optical and electrical study of undoped GaN layers obtained by metalorganic chemical vapor deposition on sapphire substrates

We investigate optical, structural and electrical properties of undoped GaN grown on sapphire. The layers were prepared in a horizontal reactor by low pressure metal organic chemical vapor deposition at temperatures of 900 °C and 950 °C on a low temperature grown (520 °C) GaN buffer layer on (0001) sapphire substrate. The growth pressure was kept at 10,132 Pa. The photoluminescence study of such layers revealed a band-to-band emission around 366 nm and a yellow band around 550 nm. The yellow band intensity decreases with increasing deposition temperature. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies show the formation of hexagonal GaN layers with a thickness of around 1 μm. The electrical study was performed using temperature dependent Hall measurements between 35 and 373 K. Two activation energies are obtained from the temperature dependent conductivity, one smaller than 1 meV and the other one around 20 meV. For the samples grown at 900 °C the mobilities are constant around 10 and 20 cm² V⁻¹ s^− 1, while for the sample grown at 950 °C the mobility shows a thermally activated behavior with an activation energy of 2.15 meV.     

The effect of temperature on toluene sorption by granular activated carbon and its use in permeable reactive barriers in cold regions

Granular activated carbon (GAC) has been used as an adsorbent for hydrocarbons in a range of permeable reactive barriers. This work investigates the influence of temperature on adsorption performance. In particular, the influence of temperature in the range of 20 °C to 4 °C on the sorption equilibrium and kinetics of toluene on GAC surface were investigated. The results show that low temperature leads to decreased toluene sorption by GAC and slower reaction kinetics. Sorption kinetics studies show that diffusion coefficients are also lower at 4 °C (3.65 × 10⁻¹³ m² s⁻¹) than 20 °C (5.112 × 10⁻¹³ m² s⁻¹).