Low Pressure Spark Research Articles

Fabrication of thermoelectric Bi2Te2·5Se0.5 with adjustable porosity

Porous structures have attracted considerable interest for their impact on the transport and mechanical characteristics of materials. The fabrication of porous materials, however, often involves intricate preprocessing steps and typically lacks the ability to tailor porosity with ease. In the present study, we present a straightforward method to prepare porous Bi2Te2·5Se0.5 samples by employing low-pressure spark plasma sintering, with the porosity regulated by preset mass density using a modified graphite mold. This approach resulted in a substantial decrease in thermal conductivity, attributed to the introduction of pores that reduce mass density and disrupt phonon transmission. An accompanying decrease in electrical conductivity was also noted, arising from reduced carrier concentration and mobility. Despite these variations, all porous samples maintained similar levels of thermoelectric performance, with a peak zT of ca. 0.9 at 373 K. The average zT within the temperature range of 298–500 K remained slightly above 0.8 for all samples. Furthermore, the porous samples exhibited greatly enhanced mechanical properties. This work demonstrates a versatile and adaptable method to produce porous materials with controllable porosity, potentially applicable to other porous thermoelectric systems.

In vitro evaluation of novel low-pressure spark plasma sintered HA-BG composite scaffolds for bone tissue engineering.

The low-pressure spark plasma sintering (SPS) technique is adopted to fabricate hydroxyapatite–bioglass (HA–BG) scaffolds while maintaining the physical properties of both components, including their bulk and relative density and hardness. However, prior to their orthopaedic and dental applications, these scaffolds must be validated via pre-clinical assessments. In the present study, scaffolds with different ratios of HA : BG, namely, 100 : 0 (HB 0 S), 90 : 10 (HB 10 S), 80 : 20 (HB 20 S) and 70 : 30 (HB 30 S) were fabricated. These scaffolds were characterized by investigating their physicochemical properties (X-ray diffraction (XRD) and surface wettability), bioactivity in a simulated body fluid (SBF) (field emission scanning electron microscopy (FESEM), Fourier-transform infrared spectroscopy (FTIR) and calcium dissolution), antimicrobial properties, biocompatibility and osteoinduction of human bone marrow-derived mesenchymal stromal cells (hBMSCs) and human monocyte immune cell response. The XRD and surface wettability results confirmed no formation of undesirable phases and the enhanced surface hydrophilicity of the scaffolds, respectively. The bioactivity in SBF indicated the formation of bone-like apatite on the surface of the scaffolds, corresponding to an increase in BG%, which was confirmed through FTIR spectra and the increasing trend of calcium release in SBF. The scaffolds showed inhibition properties against Staphylococcus aureus and Staphylococcus epidermidis. The scanning electron microscopy (SEM) micrographs and Alamar Blue proliferation assay indicated the good attachment and significant proliferation, respectively, of hBMSCs on the scaffolds. Alizarin Red S staining confirmed that the scaffolds supported the mineralisation of hBMSCs. The osteogenic protein secretion (bone morphogenetic protein-2 (BMP2), type-I collagen (COL1) and osterix (OSX)) was significant on the HB 30 S-seeded hBMSCs when compared with that of HB 0 S. The monocyte migration was significantly halted in response to HA–BG-conditioned media when compared with the positive control (monocyte chemoattractant protein-1: MCP-1). In conclusion, the HB 30 S composite scaffold has a greater potential to substitute bone grafts in orthopaedic and dental applications.

Isotropic Si3N4 ceramics fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting

Isotropic Si3N4 ceramics were fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting (DCC/LP-SPS). A novel phenomenon is observed that hardness and fracture toughness of Si3N4 lift simultaneously as temperature rises. It can be attributed to the enhanced densification and formation of β′-sialon. Rapid coagulation of the submicron ceramic suspension prepares green body with high relative density and offers the sufficient driving force for densification, while electric field facilitates particles rearrangement, thus Si3N4 ceramics can obtain the relative density above 97% under the axial pressure of 5 MPa at 1550 °C. As SPS temperature rises, there is the formation of β′-sialon phase in Si3N4 ceramics without AlN as sintering aid. This is most likely due to electric field promoting the motion and uniform distribution of charged species in liquid phase, which facilitates the replacement of Si4+ and N3− by Al3+ and O2− during the process of dissolution-precipitation. Additionally, compared with Si3N4 prepared by hot pressure sintering, there is no obvious directional alignment of grains in the samples that are spark plasma sintered under a low applied pressure. As a consequence, Si3N4 ceramics prepared by DCC/LP-SPS acquire complete transformation of α→β phase and full densification without abnormal grain growth, leading to ultimate formation of an isotropic self-reinforced bimodal structure containing β-Si3N4 and β′-sialon. Therefore, Si3N4 ceramics, coagulated by 45 vol% suspension and sintered at 1650 °C for 20 min with a uniaxial pressure of 5 MPa, simultaneously achieve the high Vickers hardness and fracture toughness as follows: 17.5 ± 1.0 GPa and 8.5 MPa m1/2.

Low pressure spark plasma sintered hydroxyapatite and Bioglass® composite scaffolds for bone tissue repair

Hydroxyapatite (HA) based composite scaffold materials were prepared by combining it with Bioglass® (BG) to overcome the major limitations of HA such as the lack of osseointegration, inadequate bioactivity and biodegradation. Earlier attempts to prepare BG reinforced HA composites via the conventional sintering process resulted excessive reactions between the constituents and/or crystallization of BG. Excessive reaction between the constituents and crystallization of BG are known to lengthen the bioactivity response. In this work, low pressure spark plasma sintering (SPS) was utilized to prepare HA-BG composite scaffold materials with BG content until 30 wt%. Compared to the conventional sintering practice, the milder processing conditions during the SPS process such as the compaction pressure, sintering time and temperature produced HA-BG composite scaffolds without the excessive reactions between the constituents and prevented the crystallization of BG. All of the developed composites were composed of calcium phosphate (HA and β-Tricalcium Phosphate phases) and glassy phases only. The effect of BG addition on the physical properties such as the bulk density, relative density and hardness are well in line with the XRD and FESEM analyses. The in vitro bioactivity investigation from the immersion of samples in simulated body fluid (SBF) confirmed the improved bioactivity of the composite samples with increased BG content.

Manifestation of Constrained Dynamics in a Low-Pressure Spark

Some features of neutron emission from dense plasma focus suggest that the participating deuterons have energy in the range of 105 eV and have a directionality of toroidal motion. Theoretical models of these devices assume that the plasma evolves through a purely irrotational flow and thus fail to predict such solenoidal flow on the scale of the plasma dimensions. Predictions of a relaxation theory are consistent with experimental data [S K H Auluck, Physics of Plasmas,18, 032508 (2011)], but the assumptions upon which it is based are not compatible with known features of these devices. There is thus no satisfactory theoretical construct which provides the necessity for solenoidal flow in these devices. This paper proposes such theoretical construct, namely, the principle of constrained dynamics, and describes an experiment which provides support for this idea. The experiment consisted of low inductance, self-breaking spark discharge in helium at a pressure ~100 hPa between two pointed electrodes separated by 30-50 mm distance kept inside a vacuum chamber mounted on a low inductance high voltage capacitor. The current derivative signal showed reproducible sharp dips at all the extrema of the damped sinusoidal discharge. A planar diamagnetic loop centered with and perpendicular to the discharge axis consistently showed a signal representing rate of change of axial magnetic flux. A qualitative explanation of observed phenomena is obtained using a simple model using the proposed principle.

Preparation of nano-grained zirconia ceramics by low-temperature, low-pressure spark plasma sintering

Ce- and/or Y-doped zirconia nanopowders having average particle sizes ranging 12–18 nm have been synthesized by a technique based on mechanochemical processing (MCP). Despite their small particle size, the powders had excellent compactibility with green densities exceeding 50% achieved under a moderate uniaxial pressure of 150 MPa. Nearly fully dense ceramics having grain sizes of around 100 nm were successfully produced from these powders by spark plasma sintering (SPS) at temperatures of 1,050–1,150 °C for 5 min under pressures of 50–80 MPa; these temperatures and pressures are considerably lower than those required for achieving near full density with conventional nanopowders. Hardness and fracture-toughness measurements showed that the ceramics prepared by SPS had superior mechanical properties to those prepared by conventional pressureless sintering. It is argued that the high sinterability of the MCP nanopowders is ascribed to their ability to form uniform powder compacts under relatively low pressure, and that that ability in turn originates in two features of the MCP powders: absence of hard agglomeration and pseudo-spherical particle morphology.

Subnanosecond Breakdown Mechanism of Low-Pressure Gaseous Spark Gaps

Subnanosecond breakdown of low-pressure gaseous spark gaps has been investigated by using a combined modeling of the volume discharge in gas and the electron emission at the electrodes. Simulation results predict minimum breakdown delays of about 400 ps for the geometries considered in this paper. These results are in good accordance with the measurements.

Pseudospark switches for the power supply circuits of pulsed lasers

A family of sealed-off low-pressure spark gap switches developed at the Institute of High Current Electronics (Tomsk) and at the Plasma Scientific Research Institute (Ryazan) with participation of researchers from the Physical Institute of the University of Erlangen (Germany) is described. The devices are of cermet construction, are capable of switching pulsed currents over wide ranges of amplitudes and durations, and have a number of unique characteristics. The principle of operation of the spark gap switches is described and their designs are given. The triggering circuits of the switches are considered.

Over recovery in low pressure spark gaps-confirmation of pressure reduction in over recovery

The over recovery in sparkgaps (Nagesh, 1997, Nagesh et al., 1999) operating along the left-hand side of Paschens' characteristics is due to pressure reduction in the gap after the first pulse discharge. This pressure reduction leading to over recovery in low pressure spark gaps has been verified using a low pressure spark gap with two spark gaps placed one above the other in the same chamber. The breakdown voltage strength characteristics of the second gap has been determined for gap spacings of 3.5 mm to 10 mm, diffusion of plasma in the direction of vacuum pumping and opposite, at a pressure of 2.1 Pa for hydrogen gas. The vacuum pumping direction has a great influence on the breakdown strength characteristics of second gap after the first gap discharge. The breakdown voltage of the second gap can exceed its self breakdown voltage only 1) when the diffusion of plasma and vacuum pumping direction are same and 2) when the second gap spacing is greater than or equal to first gap spacing. Shorter gaps can always have breakdown voltage lower than or equal to their self breakdown voltage. The experimental setups, behavior of self breakdown voltages of second gap due to breakdown in the first gap, over recovery characteristics of spark gaps, the results, and discussions are presented here.

Urinary NAG excretion after anesthesia-free extracorporeal lithotripsy of renal stones: a marker of early tubular damage

Second generation lithotripters require a higher number of shocks per session as well as an increased rate of secondary treatments for complete stone disintegration compared to the original spark gap lithotripter. The clinical relevance of biological side effects caused by such treatment are less known. We evaluated urinary excretion of N-acetyl-glucosaminidase (NAG) before and after lithotripsy in 50 patients treated with a low pressure spark gap lithotripter (Dornier HM3) and in 36 patients treated with a piezoelectric lithotripter (Wolf Piezolith 2200) in an attempt to evaluate their side effects on renal tissue. The urinary excretion of NAG increased after both spark gap lithotripsy using the modified HM3 and piezoelectric lithotripsy. These changes may be associated with slight tubular damage that would occur after anesthesia-free lithotripsy in patients subjected both to a high number of shocks and to secondary treatments.

Breakdown and Recovery of a 250-kV Low-Pressure Spark Gap

Tests of an experimental low-pressure spark gap Blumlein switch are presented. Gas flows from a pulsed valve into the gap at a steady rate through holes in the cathode and results in a steady pressure of about 0.1 torr. The pressure outside the gap over the Blumlein insulator is much lower. Trigger electrodes are mounted in the grounded cathode. Initially the current rises exponentially with time with rate constant proportional to gas density. Measurements of rise time, jitter, and voltage holding recovery time are presented for charge voltages up to 250 kV for variations of charge time, gas density, gas type, and triggering method.

Onset of self-breakdown in a low-pressure spark gap

Conditions for self-breakdown are studied with a time-dependent one-dimensional model which includes effects of ionization by electrons, ions, and energetic neutrals, charge exchange, secondary-electron emission from the cathode due to impact of ions and neutrals, and backscattering of electrons from the anode. Theoretical predictions for the low-pressure branch of the Paschen curve are reported. $\mathrm{Pd}$ scaling is explicitly verified. In addition, the rates for current buildup above the critical pressure are calculated. The relative importance of various physical effects on the onset of self-breakdown are studied.

Production of high current particle beams by low pressure spark discharges

On the basis of low pressure spark discharges a new type of a particle accelerator as been developed. It produces pulsed ion as well as electron beams of high intensities in a gas atmosphere at pressures of the order of 1 mbar. The method was used to produce in hydrogene at particle energies of about 70 keV an electron beam with a current density of more than 106 A/cm2 at 140 A total current. It is shown that magnetic confinement by the pinch effect takes place in the discharge. Discharge times smaller than 5 ns and spark frequencies up to 2 MHz can be obtained.

A theoretical investigation into the efficiency characteristics of low pressure spark chambers

Efficiency characteristics of parallel plate spark chambers were previously studied experimentally as a function of pulse voltage and argon pressure in the range 5–760 torr. Theoretical explanations of the efficiency curves obtained therefrom are considered in the present work in the frame-work of the avalanche-streamer model of gas discharge suitably modified in the region of low pressure. We obtain separate expressions for efficiencies of defined sparks and glow type discharges in the spark chambers which fairly agree with the experimental curves. We conclude that glow discharges are initiated only when the effective number of seed electrons in the chamber gap does not exceed 2 per cm. It also appears that the clustering effect plays a significant role only in the case of defined sparks in argon and not in helium.

Track characteristics of low pressure spark chambers

Argon-filled spark chambers (gap sizes 1.9 and 9.5 cm) operated under low gas pressures (80–300 torr) and short pulse lengths (30–120 ns) revealed spark tracks with a distinc structure (blobs) property. The blob density is found to be a function of the gas pressure and particle ionization. Results show that this mode of observation may be utilized with advantage particularly in large area cosmic ray experiments for the measurement of particle ionization.

Low pressure spark chamber

Dependence of a spark chamber detection efficiency on a gas pressure was investigated at pressures lower than 1 atm. Results are explained by considering not all of seed electrons, but an effective part of them is accelerated in a gap length of the spark chamber. Comparason of efficiency curves for Ar and He gases indicates that a low pressure He spark chamber can be made useful as ionization discriminator of incident particles.

Operational characteristics of low pressure spark chambers

Efficiency of parallel spark chambers having different gap widths has been measured as a function of applied voltage and gas pressure for cosmic ray muons of mean energy 4 GeV in the pressure range 5–750 torr. A linear relationship analogous to Paschen's Law for the quantities threshold voltage, argon pressure and gap width has been found to hold with a gradient K = 141.4 dyne V −1 cm −1. With a fixed plate voltage 10 kV, the efficiency remained substantially flat at nearly 100% in the pressure range 100–550 torr. Beyond these limits it was observed to fall steeply. The efficiency characteristics with argon fillings below 100 torr for the same plate voltage have shown that the threshold pressure down to which only defined sparks are possible was 30 torr; with an alcohol contamination of 20% with argon this pressure could be further lowered to about 22 torr. Below this pressure undefined sparks in addition to defined ones appeared across the spark gaps along the path of the ionizing radiation. The behaviour towards the net efficiency of defined and undefined sparks was analysed.

Low pressure spark chambers — high resolution spectrometer for 1 GeV protons

A low pressure magnetic spectrometer for 1 GeV protons utilizing wire spark chambers operated at 0.1 normal atmosphere is described. The reduced multiple scattering of this system produced a distinct improvement in energy resolution over that of a comparable spectrometer operated at normal atmospheric pressure. Details on the operating characteristics of the low pressure spark chambers are given, and an outline of the pressure regulating system is included.

423. Triggering mechanism of low-pressure spark gaps: R. Hancox, Rev. Sci. Instrum., 33 (11), Nov. 1962, 1239–1244

423. Triggering mechanism of low-pressure spark gaps: R. Hancox, Rev. Sci. Instrum., 33 (11), Nov. 1962, 1239–1244

Triggering Mechanism of Low-Pressure Spark Gaps

The triggering delay in two low-pressure spark-gap switches, operating in the pressure range 10−3 to 3×10−2 mm Hg, has been measured under a wide range of conditions. When the trigger pin is in the negative electrode, the delay is found to consist of two components. The first part depends on the construction of the trigger pin, the trigger voltage, and the impedance of the trigger circuit, while the second part depends on the nature and pressure of the gas in the gap, and the voltage and impedance of the circuit being switched. If the trigger pin is in the positive electrode a further delay is added which is approximately equal to the transit time of an ion across the gap. A mechanism for the breakdown is proposed which is consistent with the measurements and with previously reported results.