Sintering In H2 Research Articles

Influence of Fe2O3 and sintering atmosphere on the phase evolution, and electrical resistivity of porous SiC

The effects of Fe2O3 addition on the electrical resistivity, thermal conductivity, and microstructure formation of SiC processed under Ar, N2, and N2 + 5% H2 sintering atmospheres were investigated. The use of N2 + 5% H2 resulted in the lowest electrical resistivity, improved thermal conductivity and enhanced flexural strength of SiC as compared to Ar and N2. The use of 10 wt% Fe2O3 resulted in an electrical resistivity of 3.8 × 10−1 Ω cm, a thermal conductivity of 20 W/m.K, and a flexural strength of 40 MPa with 33% porosity of samples sintered at 1250 °C under N2 + 5% H2. The in-situ formation of a network-like structure of iron silicide (Fe3Si) at contact points of SiC yielded a conductive path and reduced the electrical resistivity, while the formation of Si3N4 improved the mechanical property. Although, all sintering atmospheres resulted in the formation of electrically conductive Fe3Si, but the use of N2 + 5% H2 produced samples with highest Si3N4 content, lowest amount of oxide phase (SiO2 and Si2N2O), and additionally densified the microstructure.

Effect of the pressureless post-sintering on the hot isostatic pressed Al2O3 prepared from the oxidized AlN powder

The effect of the pressureless post-sintering in hydrogen on the structural and mechanical properties of the hot isostatic pressed Al2O3 prepared by oxidized AlN powder has been studied. The micrometer size AlN powder has been oxidized in air at 900° C and sintered by hot isostatic pressing (HIP) at 1700 °C, 20 MPa nitrogen atmosphere for 5 h. Pressureless sintering (PS) has been applied for all HIP sintered samples in H2 gas at 1800° C for 10 h. It has been shown that the oxidation caused a core–shell AlN/Al2O3 structure and the amount of Al2O3 increased with increasing of the oxidation time of the AlN powder. For the first time, the green samples obtained from oxidized AlN powder have been successfully sintered first by HIP followed by post-sintering by PS under hydrogen without adding any sintering additives. All post-sintered samples exhibited the main α-Al2O3 phase. Sintering in H2 caused the full transformation of AlN to α-Al2O3 phase and their better densification. Therefore, the hardness values of post-sintered samples have been increased to 17–18 GPa having apparent densities between 3.11 and 3.39 g/cm3.

Synthesis, crystal structure and photoluminescence properties of novel Ba3Lu4O9:Ce3+ orange-red phosphors for white light emitting diodes

The exploration of red phosphors is very meaningful for enhancing the color rendering index (CRI) of white-light-emitting diodes (WLEDs). Herein, the novel BLO:Ce3+ orange-red phosphors are successfully synthesized by the high-temperature solid-state reaction via introducing Ce3+ ions into the Ba3Lu4O9 host. Structural analysis shows there was only the BLO:Ce3+ phosphors in the obtained product according to the standard diffraction card. The optimal preparation condition is the single-pass sintering in H2 at 1873 K for 3 h. In the spectral aspect, the optimal Ce3+-doped concentration is 0.5 atom %, and the strongest excitation and emission peaks of are located at 410 nm and 584 nm, respectively. Compared to reported Ba3Sc4O9:Ce3+ phosphors, the emission peak of BLO:Ce3+ phosphors is effectively red-shifted to about 21 nm. The fluorescent lifetime of BLO:Ce3+ phosphors is about 11.45 ns. Based on the temperature-dependent spectral and structural investigations, the thermal fluorescent quenching of BLO:Ce3+ phosphors originates mainly from the surrounding change of Ce3+ ions, and it is more evident at the temperatures below 340 K. Moreover, the crystallographic sites of the Ce3+ ions in Ba3Lu4O9 are also verified according to the temperature-dependent photoluminescence characterization with the excitation of 355 nm and empirical theoretical analysis, suggesting the observed luminescence mainly comes from the Ce3+ ions located in the LuO6 and BaO6 polyhedra. Finally, utilizing the as-prepared BLO:Ce3+ phosphors, commercial green phosphors and a blue LED, the WLED device is fabricated with a warm white-light performance in the color rendering (Ra = 91.2 and R9 = 88) and color temperature (3758 K) aspects. This study indicates the potential application of BLO:Ce3+ orange-red phosphors in the white-light illumination, and also provides some guiding inspirations in the exploration new red oxide phosphors.

Nanostructured oxide dispersion strengthened Mo alloys from Mo nanopowder doping with oxide nanoparticles

A simple and efficient pathway was developed to synthesize ultrafine grained oxide dispersion strengthened (ODS) Mo alloys doped with nano-scaled oxide particles. Firstly, dispersed La(NO3)3 or Al(NO3)3 nanoparticles were introduced into commercial MoO3 via a solution spraying method. Then, Mo nanopowders containing different amounts of oxides nanoparticles (La2O3 (0%, 0.5%, 1% and 2%) or Al2O3 (0.5%, 1%)) were successfully prepared by reducing the doped MoO3 with carbon black at 600 °C and 1050 °C, and hydrogen at 800 °C. Due to the high sintering activity of prepared doped Mo nanopowder, after sintering in H2 at 1300 °C for 3 h, the relative density of sintered products containing 0.5% La2O3 or Al2O3 reached to above 95.4%. Nanostructured ODS Mo alloys with Mo and La2O3 (or Al2O3) grain sizes of about 0.5 μm and 75 nm (or 50 nm for Al2O3) were successfully prepared at around 1300 °C. The obtained nanostructured Mo-oxides alloys had much high hardness values. For the Mo-La2O3 alloys, the highest value of hardness can reach to 338 HV (1% La2O3, Mo grain size, 0.51 μm; La2O3 grain size, 71.3 nm; density, 93.0%), while the highest hardness of Mo-Al2O3 alloys reached to 385 HV (1% Al2O3, Mo grain size, 0.55 μm; Al2O3 grain size, 55.9 nm; density, 94.6%), which were much higher than the ODS-Mo alloys prepared from the traditional methods. This method could also have great potential for industrially producing nanostructured ODS Mo alloys and W alloys.

Hybrid Atmosphere Processing of Lead-Free Piezoelectric Sodium Potassium Niobate-Based Ceramics

K0.5Na0.5NbO3-based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from alkali carbonates and hydroxides, and oxidizing atmospheres is used during sintering to avoid alkali evaporation. Discs of Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were sintered in air, N2, 9% H2 in N2, or 9% H2 in N2 during heating and air during sintering (hybrid atmosphere processing). The highest density was obtained by sintering in 9% H2 in N2, but resulted in high alkali loss and decomposition of the surface, followed by low piezoelectric response. However, with the hybrid H2/air processing it was possible to both avoid surface decomposition and leakage currently associated with alkali evaporation during sintering in H2, and to obtain a denser, more phase-pure and small-grained KNNLTM ceramic with a higher piezoelectric response than obtained by sintering in air or N2.

Oxidation Characteristics of Nano-Oxide Dispersed Ferritic Stainless Steel Alloys for Solid Oxide Fuel Cell Interconnects

Ferritic stainless steel based metallic interconnects for intermediate temperature solid oxide fuel cells (SOFC) show performance degradation due to the continuous growth of chromia scale. Modification of stainless steel alloy is considered to be one of the viable options to utilize ferritic stainless steel for long-term application in SOFC as interconnect. In this study, we developed the nano-oxide dispersed alloys of ferritic stainless steel SUS430 and studied the oxidation properties at 800 °C in oxidizing conditions. Nano-oxides of CeO2, La2O3, Y2O3, and LaCrO3 were dispersed in commercial SUS430 by high-energy ball milling and the specimen were fabricated by uniaxial and isostatic pressing and sintering in H2 at 1400 °C. The sintered samples were tested for oxidation properties and long-term area specific resistance (ASR) in the air at 800 °C for 1000h and post-test SEM and XRD analysis was performed to analyze the oxide scale. It was found that the addition of up to 3wt. % nano-CeO2 is the most beneficial among the added nano-oxides in order to improve the oxidation properties of SUS430 due to the reduced thickness of the chromia scale, and improved adhesion of the oxide scale and the alloy substrate.

Laser generation on Yb:LuAG ceramics produced by nanocrystalline pressure-less sintering in H2

The spectral and laser properties of 3 at.% Yb3+:Lu3Al5O12 ceramics produced by a new method of nanocrystalline pressure-less sintering in H2 were investigated. The studied sample had a transmittance close to the theoretical value (83.3% at λ = 1070 nm) and a laser level lifetime of 0.93 ms. Laser generation in different regimes with a maximum slope efficiency of 54% and a maximum average power of 206 W was attained, which makes this method highly promising for producing ceramics for lasers with multi-kilowatt average power.

A study on the sintering of ultrafine grained tungsten with Ti-based additives

Tungsten (W) is a brittle material at room temperature making it very difficult to fabricate. Although the lack of ductility remains a difficult challenge, nano-sized and ultrafine grain (UFG) microstructures offer potential for overcoming tungsten's room temperature brittleness. One way to manufacture UFG W is to compact and sinter nanosized W powder, however, it is a non-trivial task to control grain growth during sintering. In an effort to inhibit grain growth, the effect of Ti-based additives on the densification and grain growth of nano-W powders was investigated in this study. The addition of 1wt.% Ti into tungsten led to more than a 63% decrease in average grain size of sintered samples at comparable density levels. It was also found that sintering in Ar yielded a finer grain size than sintering in H2 at similar densities. Compared to conventional high temperature sintering, a lower temperature sintering cycle for a longer hold time resulted in both near-full density and fine grain size. The roles of the Ti additive include not only the inhibition of grain growth, but also the potential absorption of oxygen from W particles.

The study on low temperature sintering of nano-tungsten powders

Ultrafine grain tungsten is a promising candidate for plasma facing component applications in future fusion reactors. The conventional press-and-sinter powder metallurgy method is a preferred route for producing ultrafine grain tungsten from nanosize tungsten powders. However, controlling grain growth, while simultaneously attempting to achieve maximum densification, has been a challenge for making UGF tungsten by sintering. In this study, the effect of processing parameters on the densification and grain growth of nano-W powders was investigated. Near-fully densified tungsten was obtained at sintering temperatures between 1100 and 1300°C, and both Ar and H2 sintering atmospheres were investigated. The Ar sintering atmosphere was determined to more favorably promote densification and minimize grain growth. The nanosized tungsten powder compacts were subjected to reduction in H2 as a part of the sintering cycle. The reduction temperature was found to have significant effects on the sintering of nano-W powder, primarily as a result of grain coarsening, which was seen at temperatures as low as 700°C.

The Effects of Atmosphere on the Sintering of Ultrafine-Grained Tungsten with Ti

Tungsten (W) is a brittle material at room temperature making it very difficult to fabricate. Although the lack of ductility remains a difficult challenge, nano-sized and ultrafine-grained (UFG) structures offer the potential to overcome tungsten’s room-temperature brittleness. One way to manufacture UFG W is to compact and sinter nano-sized W powder. It is challenging, however, to control grain growth during sintering. As one method to inhibit grain growth, the effect of Ti-based additives on the densification and grain growth of nano-W powders was investigated in this study. Addition of 1% Ti into tungsten led to more than a 63% decrease in average grain size of sintered samples at comparable density levels. It was found that sintering in Ar yielded a finer grain size than sintering in H2 at similar densities. The active diffusion mechanisms during sintering were different for W-1% Ti nano powders sintered in Ar and H2.

Manufacturing of self-passivating tungsten based alloys by different powder metallurgical routes

Self-passivating tungsten based alloys will provide a major safety advantage compared to pure tungsten when used as first wall armor of future fusion reactors, due to the formation of a protective oxide layer which prevents the formation of volatile and radioactive WO3 in case of a loss of coolant accident with simultaneous air ingress. Bulk WCr10Ti2 alloys were manufactured by two different powder metallurgical routes: (1) mechanical alloying (MA) followed by hot isostatic pressing (HIP) of metallic capsules, and (2) MA, compaction, pressureless sintering in H2 and subsequent HIPing without encapsulation. Both routes resulted in fully dense materials with homogeneous microstructure and grain sizes of 300 nm and 1 μm, respectively. The content of impurities remained unchanged after HIP, but it increased after sintering due to binder residue. It was not possible to produce large samples by route (2) due to difficulties in the uniaxial compaction stage. Flexural strength and fracture toughness measured on samples produced by route (1) revealed a ductile-to-brittle-transition temperature (DBTT) of about 950 °C. The strength increased from room temperature to 800 °C, decreasing significantly in the plastic region. An increase of fracture toughness is observed around the DBTT.

New Alloying Systems for Sintered Steels: Critical Aspects of Sintering Behavior

Oxygen-sensitive alloying elements such as Mn, Si, and Cr have a high potential for improving the properties of low alloyed sintered steels while reducing the alloying cost. However, it is necessary to find a way for avoiding, or at least minimizing, the oxidation of these elements especially during the early stages of the sintering cycle. In this study Mn, Si, and Cr were introduced in the form of a master alloy powder designed to be mixed with the iron base powder and provide the final composition of the steel during the sintering process. The reduction/oxidation phenomena taking place during the heating stage were studied by thermogravimetry, dilatometry, and mass spectroscopy, using either reducing (H2) or inert (Ar) atmospheres. The results show how the difference in chemical activity between base iron powder and master alloy causes the so called “internal-getter” effect, by which the reduction of less stable iron oxides leads to oxidation of the elements with higher affinity for oxygen. This effect can be somehow minimized when sintering in H2, since the iron oxides are reduced at lower temperatures at which the reactivity of the elements in the master alloy is lower. However, H2 concentration in the processing atmosphere needs to be carefully adapted to the specific composition of the materials being processed in order to minimize decarburization by methane formation during sintering.

Improvement in pH Sensitivity of Low-Temperature Polycrystalline-Silicon Thin-Film Transistor Sensors Using H2 Sintering

In this article, we report an improvement in the pH sensitivity of low-temperature polycrystalline-silicon (poly-Si) thin-film transistor (TFT) sensors using an H2 sintering process. The low-temperature polycrystalline-silicon (LTPS) TFT sensor with H2 sintering exhibited a high sensitivity than that without H2 sintering. This result may be due to the resulting increase in the number of Si–OH2+ and Si–O− bonds due to the incorporation of H in the gate oxide to reduce the dangling silicon bonds and hence create the surface active sites and the resulting increase in the number of chemical reactions at these surface active sites. Moreover, the LTPS TFT sensor device not only offers low cost and a simple fabrication processes, but the technique also can be extended to integrate the sensor into other systems.

The incorporation of plutonium in lanthanum zirconate pyrochlore

The incorporation of plutonium (Pu) within lanthanum zirconate pyrochlore was investigated using air, argon, and N2–3.5%H2 sintering atmospheres together with Ca2+ and Sr2+ incorporation for charge compensation. The samples have been characterised in the first instance by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results show Pu can be exchanged for La3+ on the A-site with and without charge compensation and for Zr4+ on the B-site. DRS measurements were made over the wavenumber range of 4000–19,000cm−1 and the Pu in all air- and argon-sintered samples was found to be present as Pu4+ while that in samples sintered in N2–3.5%H2 was present as Pu3+. The Pu valence was confirmed for three of the samples using X-ray near-edge absorption spectroscopy (XANES). Pu valences >4+ were not observed in any of the samples.

Microwave sintering of W-15Cu ultrafine composite powder prepared by spray drying & calcining-continuous reduction technology

The effects of microwave sintering and conventional H2 sintering on the microstructure and properties of W-15Cu alloy using ultrafine W-15Cu composite powder fabricated by spray drying & calcining-continuous reduction technology were investigated. In comparison to the conventional H2 sintering processing, microwave sintering to W-15Cu can be achieved at lower sintering temperature and shorter sintering time. Furthermore, higher performances in microwave sintered compacts were obtained, but high microwave sintering temperature or long microwave sintering time could result in coarser microstructures.

Effect of the Si Additions on the Mechanical Behaviour and High-Temperature Performance of Hydrogen Sintered 434L Stainless Steels

Non-commercial, 434L ferritic stainless steel powders prealloyed with 2% of Si were uniaxially compacted at 700 MPa and sintered in 100% H2 atmosphere for 30 min at three different temperatures (1225, 1250 and 1275 °C). For comparison, standard, commercial 434L powders were sintered in the same conditions, and 434L+2%Si powders were also sintered in vacuum. The porosity and grain size of the 9 sintered stainless steels were measured by image analysis methods. Tensile strength and Vickers hardness tests show that the steels with 2% of Si exhibit higher mechanical properties. Oxidation tests were carried out in air at 800 and 900 °C. H2-sintered 434L steels prealloyed with Si exhibit higher weight gains per unit of apparent surface than 434L steels sintered in the same conditions, due to the higher porosity of the former. The sintering atmosphere affects the nature and morphology of the scales observed by XRD and SEM. Vacuum sintering of 434L+2%Si stainless steels promotes the formation of more protective scales and lower mass gains (for identical porosity volume) than H2 sintering.

Acceptor segregation and nonlinear current-voltage characteristics in H2-sintered SrTiO3

The current-voltage characteristics with acceptor segregation at grain boundaries have been investigated in H2-sintered SrTiO3. Al-added SrTiO3 was sintered in H2 and then annealed in air for selective oxidation of grain boundaries. The samples showed nonlinear current-voltage characteristics, and both the breakdown voltage and nonlinearity coefficient increased with Al concentration. An energy dispersive spectroscopy analysis revealed that Al ions were segregated at the grain boundary, suggesting the formation of Schottky barriers at the boundary. The present results thus point toward the possibility of modifying the grain boundary composition in SrTiO3 as well as fabricating effective switching devices by H2 sintering.

Novel Back End-of-Line Process Scheme for Improvement of Negative Bias Temperature Instability Lifetime

A novel back end-of-line (BEOL) process scheme is proposed to improve negative bias temperature instability (NBTI) characteristics through the characterization of the impact of each BEOL process on NBTI of p+ gate metal oxide semiconductor field-effect transistor (PMOSFETs). It is demonstrated that NBTI is strongly dependent on the plasma enhanced nitride (PE-SiN) passivation film and H2 sintering anneal. A new process scheme of N2 annealing instead of H2 annealing prior to PE-SiN deposition is proposed and proven to be highly efficient in improving NBTI without degradation of device performance and n+ gate metal oxide semiconductor (NMOS) hot carrier lifetime.

Quantitative XPS determination of Pt coverage on SrBi2Ta2O9 thin films for ferroelectric memories

AbstractX‐ray photoelectron spectroscopy measurements were conducted for a spin‐coated SrBi2Ta2O9 (SBT) system with sputter‐deposited 5 nm thick Pt film on it. The spectral peaks were analysed quantitatively under several surface structure models. The XPS results also were compared with SEM observation results. The Pt coverage was lowest (∼15%) on a specimen prepared from complexed alkoxide solution followed by a second annealing at 800 °C in O2 and sintering in a hydrogen atmosphere at 400 °C (H2 sintering), whereas it was highest (97%) on a specimen prepared from a sol–gel solution without any other treatment except crystallization. From regression analysis of all the data, the effects of the solution property, second annealing and H2 sintering on Pt coverage were evaluated. It was found that the effect of the second annealing in O2 was very large, whereas that of the H2 sintering does not give a large effect but it certainly causes a decrease in the apparent Pt coverage. Even the difference in solution for spin‐coating has an influence on the Pt coverage, i.e. sol–gel and complex solutions give the highest and lowest values, respectively. It was clarified that the Pt‐coverage had a clear correlation with fabrication factors, such as solution property, second annealing in O2 and H2 sintering. There was an excellent agreement between XPS and SEM results. Copyright © 2001 John Wiley & Sons, Ltd.

Sintering of Translucent Alumina in a Nitrogen–Hydrogen Gas Atmosphere

The sintering of alumina doped with magnesia‐based additives to translucency previously has been possible only in atmospheres of hydrogen or oxygen or under vacuum. This paper reports on the sintering of alumina in N2–H2 atmospheres that contain as little as 2% H2, where optical transparency that is equivalent to that of normal H2 sintering is achieved. In an effort to explain these results, experiments that vary the content of H2 and H2O, as well as the changes in the dynamic atmosphere, have been conducted. The evidence indicates that the final‐stage sintering involves a point‐defect pair mechanism that includes hydrogen interstitials and nitrogen solutes.