Effect of the target bias voltage during off-pulse period on the impulse magnetron sputtering

With a high-power impulse magnetron sputtering (HiPIMS) apparatus, it has been studied how the target bias voltage during the off-pulse period affects the stability of the generated plasma. We have prepared an electrical pulse power source which can control the target voltage during the pulse off period, in addition to the pulse voltage, repetition frequency and a duty ratio of the pulse. Time-resolved current-voltage characteristic was monitored by an oscilloscope, and plasma generation behavior was elucidated. With titanium target and at Ar gas pressures of 0.6-5 Pa, pulse-off bias voltage was changed between −300 and +100 V, and the I-V characteristics were recorded. On increasing the negative bias voltage, the time at which the target current began to rise was gradually delayed. And at a certain voltage, the delay suddenly disappeared. This voltage was found to be the sustain voltage of the dc discharge in the same condition. Applying positive bias voltage resulted in a much longer delay. These results suggest that the minimal discharge during the pulse-off period helps the initiation of high-density plasma, while the bias voltage which can not maintain the plasma contrarily hampers it.     

Oxygen implantation and diffusion in pure titanium by an rf inductively coupled plasma

The superficial oxidation of pure titanium, 9 mm diameter, 5 mm thick disc samples by implantation and diffusion from inductively coupled plasmas is reported. Such rf plasmas were generated in a 15 l cylindrical Pyrex-like glass chamber containing pure circulating oxygen. A quarter wavelength solenoidal antenna capable of transmitting 500 W at 13.54 MHz was externally wound around the chamber and connected to an rf generator capable of up to 1200 W through an automatic matching network. The oxidation process was carried out for 6 h periods while varying the gas pressure between 1 × 10² and 5 × 10^?1 Pa and the sample bias up to ?3000 V DC. It was found that the sample temperature was a function both of the plasma density and the bias voltage. Without bias, the plasma heated the sample up to ～200 °C, and with maximal bias voltage, the substrate was heated to 680 °C. At the latter temperature, the presence of the rutile phase was particularly evident in X-ray diffraction patterns. According to EDX data, the average oxygen to titanium ratio rose, from ～0.06 for an untreated reference sample, to a ～1.7 value for samples treated up to 680 °C.     

Synthesis of porous titanium implants by environmental-electro-discharge-sintering process

Porous Ti implants with various porosities were first fabricated by environmental-electro-discharging-sintering (EEDS) of atomized spherical Ti powders. Powders in two size range (50–100 and 200–250 μm) were settled by vibration into a quarts tube and subjected to a high voltage and high density current pulse. A single pulse of 0.75–2.0 kJ/0.7 g-powder, using 150, 300 and 450 μF capacitors, was applied to produce fully porous and porous-surfaced Ti implant compacts. The solid core was automatically formed in the center of the compact after discharge and porous layer consisted of particles connected in three dimensions by necks. The solid core and neck sizes increased with an increase in input energy and capacitance. On the other hand, pore volume decreased with increased capacitance and input energy due to the formation of a solid core. Capacitance and input energy are the only controllable discharge parameters even though the heat generated during a discharge is the unique parameter that determines the porosity of compact. It was shown that EEDS of spherical Ti powders can efficiently produce fully porous and porous-surfaced Ti implants with various porosities in short times (<400 μs) by manipulating the discharging conditions such as input energy and capacitance including powder size.     

Investigations of the electron-induced ablation in the surface wave plasma

High-density plasmas produced by the microwave power coupled in surface wave modes have been studied in order to apply to the material processing. Small targets biased positively beyond a transition voltage are found to emit their materials to the plasma and to form a plume, as seen in Laser ablation, even at the electron current of 2.5 A with the target voltage as low as 60 V. Deposition of films, e.g. carbon and metal oxides was performed on substrates exposed near the plume in the plasma. The relevance of the target current density to the plume formation is addressed. Probe measurements of the plasma parameters lead to an interpretation of the condition for the plume to be induced in terms of the particle balance between the net electron current to the target and the theoretical Bohm ion current to the vessel wall.     

The effects of pulse frequency and substrate bias to the mechanical properties of CrN coatings deposited by pulsed DC magnetron sputtering

The chromium nitride coatings have been prepared by the bipolar symmetric pulsed DC magnetron reactive sputtering process at 2 kHz and 20 kHz pulse frequencies, respectively. Different substrate bias was applied with a pulsed DC bias unit with 50 kHz pulse frequency. Oscilloscope traces of the I–V waveforms indicate high power and high current density outputs during the symmetric bipolar pulsed mode. It is concluded that the (200) orientation of CrN films is observed. The grain size decreases with increasing pulse frequency and substrate bias. The substrate bias has a strong influence on the mechanical properties of CrN films. The scratch tests of the CrN coatings show that almost only tiny chipping failure is occurred. Sufficient adhesion strength quality of the coating is also observed. The substrate bias for the deposition of CrN films with sufficient hardness and adhesion properties combination is − 290 V at 20 kHz and − 408 V at 2 kHz pulse frequency, respectively.     

Optimization of electrolyte concentration and voltage for effective formation of Sn/SnO2 nanoparticles by electrolysis in liquid

This work investigates the optimum experimental conditions required for the synthesis of Sn nanoparticles (Sn-NPs) via surfactant-free direct-current electrolysis using KCl as the electrolyte. Metallic Sn wire was used as a cathode, which was melted by the local concentration of current upon the application of a direct-current voltage. The effect of electrolyte concentration was analyzed by varying the concentration from 0.01 to 1.0 M, under constant electric power of 40 W. Results indicated that the applied voltage required for plasma generation increased with a decrease in the electrolyte concentration and the particle size decreased at high applied voltage with low electrolyte concentration; particles with a mean diameter of 258.5 nm formed at 0.05 M. However, coarse Sn₆O₄(OH)₄ crystals were precipitated at a concentration of 0.01 M. Therefore, the optimum concentration required for the formation of smaller particles was determined to be 0.05 M. Subsequently, the effect of voltage was analyzed by varying the applied voltage from 70 to 190 V. As a result, the effective production energy of 45 W h/g was obtained at voltages ranging from 110 to 130 V.     

Electrical-driven disaggregation of the two-dimensional assembly of colloidal polymer particles under pulse DC charging

Two-dimensional structures of polymer (polystyrene) colloidal particles were formed on a substrate by electrophoretic deposition in an aqueous suspension. The prepared suspension is stable for a few days; therefore, it was not necessary to place an additional mixing during the deposition process. The particle aggregation/disaggregation formation was observed on the metal surface in order to study the effect of pulse direct current (DC) charging in comparison with conventional DC charging. At the charging frequency of 80 Hz, it was observed that pulse DC charging with a maximum applied voltage of 3.3 V/cm (50% duty cycle) clearly reduced the degree of aggregation of particles having diameters of 50 and 300 nm comparing to those of DC charging. Pulse charging at high frequency is thus found to be more effective in preventing particle aggregation during electrophoretic deposition than conventional DC charging.     

Role of environmental and annealing conditions on the passivation-free in-Ga–Zn–O TFT

We examined the characteristics of passivation-free amorphous In–Ga–Zn–O thin film transistor (a-IGZO TFT) devices under different thermal annealing atmospheres. With annealing at higher temperature, the device performed better at the above-threshold operation region, which indicated the film quality was improved with the decrease of defects in the a-IGZO active region. The mobility, threshold voltage and subthreshold swing of a-IGZO TFT annealed at 450 °C was 7.53 cm²/V s, 0.71 V and 0.18 V/decade, respectively. It was also observed that the a-IGZO was conductive after thermal annealing in the vacuum, due to the ease of oxygen out-diffusion from the a-IGZO back channel. The oxygen deficiency resultantly appeared, and provided leaky paths causing electrical unreliability when TFT was turned off. In contrast, the annealing atmosphere full of O₂ or N₂ would suppress the oxygen diffusion out of the a-IGZO back channel. The worst V_th degradation of a-IGZO TFT after positive gate bias stress and negative gate bias stress (NGBS) was about 2 V and ? 2 V, respectively. However, the V_th shift in the NGBS testing could be suppressed to ? 0.5 V in vacuum chamber. Material analysis methods including X-ray photoelectron spectroscopy and scanning electron microscopy were used to investigate the change of a-IGZO film after different thermal annealing treatments. The variation of O 1s spectra with different annealing atmospheres showed the consistence with our proposed models.     

A new strategy to diminish the 4 V voltage plateau of LiNi0.5Mn1.5O4

As for spinel LiNi_0.5Mn_1.5O₄, there is 4 V voltage plateau in the charge–discharge profiles. This voltage plateau can be reduced by an annealing process, however it is hard to avoid it completely. In this study, a new strategy of partial substitution for Mn by Mg is applied. There is no 4 V voltage plateau in the charge–discharge profiles of Mg-doped compound LiNi_0.5Mn_1.45Mg_0.05O₄. This compound exhibits good electrochemical properties which can be used as cathode material of lithium ion batteries. At 1 C rate, it can deliver a capacity of around 129 mAh g⁻¹ and remain good cycle performance.     

An investigation of nitrided layer prepared by direct current nitrogen arc discharge

A simple direct current (DC) nitrogen arc discharge method is presented, which allows for in situ nitriding of titanium at atmospheric pressure. The microstructure and microhardness of the nitrided layer and effects of the arc discharge current were investigated. The nitrided layer was mainly composed of TiN dendrites and small amounts of TiN_0.3. The density and size of the TiN dendrites gradually decreased from the surface towards the titanium substrate. The layer had a good adhesion with titanium. With an increase of the arc discharge current from 40 to 80 A, the TiN dendrites coarsened, the layer thickness and amount of TiN increased and the layer hardness enhanced. The nitrided layer with the highest hardness value of 1600 HV and thickness of 1800 μm was obtained for an arc discharge current of 80 A.     

Count rate and dynamic range of an X-ray imaging system with MCP detector

X-ray imaging system with edge-on MCP detector has recently been evaluated for potential application in medical imaging. Although this system has been able to provide good quality images, its clinical implementation is limited by its low count rate. The system count rate was limited by a delay line position encoding electronics, which processes the events serially. The count rate of the MCP was also limited because of high gain required for delay line electronics. In this work, the count rate and dynamic range of the system was evaluated. The count rate limitation due to readout electronics and MCP was measured separately. Pulse counting and charge integrating modes of the MCP detector was used. Photon beams for these measurements were generated using an X-ray tube at 50 and 90 kVp peak potentials with 41 and 75 keV average photon energies, respectively. The electronics dead time was measured to be 20% at 200 kHz total system count rate. In addition, 15–30% of the photon signals were not detected by delay line electronics due to exponential pulse height distribution of the MCP signals. The exponential pulse height distribution of the MCP was due to the edge on illumination mode of the MCP and the high energy X-ray photon beam. Image artifacts due to dead time and signal loss were evaluated. Multichannel ASIC readout electronics was considered as a potential solution to the count rate and dynamic range problem. It was shown that the edge-on MCP detector can operate in pulse counting mode at low gains of 10⁴–10⁵ e/photon required for ASIC electronics. It was also shown that at this low gain, the MCP count rate can be increased up to 1.33×10⁶ count/s/pixel, for 100 μm detector pixels, which is appropriate for clinical X-ray imaging.     

Current-induced butterfly shaped domains and magnetization switching in magnetic tunnel junctions

Patterned magnetic tunnel junctions (MTJs) with the layer structure of Ta (5 nm)/Ni₇₉Fe₂₁ (5 nm)/Cu (20 nm)/Ni₇₉Fe₂₁ (5 nm)/Ir₂₂Mn₇₈ (10 nm)/Co₇₅Fe₂₅ (4 nm)/Al (0.8 nm)-oxide/Co₇₅Fe₂₅ (4 nm)/Ni₇₉Fe₂₁ (20 nm)/Ta (5 nm) were fabricated using magnetron sputtering deposition and lithography. High tunnelling magnetoresistance ratios of 22 and 50% were obtained at room temperature before and after annealing, respectively. The evolution of leaf shaped images was observed via Lorentz transmission electron microscopy (LTEM) on the MTJs, which were deposited on a patterned and carbon-coated transmission electron microscopy grid. These leaf-like LTEM images correspond to a butterfly shaped domain structure that was confirmed by a micromagnetics simulation. When a large DC current or bias voltage was applied across the MTJ, the butterfly-like vortex domain structures could be induced to form in the free layer of the MTJ, resulting in a significant decrease of magnetization in the free layer. The existence of these butterfly shaped domains could be one of the major causes of the bias voltage dependence of the TMR ratio.     

Observation of self-magnetic field relaxations in Bi2223 and Y123 HTS tapes after over-current pulse and DC current operation

The development of power transmission lines based on long-length HTS tapes requires the production of high quality tapes. Due to fault conditions, technical mistakes and human errors during the operation of a DC power transmission line, an over-current pulse, several times larger than the rated current, could occur. To study the effect of such over-current pulses on the transport current density distribution in the HTS tapes, we simulated two start-up scenarios for one BSCCO and two YBCO tapes. The first start-up scenario is an initial over-current pulse during which the transport current was turned on rapidly, rising to 900 A during the first milliseconds, then reduced to a 100 A DC current. The second start-up scenario is normal operation, and involved increasing the transport current slowly from 0 A to 100 A at a rate of 1 A/s. For both scenarios, we then measured the vertical component of the self-magnetic field by means of a Hall probe above the tape, and afterward, by solving a linear equation of the inverse problem we obtain the current density profiles. We observe a change of the self-magnetic field above the edge of the BSCCO and YBCO tapes during 30 min after the 5 ms of over-current pulse and during the normal operation. The current density profiles are peaked in the centre for over-current pulse, and more peaked around the edge of the HTS tape for normal operation, which means that the limited time over-current pulse changes the current density profiles of the HTS tapes. We observe also a loop of current for YBCO tapes and we show the role of the HTS tape stabilizer.     

Formation and investigation of p–n diode structures based on lanthanum manganites and Nb-doped SrTiO3

High quality La_2 / 3Ba_1 / 3MnO₃ (LBMO), La_2 / 3Ca_1 / 3MnO₃ (LCaMO) and La_2 / 3Ce_1 / 3MnO₃ (LCeMO) thin films were grown on Nb 0.1 wt.% doped conducting SrTiO₃(100) (STON) substrates. Asymmetric current–voltage relations measured for the LBMO/STON, LCaMO/STON and LCeMO/STON heterostructures at T = 78/300 K certified hole-doping of the manganite films. The diffusion voltage, corresponding to a steep current increase at forward bias has been estimated. The LCaMO/STON heterojunction showed possible impact of interfacial strain on the rectifying behavior, meanwhile, the LaCeMO/STON heterostructures demonstrated evidence of phase separation of the manganite film at the interface.     

Effect of microwave irradiation on TATB explosive (II): Temperature response and other risk

TATB (1,3,5-triamino-2,4,6-trinitrobenzene) explosives were safely irradiated with microwave and showed no visible change according to XPS and XRD spectra. Temperature of TATB sample increased quickly at the beginning and gently during sequent continuous irradiation with temperature less than 140 °C after 60 min, 480 W irradiation, and increased more quickly in 300 g at 480 W than in 150 g at 480 W, both implied that heat dissipation was in the majority of microwave energy. Two major risk factors in microwave irradiation were concerned including overheating which should be avoidable with temperature monitor and microwave discharge which should be controllable experimentally though dielectric breakdown mechanism was not elucidated theoretically yet.     

Tuning of charge carrier density of ZnO nanoparticle films by oxygen plasma treatment

In this work, the charge carrier density of ZnO nanoparticle films was modified after deposition and annealing by an oxygen plasma treatment. The respective films were utilized as active layers in thin film transistors. For a discussion of the plasma–surface interaction on the molecular level, the electrical behavior of the layers was investigated which in general is highly sensitive to low level variations in defect or doping densities. A treatment with remote oxygen plasma at 400 W for 10 s led to a shift of the turn-on voltage from ?12 to 4 V and a reduction of the off-current by more than two orders of magnitude. A model for the influence of oxygen species adsorbed to ZnO nanoparticle surfaces on electrical characteristics of ZnO nanoparticle thin film transistors is introduced.     

Residual stresses in titanium nitride thin films deposited by direct current and pulsed direct current unbalanced magnetron sputtering

This work presents a study on the effect of deposition parameters on the residual stresses developed in titanium nitride (TiN) thin films deposited onto cemented carbide (WC-Co) substrates. Depositions were conducted by reactive unbalanced magnetron sputtering of a single titanium target. Six different conditions were selected, varying parameters such as bias (0, − 50 or − 100 V), power applied to the target (direct current or pulsed direct current) and, in the cases where substrate bias was zero, substrate condition (ground or floating). Pulsed power was applied at a frequency of 50 kHz and with a reverse pulse time of 1 μs. Residual stresses were evaluated through X-ray diffraction, using the sin²ψ method. Results confirmed the effect of substrate bias on the residual stresses of thin films. Additionally, it was possible to observe that by pulsing the power to the target, residual stress varies as a consequence of the increased ion energy.     

Deposition of B4C/BCN/c-BN multilayered thin films by r.f. magnetron sputtering

Thin films of cubic boron nitride (c-BN) and B₄C/BCN/c-BN multilayers, were deposited by r.f. (13.56 MHz) multi-target magnetron sputtering from high-purity (99.99%) h-BN and a (99.5%) B₄C targets, in an Ar (90%)/N₂ (10%) gas mixture. Films were deposited onto silicon substrates with (100) orientations at 300 °C, with r.f. power density near 7 W/cm². In order to obtain the highest fraction of the c-BN phase, an r.f. substrate bias voltage between − 100 and − 300 V was applied during the initial nucleation process and − 50 to − 100 V during the film growth. Additionally, B₄C and BCN films were deposited and analyzed individually. For their deposition, we varied the bias voltage of the B₄C films between − 50 and − 250 V, and for the BCN coatings, the nitrogen gas flow from 3% to 12%. A 300-nm-thick TiN buffer layer was first deposited to improve the adhesion of all samples. X-ray diffraction patterns revealed the presence of c-BN (111) and h-BN phases. FTIR spectroscopy measurements indicate the presence of a peak at 780 cm^− 1 referred to as “out-of-plane” h-BN vibration mode; another peak at 1100 cm^− 1 corresponds to the c-BN TO mode and the “in-plane” vibration mode of the h-BN at 1400 cm^− 1. BN films deposited at 300 °C at a pressure of 4.0 Pa and under − 150 V of nucleation r.f. bias, applied for 35 min, presented the highest c-BN fraction, near 85%. By using 32 layers, it was possible to deposit a 4.6-μm-thick c-BN film with adequate mechanical properties and good adhesion to the substrate.     

Experimental study of the influence of cold heat exchanger geometry on the performance of a co-axial pulse tube cooler

Improving the performance of the pulse tube cooler is one of the important objectives of the current studies. Besides the phase shifters and regenerators, heat exchangers also play an important role in determining the system efficiency and cooling capacity. A series of experiments on a 10 W @ 77 K class co-axial type pulse tube cooler with different cold heat exchanger geometries are presented in this paper. The cold heat exchangers are made from a copper block with radial slots, cut through using electrical discharge machining. Different slot widths varying from 0.12 mm to 0.4 mm and different slot numbers varying from around 20–60 are investigated, while the length of cold heat exchangers are kept the same. The cold heat exchanger geometry is classified into three groups, namely, constant heat transfer area, constant porosity and constant slot width. The study reveals that a large channel width of 0.4 mm (about ten times the thermal penetration depth of helium gas at 77 K, 100 Hz and 3.5 MPa) shows poor performance, the other results show complicated interaction effects between slot width and slot number. These systematic comparison experiments provide a useful reference for selecting a cold heat exchanger geometry in a practical cooler.