Boundary area between gas and vacuum breakdown mechanism

This paper considers the transition from the gas breakdown mechanism to the vacuum breakdown mechanism. Gas breakdown mechanisms at the points of Paschen minimum and on the left from it, as well as vacuum breakdown mechanisms and their dependence on experimental parameters have been analysed. In accordance with this analysis, an experiment with the following variable parameters has been conceived and carried out: type of gas (i.e. residual gas); gas pressure; inter-electrode gap; electrode material; electrode surface topography; type of voltage load; and position in the insulation system of an additional source of ion-electron pairs i.e. of the α-radiation Bragg peak. By a statistical analysis of experimentally obtained statistical samples of the random variable breakdown voltage (dc and impulse), it has been established that the transition from gas to vacuum breakdown mechanism takes place in a relatively broad region, in which the gas breakdown mechanism of an anomalous Paschen type and the avalanche mechanism of vacuum breakdown are present simultaneously.     

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.     

On morphological, structural and electrical properties of vacuum deposited pentacene thin films

Films of different thickness (50, 100, 150 and 200 nm) were deposited by thermal evaporation in vacuum on two types of substrates glass and ITO. The deposition was performed under a pressure of 10⁻⁶ mB with a rate of 0.25 nm/s. Films surface investigations showed morphological and structural changes in function of films thickness and the nature of the substrate. Films optical transmission was analysed in the 280-1600 nm spectral range and the electrical measurements were done in low vacuum (10⁻¹:10⁻² mB) and in dark.     

Composition, structural, dielectric and DC characterization of vacuum deposited ZnSe thin films

ZnSe thin films with different thicknesses are deposited onto glass substrates under a vacuum of 4×10⁻⁵ mbar by vacuum evaporation. Rutherford backscattering spectrometry is used to identify the composition of the deposited films. The composition of the deposited films is found to be nearly stoichiometric. The X-ray diffractogram reveals a cubic structure with preferential orientation along the (1 1 1) direction and structural parameters such as crystallite size D, dislocation density δ, strain ε, and lattice parameters are calculated. It is observed that the crystallite size increases from 20.11 to 55.56 nm with increase of film thickness. In the DC conduction studies the conduction mechanism is found to follow an exponential trap distribution with density of states 3.251×10⁴⁸ J⁻¹ m⁻³. The dielectric constant is calculated as 8.11 [306 K].     

Vacuum insulation panels—From research to market

Vacuum insulation panels (VIPs) have a thermal resistance about a factor of 10 higher than that of equally thick conventional polystyrene boards. Similar to thermos flasks these systems make use of ‘vacuum’ to suppress the heat transfer via gaseous conduction. While thermos flasks are to be pumped down to a high vacuum, filling material integrated in the flat VIP elements, which bears the atmospheric pressure load, reduces the requirements on the vacuum and thus on the tightness of the vacuum casing. Optimal in this respect is a kernel of fumed silica. This kernel is evacuated to below 1 mbar and sealed in a high-barrier laminate, which consists of several layers of Al-coated polyethylene (PE) and polyethylene terephthalate (PET). The laminate is optimized for low air and moisture leakage rates and thus for a long service life. The evacuated silica kernel has a thermal conductivity of about 0.004 W m⁻¹ K⁻¹ at room temperature, mainly resulting from solid thermal conduction along the tenuous silica backbone. As the kernel is nanoporous, the gaseous thermal conductivity becomes noticeable only for pressures above 10 mbar. At about 200 mbar the thermal conductivity measures about 0.008 W m⁻¹ K⁻¹. Such a gas pressure could occur after several decades of usage in a middle European climate. With VIP, slim yet highly insulating façade constructions can be realized. A centre U-value of 0.2 W m⁻² K⁻¹ can be achieved for a VIP thickness of only 2 cm, if optimized kernels and barrier laminates as well as stringent quality control are employed. A successful “self-trial” using VIPs within a façade of the ZAE-building in Würzburg in 1999 was the starting point for new applications of evacuated insulations in the building sector.     

Characteristics of a magnetron cold cathode gauge

The discharge characteristics of a cold-cathode gauge of the non-inverted magnetron type were studied in ultra-high vacuum. The experimental magnetron cell of length 56 mm and diameter 32 mm was made of stainless steel. The cathode with a diameter of 6 mm was placed along the anode axis. The diameter of the anode was 25 mm and the length was 50 mm. Discharge current versus voltage and magnetic field was measured in the pressure range between 1×10⁻⁸ and 1×10⁻⁶ mbar. It was found that the current at first slowly increased with increasing voltage, reached a maximum at a certain voltage, and decreased rapidly with further increase of the voltage. The voltage, at which the current reached the maximum, depended on the magnetic field density and slightly on the pressure. A novel type of a cold cathode gauge with a self-adjusting power supply is suggested.     

Study on the distillation rates of LiCl-KCl eutectic salt under different vacuum conditions

A study on the distillation rate of LiCl-KCl eutectic salt under different vacuums from 0.5 to 50 Torr was performed by using thermogravimetric (TG) method. A distillation rate of the order of 10⁻⁴-10⁻⁵ mol cm⁻² s⁻¹ was obtainable at temperatures of 1200-1300 K and vacuums of 5-50 Torr. Based on the non-isothermal TG data, model distillation flux equations could be derived as a function of temperature. Pure gas-phase and gas-liquid interfacial resistances at different vacuum conditions were evaluated from the comparison of experimental vaporization fluxes with the maximum flux obtained from the kinetic theory of gas. The difference between interfacial mass transfer coefficients and gas-phase ones increases with the temperature. Gas-phase resistance is much greater than that of the phase transition between condensed and gas phases at tested vacuum conditions of 0.5-50 Torr.     

Field emission from oriented tin oxide rods

Tin oxide (SnO₂) films were grown on silicon substrates by a wet chemical route. It was found from scanning electron microscopy investigations that oriented SnO₂ rods normal to the substrates were obtained. Field emission studies were carried out in diode configuration in an all metal ultra high vacuum chamber at a base pressure ∼ 1.33 × 10^− 8 mbar. The ‘onset’ field required to draw 0.1 μA/cm² current density from the emitter cathode was found to be ∼ 3.4 V/μm for SnO₂ rods. The field emission current and applied field follows the Folwer-Nordheim relationship in low field regime. The observed results indicate that the field emission characteristics of chemically grown SnO₂ structures are comparable to the vapor grown nanostructures.     

Validity of the Space-Time Enlargement Law for vacuum breakdown

This paper investigates, theoretically and experimentally, the applicability of the Space-Time Enlargement Law to vacuum-insulated systems. A discussion on how characteristics of possible vacuum breakdown mechanisms determine the distribution function of the breakdown voltage random variable is presented. By superimposing effects of electrode surface enlargement and inter-electrode gap enlargement, expressions for the mean value and standard deviation of the breakdown voltage random variable are obtained. In the case of time extensions, the assumption of complete independence of consecutive discharge processes is discussed. Experimental testing of the Enlargement Law was performed on Rogowski type two-electrode systems, with different electrode surface areas, inter-electrode gaps and vacuum pressures. Measurements were conducted using industrial ac voltage, dc voltage with 50 V/s rate of rise, standard atmospheric pulse voltage (1,2/50 μs), and commutational pulse voltage (250/2500 μs). The final conclusion, based on the comparison of theoretical considerations and the experimental results, is that the Space-Time Enlargement Law can be applied in the design phase during the development of vacuum devices, with certain limitations, regardless of the type of the applied voltage.     

Al and Ni co-doped ZnO films with room temperature ferromagnetism,low resistivity and high transparence

Zn_0.91Al_0.07Ni_0.02O and Zn_0.90Al_0.05Ni_0.05O films of about 250 nm thick were deposited on glass substrates at 300 K by co-sputtering with ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h and then cooled down to room temperature under a magnetic field of 4.8 × 10⁴ A m⁻¹ applied along the film plane. After this process the films showed room temperature ferromagnetism, a resistivity of about 2 × 10⁻³ Ω cm and an average transmittance of 75% in the visible wavelength range. The films have a wurtzite structure with the c-axis orientation in the film growing direction and consist of thin columnar grains perpendicular to the substrate. A temperature dependence of the resistivity from 2 K to 300 K reveals that the carrier transport mechanism is thermally activated band conduction above 150 K and Mott's variable range hopping below 70 K.     

Effects of aluminium doping on zinc oxide transparent thin films grown by filtered vacuum arc deposition

Thin n-type ZnO films doped with different atomic concentrations of aluminium were grown by filtered vacuum arc deposition (FVAD) on glass substrates. The films were deposited using an oxygen working pressure of 2.0 mTorr with an arc current running at two 100 ms pulses s⁻¹. Structural, optical and electrical properties were investigated to understand the effect of Al doping on ZnO films. The best values were found for an ideal aluminium percentage between 4 and 6 at.%.     

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10⁻⁴ to 10⁻¹ Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.     

Calibration of ultrahigh vacuum gauge from 10 Pa to 10 Pa by the two-stage flow-dividing system

A new two-stage flow-dividing system has been developed for the calibration of ultrahigh vacuum gauges from 10⁻⁹ Pa to 10⁻⁵ Pa for N₂, Ar, and H₂. This system is designed based on the techniques for our previously developed calibration system in the range from 10⁻⁷ Pa to 10⁻² Pa. Three modifications were performed to extend the calibration pressure to a lower range. The relative standard uncertainty of the generated pressure (k = 1) is in the range from 2.3% to 2.6%, from 10⁻⁹ Pa to 10⁻⁵ Pa. The characteristics of ultrahigh vacuum gauges were also examined by using this system. The stabilities of the pressure reading, the linearity, the temperature dependence, and the long-term stability were examined. These results show that the calibration of ultrahigh vacuum gauges is possible in the range from 10⁻⁹ Pa to 10⁻⁵ Pa for N₂, Ar, and H₂ with the uncertainty of about 6.0% (k = 2) by this new system.     

Diffusion bonding of Fe-28Al(Cr) alloy with low-carbon steel in vacuum

Fe-28Al(Cr) alloy and low-carbon steel were diffusion bonded in a vacuum of 10⁻⁴-10⁻⁵ Pa. The relationship of the bond parameters and shear strength at the interface was discussed. Microstructure characteristics and the reaction products at the interface were investigated by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The thickness of the diffusion reaction layer was measured with electron probe microanalysis (EPMA). The results indicated that controlling bonding temperature 1333 K for 3.6 ks, shear strength at the interface can be up to 112 MPa. Three kinds of reaction products were observed to have formed during the vacuum diffusion bonding, namely FeAl, Fe₃Al and α-Fe (Al) solid solution. The thickness (X) of the diffusion reaction layer increases with bonding time (t) according to a parabolic law X²=6.4×10³ exp(−104.1/RT)(t-t₀) (μm²).     

Status of the cyclotron vacuum system at TRIUMF

The world's largest cyclotron was built at TRIUMF in 1972 and commissioned to full energy in 1974 [Harwood VJ, Yandon JC. TRIUMF Design Note, TRI-69-7, 1969 [1], Blakely RG, Moore RW, Harwood VJ. TRIUMF Design Note, TRI-69-9, 1969 [2]]. The cyclotron accelerates negatively charged hydrogen ions up to 500 MeV, and protons are produced by inserting a stripping foil in the beam which removes two electrons from each negatively charged hydrogen ion and allows the remaining bare protons to be channeled out of the accelerator. By making these protons strike different kinds of targets, intense beams of neutrons, pions and muons can also be created, thus making possible many different experiments. The volume of the cyclotron vacuum tank is about 100 m³ and operates at 2×10⁻⁸ Torr pressure during beam production. Most of the vacuum is achieved by cryopumping with a B-20 cryogenerator and six cryopumps. The B-20 is a Stirling cycle refrigerator, which supplies helium gas at 16 and 70 K to the cryopanels in the tank. The tank is also equipped with two turbo pumps. The vacuum system went through a few modifications during more than 30 years of operation. This paper presents the status of the cyclotron vacuum system and discusses the latest upgrades.     

INRIM continuous expansion system as high vacuum primary standard for gas pressure measurements below 9 × 10 Pa

The continuous expansion system can be considered the state of the art for pressure measurement in the ultra high vacuum range. In the last years, at INRIM, a new continuous expansion system was designed, assembled and characterized. The system is the high vacuum primary standard in the pressure range from 1 × 10⁻⁶ Pa to 9 × 10⁻² Pa with relative standard uncertainty ranging from 2.1% at 1 × 10⁻⁶ Pa down to 0.4% at 9 × 10⁻² Pa. The system is based on the passing of a measured gas flow through a fixed and known conductance. The gas flow is generated and measured by a primary gas flowmeter based on the constant-pressure-variable-volume method.In the first part of the paper both a correction for the effect of transitional flow through the orifice and a new analytical evaluation of orifice conductance are presented. In the second part the accuracy of the system and the pressure uncertainty evaluation are described.     

Investigation of Sb diffusion in amorphous silicon

Amorphous silicon materials and its alloys became extensively used in some technical applications involving large area of the microelectronic and optoelectronic devices. However, the amorphous-crystalline transition, segregation and diffusion processes still have numerous unanswered questions. In this work we study the Sb diffusion into an amorphous Si film by means of Secondary Neutral Mass Spectrometry. Amorphous Si/Si_1−xSb_x/Si tri-layer samples with 5 at% antimony concentration were prepared by direct current magnetron sputtering onto Si substrate at room temperature. Annealing of the samples was performed at different temperatures in vacuum (p<10⁻⁷ mbar) and 100 bar high purity (99.999%) Ar pressure. During annealing a rather slow mixing between the Sb-alloyed and the amorphous Si layers was observed. Supposing concentration independent of diffusion, the evaluated diffusion coefficients are in the range of ∼10⁻²¹ m²s⁻¹ at 550 °C.     

Characteristics of traveling wave ultrasonic motor under atmosphere and vacuum cycle condition

Under atmosphere and vacuum cycle condition, characteristics of a disc-type traveling wave ultrasonic motor (TWUSM) are tested, variations of rotation speed with time and load are also obtained. Two vacuum conditions used are low vacuum of 2 Pa and high vacuum of 5 × 10⁻³ Pa. The lower the environment pressure is, the smaller the rotation speed of TWUSM. And load characteristics of TWUSM in vacuum are poorer than that in atmosphere. Then, using polyfluortetraethylene (PTFE) composite and p-hydroxybenzoic acid polymer (Ekonol) composite as friction materials, load characteristics of TWUSM are compared. As a result, the load characteristics of Ekonol composite are better than the former one, which can hold bigger torque with higher speed. Both principle of ultrasonic motor and knowledge of tribology are applied to analyze the reason that stall torque of TWUSM in vacuum is smaller than in atmosphere.     

Nanocrystalline diamond film as cathode for gas discharge sensors

Nanocrystalline diamond (NCD) film was deposited on a silicon substrate utilizing microwave plasma-enhanced chemical vapor deposition in a mixed flow of methane, hydrogen and argon. The deposited film had a cauliflower-like morphology, and was composed of NCD, carbon clusters and mixed sp²- and sp³-bonded carbon. Electron field emission (EFE) in vacuum and electrical discharges in Ar, N₂ and O₂ using the NCD film as the cathode were characterized. The turn-on field for EFE and the geometric enhancement factor for the NCD film were 8.5 V/μm and 668, respectively. The breakdown voltages for Ar, N₂ and O₂ increased with pressures from 1.33 × 10⁴ Pa to 1.01 × 10⁵ Pa, following the right side of the normal Paschen curve.