Showing papers on "Sintering published in 2009"

Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide - A review

Abstract: Nanocrystalline WC–Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC–Co (average WC grain size

Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys

Abstract: We report a melt spinning technique followed by a quick spark plasma sintering procedure to fabricate high-performance p-type Bi0.52Sb1.48Te3 bulk material with unique microstructures. The microstructures consist of nanocrystalline domains embedded in amorphous matrix and 5–15 nm nanocrystals with coherent grain boundary. The significantly reduced thermal conductivity leads to a state-of-the-art dimensionless figure of merit ZT∼1.56 at 300 K, more than 50% improvement of that of the commercial Bi2Te3 ingot materials.

High Total Proton Conductivity in Large-Grained Yttrium-Doped Barium Zirconate

Abstract: Barium zirconate has attracted particular attention among candidate proton conducting electrolyte materials for fuel cells and other electrochemical applications because of its chemical stability, mechanical robustness, and high bulk proton conductivity. Development of electrochemical devices based on this material, however, has been hampered by the high resistance of grain boundaries, and, due to limited grain growth during sintering, the high number density of such boundaries. Here, we demonstrate a fabrication protocol based on the sol−gel synthesis of nanocrystalline precursor materials and reactive sintering that results in large-grained, polycrystalline BaZr_(0.8)Y_(0.2O3−δ) of total high conductivity, 1 × 10^(−2) Scm^(−1) at 450 °C. The detrimental role of grain boundaries in these materials is confirmed via a comparison of the conductivities of polycrystalline samples with different grain sizes. Specifically, two samples with grain sizes differing by a factor of 2.3 display essentially identical grain interior conductivities, whereas the total grain boundary conductivities differ by a factor of 2.5−3.2, depending on the temperature (with the larger-grained material displaying higher conductivity).

Intense pulsed light sintering of copper nanoink for printed electronics

Abstract: An intense pulsed light (IPL) from a xenon flash lamp was used to sinter copper nanoink printed on low-temperature polymer substrates at room temperature in ambient condition. The IPL can sinter the copper nanoink without damaging the polymer substrates in extremely short time (2 ms). The microstructure of the sintered copper film was investigated using X-ray powder diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), X-ray micro tomography, and atomic force microscopy (AFM). The sintered copper film has a grainy structure with neck-like junctions. The resulting resistivity was 5 μΩ cm of electrical resistivity which is only 3 times as high as that of bulk copper. The IPL sintering technique allows copper nanoparticles to be used in inkjet printing on low-temperature substrates such as polymers in ambient conditions.

Generation of nanoparticles by spark discharge

Abstract: The production of nanoparticles by microsecond spark discharge evaporation in inert gas is studied systematically applying transmission electron microscopy, mobility analysis and BET surface area measurement. The method of spark discharge is of special interest, because it is continuous, clean, extremely flexible with respect to material, and scale-up is possible. The particle size distributions are narrow and the mean primary particle size can be controlled via the energy per spark. Separated, unagglomerated particles, 3–12 nm in size, or agglomerates can be obtained depending on the flow rate. The nanoparticulate mass produced is typically 5 g/kWh. A formula is given, which estimates the mass production rate via thermal conductivity, evaporation enthalpy and the boiling point of the material used. We showed that with gas purified at the spot, the method produced gold particles that were so clean that sintering of agglomerated particles occurred at room temperature. The influence of a number of parameters on the primary particle size and mass production rate was studied and qualitatively understood with a model of Lehtinen and Zachariah (J Aerosol Sci 33:357–368, 2002). Surprisingly high charging probabilities for one polarity were obtained. Spark generation is therefore of special interest for producing monodisperse aerosols or particles of uniform size via electrical mobility analysis. Qualitative observations in the present study include the phenomenon of material exchange between the electrodes by the spark, which opens the possibility of producing arbitrary mixtures of materials on a nanoscale. If spark generation of nanoparticles is performed in a standing or almost standing gas, an aerogel of a web-like structure forms between surfaces of different electrical potential.

AlON: A brief history of its emergence and evolution

Abstract: In the early 1970s in Japan, the United States and France it was found that additions of nitrogen into aluminum oxide resulted in new spinel-like phases. At about the same time there was much increased interest in oxynitrides, stimulated by Professor K. Jack in the UK and Y. Oyama in Japan. Following these activities a major research program in this area was initiated at the Army Materials and Mechanics Research Center in Watertown, Massachusetts in 1974. These efforts resulted in the first complete Al2O3–AlN phase equilibrium diagram and a process to reactively sinter to nearly full density, translucent aluminum oxynitride spinel ceramic, which was named AlON. Subsequently, the Raytheon Company further developed AlON into a highly transparent material (ALON™) with many applications including transparent armor and EM domes and windows, among others—the technology was recently transferred to the Surmet Corporation. This paper will review the early history, phase equilibrium, crystal chemistry, and properties of this material, along with more recent work in our laboratory on transient liquid phase sintering and new data on lattice parameter measurements. In addition, recent results of collaborative work on AlON's dynamic mechanical properties using plate impact, Kolsky bar and edge-on impact (EoI) experimental techniques, including preliminary modeling at the microstructural scale of AlON in the EoI test, will be presented.

Sintering and crystallisation of 45S5 Bioglass® powder

Abstract: The sintering process of 45S5 Bioglass ® powder (mean particle size ® crystallises at temperatures between 600 and 750 °C. The characteristic crystalline phases were identified by Fourier Transformed Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). The crystallisation kinetics was studied by DTA, using a non-isothermal method. The Kissinger plot for Bioglass ® powder heated at different heating rates between 5 and 30 °C/min yielded an activation energy of 316 kJ/mol. The average value of Avrami parameter determined using the Augis–Bennett method was 0.95 ± 0.10, confirming a surface crystallisation mechanism. After sintering at 1050 °C for 140 min, the main crystalline phase was found to be Na 2 Ca 2 Si 3 O 9 . The results of this work are useful for the design of the sintering/crystallisation heat treatment of Bioglass ® powder which is used for fabricating tissue engineering scaffolds with varying degree of bioactivity.

Microwave flash sintering of inkjet-printed silver tracks on polymer substrates

Abstract: Microwave flash sintering of inkjet printed colloidal silver dispersions on thin polymer substrates was studied as a function of the antenna area and initial resistance. The presence of conductive antennae promotes nanoparticle sintering in predried ink lines. For dried nanoparticle inks connected to antennae, sintering times of 1 s are sufficient to obtain pronounced nanoparticle sintering and conductivities between 10 and 34% compared to bulk silver. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.

Microstructural and mechanical characterization of Al–MWCNT composites produced by mechanical milling

Abstract: Novel Al-based nanocomposites reinforced with multi-walled carbon nanotubes were produced by mechanical milling. Next, pressure-less sintering at 823 K under vacuum and hot extrusion at 773 K were carried out. The interface between Al matrix and the multi-walled carbon nanotubes was examined using transmission electron microscopy. The values of yield strength (σy), maximum strength (σmax) and microhardness Vickers (HVN) of the composites were evaluated and reported as a function of carbon nanotubes content. The concentration of multi-walled carbon nanotubes has an important effect on the mechanical properties of the nanocomposite. Formation of aluminum carbide in the nanocomposites was observed. Possible strengthening mechanisms are presented and discussed.

A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)

Abstract: A review of the densification mechanisms and the microstructural development for transparent spinel made by free sintering and by hot pressing is given. The paper is divided into two main parts. The first part considers spinel without any sintering additives because there still is some controversy concerning the role of cation stoichiometry on sintering and grain growth. The second part discusses the role of the classic sintering aid, LiF, in processing transparent spinel. LiF is shown to have multiple behaviors: (1) it initially wets spinel and forms a liquid phase at relatively low temperatures, which affects early-stage densification and also grain growth; (2) upon cooling from intermediate temperatures, or even from higher temperatures if microstructure evolution (e.g., formation of closed porosity) prevents volatization, the LiF-containing liquid dewets and resides in isolated pockets; (3) LiF alters the cation stoichiometry, thereby enhancing diffusion via an increase in the concentration of oxygen vacancies; this affects both the densification rate and grain growth; and (4) it reacts with impurities in the system, thereby acting as a cleanser. For the production of transparent spinel, it is critical that LiF or associated reaction products not be retained as a secondary phase.

Size-Dependent Kinetic Enhancement in Hydrogen Absorption and Desorption of the Li−Mg−N−H System

Abstract: High operating temperature and slow kinetics retard the practical applications of the Li-Mg-N-H system for hydrogen storage To alleviate these problems, a first attempt was carried out by synthesizing Li(2)MgN(2)H(2) through sintering a mixture of Mg(NH(2))(2)-2LiNH(2) and investigating its size-dependent hydrogen storage performance A dramatically enhanced kinetics for hydrogen absorption/desorption was achieved with a reduction in the particle size For the dehydrogenation reaction, a three-dimensional diffusion-controlled kinetic mechanism was identified for the first time by analyzing isothermal hydrogen desorption curves with a linear plot method The experimental improvement and mechanistic understanding on the dehydrogenation kinetics of the Li-Mg-N-H system shed light on how to further decrease the operating temperature and enhance the hydrogen absorption/desorption rate of the amide/hydride combined materials

Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process

Abstract: The paper describes the fabrication process of high temperature oxides, such as Y 2 O 3 , HfO 2 and La 2 O 3 , dispersed tungsten composites by spark plasma sintering. The oxide contents varied from 0 to 5 wt% and sintering was conducted for 3 min at 1700 °C. Among three kinds of oxides, Y 2 O 3 is the most efficient element to consolidate W powder. As dispersed up to 5 wt% Y 2 O 3 into the matrix, the relative density of the W composite is increased up to nearly 100% of theoretical value. In order to analyze the effect of Y 2 O 3 particles on the densification of W powders, the microstructure of W–Y 2 O 3 composite is observed using the transmission electron microscopy. By this experiment, it is found that dark phases, which had been known as Y 2 O 3 phase, are composed of W, Y and O. Therefore, during sintering, W atoms move through Y 2 O 3 phases as well as W grain boundaries, thereby W and Y 2 O 3 are soluble, and so sinterability of W is enhanced. The hardness of the composite is increased from 350 to 510 kg/mm 2 with increasing Y 2 O 3 contents since the relative density is increased and the grain size is reduced from 20 to 4 μm. However, in case of HfO 2 and La 2 O 3 , the hardness of the composites is decreased even though the grain size is reduced because of their lower relative densities.

Spark Plasma Sintering as a Useful Technique to the Nanostructuration of Piezo-Ferroelectric Materials**

Abstract: Piezoelectric andferroelectric ceramicmaterialsarematureand ubiquitous materials for advanced technology. Theseceramics are the active elements in a range of piezoelectricdevices and perform functions such as sensing and actuation.The performance of these materials is closely related to theirmicrostructures and, for this reason, to the ways they havebeen processed. The first step in obtaining high-performanceceramics with a homogeneous microstructure and controlledgrain size that meet the requirements of industry isto preparepowderswithcontrolledstoichiometryandsmallparticlesize.However, even if a small-size powder is used, conventionalsintering is often unable to provide dense, very fine-grainedceramics, due to the high temperatures still required fordensification,andthefactthatthelowestgrainsizeachievableby classical techniques remains about 0.5mm or even higher,depending on the system. To solve this problem, the masstransport during the sintering step must be enhanced, sincethe temperature and time needed for consolidation must bereduced in order to achieve smaller grain sizes. Among themethods reported for activation of the mass transportduring the sintering process, the application of an electricalcurrent through the sample during heating represents apromising technique for rapid densification of ceramics atrelatively low temperatures. The most novel and increas-ingly used method is spark plasma sintering, which hasclear advantages over conventional sintering methods,making it possible to sinter nanometric powders to nearfull densification with little grain growth. This has becomeincreasingly important recently, with the miniaturization ofelectronic devices and the need to investigate size effects onthe properties in the sub-micrometer range and approach-ing the nanometer scale ( 100nm).In many material applications there is a need for densematerials, often being very close to their theoretical density.Unfortunately, taking account the fragility and refractoryproperties of ceramic materials and several difficultiesinherent in the sintering process, the compaction withoutany additives becomes a real challenge from both practicaland theoretical aspects.The final electromechanical properties of piezoelectricceramic components greatly depend upon the history of theceramic. Each step in the preparation of the material has to becarefully monitored and controlled toobtain the best product.The primary steps of the preparation of the ceramic materialaresynthesisoftheprecursor,fabricationofgreenbodiesand,last but not least, sintering of the pellet to achieve properdensification. This third step, which follows powder prepara-tion, consists of thermal treatment with the aim of strengthen-ing the desired piece. It occurs via bonding of the compactgrains without melting. Such ‘‘welding’’ may be followed

Nickel–Zirconia Anode Degradation and Triple Phase Boundary Quantification from Microstructural Analysis

Abstract: Microstructural evolution of anode supported solid oxide fuel cells (SOFC) during medium-term stack testing has been characterized by scanning electron microscopy (SEM). Low acceleration voltage SEM imaging is used to separate the three anode phases (nickel, yttria-stabilized zirconia and porosity). Microstructural quantification is obtained using a software code that yields phase proportion, particle size, particle size distribution and a direct measure of triple phase boundary (TPB) density. In addition, an anode degradation model is proposed. The model describes the gradual degradation of the anode due to nickel particle sintering and the concomitant loss of TPB. Fundamental operational and structural parameters of the anode can be used to estimate the TPB length change with time from the degradation rate. The combination of experimental results and modeling allows separating the degradation due to sintering of nickel particles from total stack degradation. Anode degradation occurs principally during the first 500 operating hours. For stack tests carried out over more than 1000 h, anode degradation was responsible for 18 % to 41 % of the total degradation depending on initial microstructure.

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Abstract: Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

Abstract: This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO2 doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO2 levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO2, especially at higher temperatures, because the SiO2 solubility limit is exceeded.