Nitrogen Overpressure Research Articles

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y2O3–Sc2O3 sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm−1. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m−1 K−1 to 70 W·m−1 K−1, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m−1 K−1 up to 238 W·m−1 K−1 for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.

(Invited) Ion Implantation and Activation of n- and p-Type Dopants in GaN

GaN-based power switching devices are of significant interest for high efficiency power conversion circuits in medium voltage applications. However, there are still significant challenges preventing mass production and widespread adoption. Recently, GaN-based vertical and lateral power devices have attracted significant interest due to promising device results coupled with progress in native substrate, epitaxial growth, and processing technology developments. However, there are two significant limiting factors for vertical GaN power devices at present. The first issue is substrate and epitaxial layer uniformity and reliability, and the second is the lack of a planar selective-area doping process. While p-type epitaxial growth capability and the ability to stack layers are quite mature from the LED industry, power devices require planar selective-area doping, which is a routine step by ion implantation in the Si and SiC device industry. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge as implant activation typically requires annealing at temperatures ~ 2/3 of the melting point, which is a region where the GaN crystal is unstable and readily decomposes to Ga + N2 at atmospheric pressure. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been the most successful, demonstrating electrical activation of up to ~10% of the implanted Mg dopant atoms using a combination of a temporary, thermally stable capping layer, annealing in a moderate nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Advances in the SMRTA process development, including optimization of capping processes, implantation profile, and processing steps such as metallization and surface treatments will be reviewed here. In addition, the evaluation of components of the SMRTA process, such as the cap and overpressure anneal, have been applied to Si implantation for n-type doping and will be presented here. This is a critical step to reduce contact resistance in lateral GaN structures as well as form the source regions of vertical devices that is typically accomplished by selective area regrowth. The use of ion implantation and activation utilizes fewer process steps and avoids impurity incorporation at sidewalls compared to the selective area regrowth process, and has been shown to yield lower contact resistance as well. This presentation will review advances in both Mg and Si ion implantation and assess the viability of this process for practical device fabrication.

Thermal properties and laser processing of hot-pressed materials from Ti\u2013Al\u2013C system

The work concerns the characterization of polycrystalline materials composed in Ti–Al–C system such as Ti2AlC and Ti3AlC2. The starting powders were synthesized from metal powder in nitrogen overpressure with the use of SHS method. Such obtained powders were chemically described and hot-pressed at temperatures 1100 °C and 1300 °C. The densification measurements were taken on obtained sintered bodies. The chemical composition analysis and microstructural observation of hot-pressed materials were made. The obtained MAX phase Young’s modulus was examined in the temperature range 25–600 °C by resonance method. The thermal properties of the thermogravimetric analysis were made in air flow to determine the oxygen resistance. The MAX phase polycrystals were taken under laser flash analysis, which allowed to measure directly thermal diffusivity and specific heat and to calculate indirectly thermal conductivity up to the temperature of 700 °C. Additionally, the polycrystalline material was laser treated in continuous work mode. The hot-pressed MAX nanolaminates were laser ablation and laser welding processed, which is new in the literature. The laser beam-treated material places were microstructurally investigated by scanning electron microscopy and chemically by energy-dispersive X-ray spectroscopy. The result of laser processing was discussed regarding material preparation route and its thermal properties.

(Invited) Vertical GaN Devices Enabled By Selective Area P-Type Doping

A key element for enabling high performance vertical GaN power electronic devices is the capability to realize selective area p-type doping. The ability to arbitrarily define p-type regions through ion implantation eliminates the need for epitaxial regrowth on etched surfaces, which is currently a well-known technological roadblock. However, activation of implanted Mg is also a challenge, since activation of the Mg acceptors requires annealing at temperatures above the thermodynamic stability limits of GaN in atmosphere. Implementing the symmetric multi cycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms. This technique includes a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Vertical GaN junction barrier Schottky (JBS) diodes and Schottky barrier diodes (SBDs) with implanted junction termination extension (JTE) are demonstrated using the SMRTA process, as are process module development toward trench MOSFET devices.

Vertical GaN Junction Barrier Schottky Diodes

Vertical GaN junction barrier Schottky (JBS) diodes are demonstrated. The JBS p-type grid was formed by Mg-implantation into a 10 μm thick unintentionally doped GaN homoepitaxial drift layer, grown by metal organic chemical vapor deposition (MOCVD). Then, symmetrical multi-cycle rapid thermal annealing (SMRTA) repaired implantation damage and activated the Mg ions. This process includes deposition of an AlN capping layer, annealing in a nitrogen overpressure, and rapid heating and cooling pulsed annealing. In addition, PiN diodes and Schottky barrier diodes (SBDs) were fabricated on the same substrate for comparison. The JBS diode demonstrates forward conduction characteristics dominated by the Schottky barrier (turn-on voltage ∼0.5 V), and reverse breakdown voltage comparable to the PiN diode (−610 V).

High Temperature Laser Based Drop on Demand Micro Joining of Thin Metallic Layers or Foils using Bronze Braze Preforms

In this paper a drop on demand based method of joining metallic layers of about 20 µm thick- ness with Cu-conductors of 100 µm diameter is presented. The process is based on melting spherical preforms of copper based braze (CuSn12) with a liquidus temperature of 990 °C (1). In the process, a braze preform is induced into a ceramic capillary, molten by a laser pulse and expelled out of the machining head via nitrogen overpressure. After a flight phase the braze droplet wets the surface of the electrode structure and the copper resulting in a firm joint after solidification. The shear strengths of the joints were measured to be 31 MPa for joints on Al2O3 substrates and 37 MPa for joints generated on LTCC substrates with screen printed Ag electrode structures. The shear strengths after temperature shock tests according to DIN EN 60068-2-14 indicate good performance of the joints in harsh environments. The average resistivity of a joint was tested by four wire testing and found to be 1.56 ± 0.33 mΩ. The introduced setup is capable to join thin metallic layers of 30 - 15 µm thickness with cu conductors of 100 µm diameter without the need of flux.

Design of an efficient, high purity hydrogen generation apparatus and method for a sustainable, closed clean energy cycle

In this paper we present a detailed design study of a novel apparatus for safely generating hydrogen (H2) on demand according to a novel method, using a controlled chemical reaction between water (H2O) and sodium (Na) metal that yields hydrogen gas of sufficient purity for direct use in fuel cells without risk of contaminating sensitive catalysts. The apparatus consists of a first pressure vessel filled with liquid H2O with an overpressure of nitrogen (N2) gas above the H2O reactant, and a second pressure vessel that stores solid Na reactant. Hydrogen gas is generated above the solid Na when H2O reactant is introduced using a regulator that senses when the downstream pressure of H2 gas above the solid Na reactant has dropped below a threshold value. The sodium hydroxide (NaOH) byproduct of the hydrogen producing reaction, is collected within the apparatus for later reprocessing by electrolysis, to recover the Na reactant.

Damage investigation on the explosive destruction of a tank at a chlorine liquefaction plant

This paper summarizes the investigation of a serious explosion, which shattered the chemical facility of a chlorine/alkaline plant in Northern Germany in 1981. A pressure tank made from HSB 50 S steel and used as storage of liquid residues from a chlorine liquefaction process violently ruptured and the explosion threw parts of the tank approximately 100m through the air. The local county government ordered an investigation to determine the root cause of this incident.The main objective of the investigation was to determine whether a nitrogen overpressure or a chemical reaction of residuals inside the so-called ‘Taffy’ receiver caused the explosion. A fracture mechanics based analyses evaluated the conditions the material was exposed to during the damage. Metallographic and mechanical tests confirmed that the material specifications were within the values specified by the guidelines but corrosion had reduced the wall thickness considerably.Analytical calculations to determine the minimum required pressure for the rupture and to cause plastic deformation to the tank confirmed that the failure did not occur due to nitrogen overpressure and therefore, a chemical reaction must have occurred. A detailed chemical analysis confirmed that the explosive disintegration of methylnitrate, which is highly sensitive to heat and impact, and its halogenated derivatives likely caused the incident. Due to the design and operational mode of the plant, impurities could accumulate and form explosive compounds in the Taffy receivers without control.

Melting point determination of uranium nitride and uranium plutonium nitride: A laser heating study

Understanding of the behaviour of nuclear material in extreme conditions is essential for the analyses of the operation limits of nuclear fuels, and prediction of possible nuclear reaction accidents. In this context, the high temperature behaviour of uranium nitride and mixed uranium–plutonium nitrides has been studied in the present work by laser heating under controlled atmosphere coupled with fast multi-wavelength pyrometry. Such an approach has allowed performing a thermal arrest analysis and establishing the solid–liquid phase boundaries in the investigated compositions, whereby non-congruent vaporisation was avoided by setting a suitable nitrogen overpressure. In addition, the normal spectral emissivities of the current samples were determined by radiance spectroscopy. Besides revealing a slightly more metallic optical behaviour in plutonium-containing compositions, this latter characterisation led to the determination of the real melting/solidification temperatures of the investigated nitrides. It is confirmed that UN melts congruently at (3120±30) K in a nitrogen pressure of 0.25MPa (2.5bar). The melting/solidification temperatures decrease in plutonium containing samples, reaching (3045±25) K for x(PuN)=0.2, a composition of interest for potential applications of this material as a nuclear fuel. Besides their fundamental importance, the current results are useful for a deeper understanding of the nitride fuel behaviour under accidental conditions, whereby uncontrolled thermal excursions might occur in the nuclear reactor core.

Influences of Nozzle Material on Laser Droplet Brazing Joints with Cu89Sn11 Preforms

Abstract This paper presents latest results on the influences of nozzle material and geometry on the electromechanical contacting of sensitive piezoceramic actuator modules. Two nozzle types have been investigated,a standard WC/Co nozzle which is used for soldering applications and a novelceramic nozzle. Applications for active piezoceramic components integrated in structural parts are e.g. active damping, energy harvesting, or monitoring of vibrations and material failure. Anup to now unsolved problem is the electrical contacting of such components without damaging the conductor or the metallization of the ceramic substrate. Since piezoelectric components are to be integrated into structures made of casted aluminum, requirements are high mechanical strength and temperature resistance. Within this paper a method forcontacting piezoceramic modules is presented. A spherical braze preform of tin bronze Cu89Sn11 with a diameter of 600 μm is located in a ceramic nozzle and is subsequently melted by a laser pulse. The liquid solder is ejected from the nozzlevia nitrogen overpressure and wets the surface of the metallization pad and the Cu-wire, resulting in a brazing joint after solidification. The process is called laser droplet brazing (LDB). To asses the thermal evolution during one cycle WC/Co and ZTA have been simulated numerically for two different geometries enabling a proposition weather the geometry or the material properties have a significant influence on the thermal load during one cycle. To evaluate the influence of the nozzle on the joint the positioning accuracy, joint height and detachment times have been evaluated. Results obtained with the ZTA nozzle show comparable positioning accuracies to a WC/Co nozzle with a lower standard deviation of solder detachment time.

Plasma-assisted molecular beam epitaxy process combined with a liquid phase electroepitaxy, a novel method for the growth of GaN layers

We have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). We have designed and built a new growth chamber which allowed us to combine a plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from a liquid Ga melt at growth temperatures as low as ∼650°C, with low nitrogen overpressures of ∼3×10−5Torr.

Influence of nitrogen overpressure on the nitridation, densification and formation of β-SiAlONs produced by silicothermal reduction

Abstract β-SiAlON materials (with z = 1) with 2 mol% DyAG have been synthesised by silicothermal reduction under different nitrogen pressures. The possibility of rapidly nitriding silicon and forming the SiAlON phase has been investigated through its reaction sequence of formation under different pressures. β-SiAlONs were fully nitrided (degree of nitridation greater than 93%) and formed after a 1-h hold at 1400 °C, or after a straight ramp to 1500 °C under 0.7 MPa of nitrogen. This was not achievable under static nitrogen at atmospheric pressure where the degree of nitridation was only 43% and β-SiAlON phase represented only about 60% of the crystalline phase assemblage at the same temperature. The formation of β-SiAlON depended on the formation of Si 3 N 4 whose reaction rate was enhanced by nitrogen overpressures.

Plasma-assisted electroepitaxy of GaN layers from the liquid Ga melt

In this current paper we have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allowed us to combine the plasma-assisted molecular beam epitaxy process with a liquid-phase electroepitaxy system. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from liquid Ga melt at growth temperatures as low as ∼650°C, with low nitrogen overpressures of ∼3×10−5Torr.

Plasma‐assisted electroepitaxy as a novel method for the growth of GaN layers

AbstractIn the current study we have demonstrated the feasibility of a novel approach for the growth of GaN layers, namely plasma‐assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the plasma‐assisted molecular beam epitaxy process with a liquid phase electroepitaxy system. We have demonstrated that it is possible to grow GaN layers by PAEE at growth temperatures as low as ∼650 oC and with low nitrogen overpressures of ∼3×10‐5 Torr. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma-assisted electroepitaxy as a method for the growth of GaN layers

In the current study we have demonstrated the feasibility of a novel approach for the growth of the GaN layers namely plasma-assisted electroepitaxy (PAEE). In this method, we have tried to combine advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface without spontaneous crystallization on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the Molecular Beam Epitaxy process with a Liquid Phase Electroepitaxy system. We have demonstrated that it is possible to grow GaN layers by PAEE at growth temperatures as low as ∼500 °C and with low nitrogen overpressures of ∼3×10 −5 Torr. We have shown that the combination of the two processes, an N-plasma flux and a DC current, are vital for the successful growth of GaN layers by PAEE. The exact mechanisms, which lead to the growth of GaN by PAEE, still need to be investigated.

Innovative Process for the Synthesis of Nanostructured Ruthenium Catalysts and their Catalytic Performance

An efficient, reproducible process for the solvothermal synthesis of Ru/γ-Al2O3 nanocatalysts has been developed. This synthesis employs a low boiling point alcohol acting as solvent and reducing agent. The reaction is performed in autoclave under nitrogen overpressure, by heating the solution of metal precursor and stabilizing agent in the presence of the support. These systems show very promising performances, also with an application perspective, in the selective hydrogenation of phenol to cyclohexanone carried out at 160 °C under 5 MPa of hydrogen: a selectivity to cyclohexanone as high as 83% has been reached at a phenol conversion value of 88%.

Synthesis of spherical Al2O3 and AlN powder from C@Al2O3 composite powder

A novel approach has been taken to produce (1) spherical Al2O3 particles by decarbonisation and (2) spherical AlN particles by nitridation and subsequent decarbonisation of C@Al2O3 composite particles. C@Al2O3 composite particles have been prepared by heterogeneous nucleation and crystallisation of Al(NO3)3 on surfactant encapsulated carbon nano particles followed by evaporative decomposition of the nitrate. Overpressure (0.4 MPa) of nitrogen and a temperature range (1723–1873 K) have been used for nitridation. Whiskers as well as spherical particles of AlN have been observed in the final product. The final product has been characterised by X-Ray Diffraction, Scanning Electron Microscope and Carbon–Hydrogen–Nitrogen content analysis by Elemental Analyser and the mechanism of the nitridation reaction has been analysed. The average size of the spherical AlN particles consisting of crystallites in nano-dimensions (30–50 nm) could be varied from 100 nm to 8 μm by changing the composition of the sol.

Properties of InN layers grown by High Pressure CVD

ABSTRACTIndium nitride (InN) and indium-rich group III-nitride alloys are promising materials for advanced optoelectronic device applications. Indium-rich alloys, e.g. (Ga1-y-xAlyInx)N will enable the fabrication of high-efficient light emitting diodes tunable in the whole visible spectral region, as well as advanced high speed optoelectronics for optical communication operating. The present limitation in this area is the growth of high quality InN and indium-rich group III-nitride alloys as documented in many controversial reports on the true physical properties of InN. The difficulties arise from the low dissociation temperature of InN that requires an extraordinarily high nitrogen overpressure to stabilize the material up to optimum growth temperatures. We developed a novel “high-pressure chemical vapor deposition” (HPCVD) system, capable to control and analyze the vast different partial pressures of the constituents. Our results show that the chosen HPCVD pathway leads to high-quality single crystalline InN, demonstrating that HPCVD is a viable tool for the growth of indium rich group III nitride alloys. The structural analysis of InN deposited on GaN-sapphire substrate by XRD show single phase InN(0002) peaks with full width half maximum (FWHM) around 400 arcsec. Infrared reflectance spectroscopy is used to analyze the plasmon frequencies, high frequency dielectric constants, the free carrier concentrations and carrier mobilities in these layers. For nominal undoped InN layers, free carrier concentrations in the mid 1019 cm−3 and mobilities around 600 cm−2-V-1-s-1 are observed. Further improvements are expected as the growth parameters are optimized. The explored growth parameters are close to of those employed for GaN growth conditions, which is a major step towards the fabrication of indium rich (Ga1−y−xAlyInx)N alloys and heterostructures.

Real-time optical monitoring of ammonia flow and decomposition kinetics under high-pressure chemical vapor deposition conditions

Understanding the gas phase decomposition kinetics of the chemical precursors involved in the nucleation and thin-film growth processes is crucial for controlling the surface kinetics and the growth process. The growth of emerging materials such as InN and related alloys requires deposition methods operating at elevated vapor densities due to the high thermal decomposition pressure of these materials. High nitrogen overpressure has been demonstrated to suppress the thermal decomposition of InN, but has so far not been explored in chemical vapor deposition experiments. In this contribution we present research results on the decomposition kinetics of ammonia in the laminar flow regime of a high-pressure flow channel reactor. Ultraviolet absorption spectroscopy is applied to analyze absorption features of ammonia with respect to the ammonia flow rate during continuous flow and pulsed ammonia injection. Pulsed ammonia injection has been used to analyze the average gas flow velocity in the high-pressure chemical vapor deposition system as a function of the total gas flow rate and the reactor pressure. The onset of the kinetics related to the decomposition of ammonia was found to start above 900K and showed a marked decrease for higher reactor pressures, thus decreasing the discrepancy of trimethylindium and ammonia decomposition temperatures.

Effect of Liquid Phase Sintering on Microstructure and Mechanical Properties of Yb2O3‐AlN Containing SiC‐Based Ceramics

A series of silicon carbide‐based ceramics with different sintering additives were liquid‐phase sintered to high densities. Yb2O3 in combination with AlN was used as the additive, instead of the commonly used Y2O3–AlN, to improve the refractoriness of the secondary phase. Thermo‐chemical decomposition of AlN was sufficiently suppressed with the use of nitrogen overpressure and reasonable weight loss was achieved in the different additive containing SiC ceramics without a reactive powder bed. Use of the heavier rare‐earth element modified the liquid phase formed during sintering and reduced the phase transformation controlled grain growth rate, compared with Y2O3 doped materials. It also permitted microstructure tailoring through post‐sintering heat treatments in nitrogen. Materials with self‐reinforced microstructures, formed as a result of anisotropic grain growth, were obtained. Improved fracture toughness (4.5–5 MPa/m1/2) and good flexural strength retention up to 1400°C were also observed.