Showing papers on "Sintering published in 2002"

The effect of size-dependent nanoparticle energetics on catalyst sintering.

Abstract: Calorimetric measurements of metal adsorption energies directly provide the energies of metal atoms in supported metal nanoparticles. As the metal coverage increases, the particles grow, revealing the dependence of this energy on particle size, which is found to be much stronger than predicted with the usual Gibbs-Thompson relation. It is shown that this knowledge is crucial to accurately model long-term sintering rates of metal nanoparticles in catalysts.

Spark Plasma Sintering of Alumina

Abstract: A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al 2 O 3 powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al 2 O 3 could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21-22 GPa and fracture toughness values of 3.5 ± 0.5 MPa.m 1/2 were found for the compacts containing submicrometer-sized Al 2 O 3 grains.

Spark plasma sintering as advanced PM sintering method

Abstract: A short overview of works on spark plasma sintering (SPS) is given in the present paper. SPS is a newly developed rapid sintering technique with a great potential for achieving fast densification results with minimal grain growth in a short sintering time. It is proven by obtained experimental data that enhanced sinterability of powders subjected to SPS mainly associated with particle surface activation and increased diffusion rates on the contact zones caused by applied pulse current. Application of rapid heating results in bypassing of low temperature regions where surface transport controlled sintering is dominant. This preserves the powder surface area to temperature levels where bulk transport is significant. However, the nature of activation effects, especially in its regards to acceleration of diffusion processes, is not clearly established. A lot of research work reports about the occurrence of plasma during the application of pulse current. However, the appearance of thermal plasma during...

Processing and properties of zirconium diboride-based composites

Abstract: Two zirconium diboride-base composites were produced and characterised The chosen starting compositions were: 55 wt% ZrB2+41 wt%TiB2+4 wt% Ni and 83 wt% ZrB2+13 wt% B4C+4 wt% Ni The microstructure and properties of these composites were compared to those of a monolithic ZrB2+4 wt% Ni material In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering From the powder mixture ZrB2+TiB2, two solid solutions of Zr–Ti–B were obtained In the case of the other mixture, B4C particles were dispersed in the ZrB2 matrix The composite materials have better mechanical properties than those of the monolithic ZrB2 ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature Long term oxidation tests indicated that the ZrB2-based composites, particularly the composite containing B4C as the second phase, were more resistant to oxidation than the monolithic ZrB2 due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB2 matrix material

Mechanical properties of Al2O3/ZrO2 composites

Abstract: In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al2O3/(t-ZrO2+m-ZrO2) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO2 inclusions are close to each other in the Al2O3 matrix, the yttrium ion originally in t-ZrO2 particles can diffuse to nearby m-ZrO2 particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al2O3/(t-ZrO2+m-ZrO2) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al2O3 alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al2O3 alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation.

Direct Selective Laser Sintering of Hard Metal Powders: Experimental Study and Simulation

Abstract: Direct selective laser sintering (SLS) technology can be used to produce 3D hard metal functional parts from commercial available powders. Unlike conventional sintering, it does not require dedicated tools, such as dies. Hence, total production time and cost can be reduced. The large shape freedom offered by such a process makes the use of, for example, sintered carbides components viable in domains where they were not applied before. Successful results have been obtained in the production of sintered carbide or hard metal parts through SLS. The investigation focuses on tungsten carbide-cobalt (WC-Co) powder mixture. This material is characterised by its high mechanical properties and its high wear resistance and is widely used in the field of cutting tools. This paper is devoted to the experimental study and the simulation of direct selective laser sintering of WC-Co hard metal powders.

Pressure-assisted low-temperature sintering of silver paste as an alternative die-attach solution to solder reflow

Abstract: Pressure-assisted low-temperature sintering of silver paste is shown to be a viable alternative to solder reflow as a die-attachment solution. A quasihydrostatic pressure is used to lower the sintering temperature. The effect of parameters such as temperature and pressure are investigated. Characterization of the silver-attached samples shows a significant improvement in electrical conductivity, thermal conductivity and mechanical strength of the joint. Given that silver deforms with little accumulation of inelastic strains, and given the absence of large voids in the attachment layer, it is also expected that the joint to be more resistant to fatigue failure than a solder attached junction. Due to the high melting temperature of silver, this alternative process is also suitable for high temperature packages.

Silicon Nitride Ceramics

Abstract: Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Lanthanum hexaaluminate: novel thermal barrier coatings for gas turbine applications: materials and process development

Abstract: Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure is a promising competitor to yttria partially stabilized zirconia (Y-PSZ) as a thermal barrier coating (TBC), since most zirconia coatings age significantly, including undesired densification at temperatures exceeding 1100 °C. The microstructure of calcined lanthanum hexaaluminate powders and thermally sprayed coatings show a platelet structure. The magnetoplumbite structure is characterized by the highly charged La 3+ cation located in an oxygen position in the hexagonal close-packed structure of oxygen ions. Ion diffusion is strongly suppressed vertical to the crystallographic c -axis, thus hindering sintering densification. In contrast to the oxygen ion conducting zirconia, lanthanum hexaaluminate permits operating temperatures above 1300 °C because of its thermal stability and electrically insulating properties. This study describes the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying (APS) in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses. The phases were characterized by X-ray diffraction (XRD).

Influence of microstructure on the ionic conductivity of yttria-stabilized zirconia electrolyte

Abstract: Yttria-stabilized zirconia (YSZ) electrolytes with diverse microstructures were prepared by using nano-size (Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 powders as precursors through conventional sintering in air. The electrolytes were tested by AC impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity. The intragranular conductivity and intergranular conductivity were correlated with the microstructures of the electrolyte to interpret the transportation of oxygen ions through the electrolyte. The intragranular conductivity was found to be dominated mainly by the relative density while the intergranular conductivity strongly depended on the grain size and grain boundary area of the electrolyte. The sintering temperature and isothermal time dependence of ionic conductivity reached a maximum value of 0.105 S/cm at a sintering temperature of 1350 °C for 4 h and 0.112 S/cm at a holding time of 8 h at 1250 °C when measured at 1000 °C, respectively. Concepts for improving the ionic conductivity of YSZ electrolyte were reviewed.

Influence of hot pressing sintering temperature and time on microstructure and mechanical properties of TiB2 ceramics

Abstract: In this paper, a titanium diboride ceramic was produced by the hot pressing sintering method. The effects of hot pressing parameters on the TiB 2 ceramic microstructure and mechanical properties were studied. The bending strength and fracture toughness were measured by three point bending testing and single edge notched bending tests (SENB), respectively. The microstructure features of the TiB 2 sintered material were revealed by means of SEM and TEM. The results show that the TiB 2 grain size increases quickly with the increasing temperature and time during hot pressing sintering. The density and the TiB 2 grain size have a great influence on the mechanical properties. The bending strength decreases with increasing TiB 2 grain size, whilst the fracture toughness increases.

Promising ceramic laser material: Highly transparent Nd3+:Lu2O3 ceramic

Abstract: A solid-state laser material based on highly transparent cubic Nd3+:Lu2O3 ceramic was developed using nanocrystalline technology and a nonpress vacuum sintering method. Spectroscopic properties of this ceramic laser material were investigated. At room temperature under single-laser diode pumping, efficient continuous wave laser oscillation was demonstrated at two wavelengths of the 4F3/2→4I11/2 channel. The potential application of such a laser material was also discussed.

Study of temperature field in spark plasma sintering

Abstract: The paper presents a brief derivation of the temperature distribution in the sample and the die in spark plasma sintering. It aims to point out that temperature difference is inevitable in the samples of SPS system. Under certain conditions, the difference may reach a few hundreds of degrees of centigrade. Experiments also proved the existence of such a difference, which is larger than the one under steady-state temperature distribution.

Implications of heat treatment on the properties of a magnetic iron oxide–titanium dioxide photocatalyst

Abstract: Magnetic iron oxide–titania photocatalysts (Fe 3 O 4 –TiO 2 ) were prepared using a coating technique in which the photoactive titanium dioxide was deposited onto the surface of a magnetic iron oxide core. These particles had a core–shell structure. The prepared particles were heat treated at high temperature in order to transform the amorphous titanium dioxide into a photoactive crystalline phase. The heat treatment temperature and duration were varied, and the correlation between the heat treatment and the observed photoactivities was investigated. An increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatment duration, led to a decrease in the activities of the catalyst particles. A decrease in surface area due to sintering, along with the diffusion of Fe ions into the titanium dioxide coating are seen as contributing factors to the decline in photoactivity which accompanied an increase in the heat treatment. Differential scanning calorimetry analysis (DSC) results confirmed that the presence of the iron oxide core did not have an effect on the phase transformation of titania under the experimental conditions investigated. In this study we also present surface charge measurements which show that the surface of the particles became more positive as the heat treatment was increased. This is an indication of changing surface properties as heat treatment is applied. For single-phase TiO 2 powders, this is postulated to be due to a decrease in the surface hydroxyl (OH) groups and/or residual organics (OR) groups. For the Fe 3 O 4 –TiO 2 powders, in addition to the loss of OH and OR groups, the diffusion of the Fe into the titania shell is postulated to also play a role in the changing surface properties with applied heat treatment. Experiments aimed at reducing the duration of the heat treatment revealed that a heat treatment duration of 20 min at 450 °C was sufficient to transform amorphous titanium dioxide into a photoactive crystalline phase. This does not only minimise loss of surface hydroxyl groups but it also has the potential to limit the oxidation of the magnetic core, which occurs due to the porosity of the coating. This has practical implications in terms of separating the magnetic particles from the treated waste waters under the application of an external magnetic field. It also presents an opportunity to produce photoactive composite particles while limiting the interactions between the core and the shell during the heat treatment.

On the delamination of thermal barrier coatings in a thermal gradient

Abstract: Thermal barrier coating (TBC) systems are susceptible to delamination failures in the presence of a large thermal gradient. These failures, which occur within the TBC layer, are very different in character from those associated with the thermally grown oxide. Three possible causes of internal delamination are analyzed. In all cases, the thermomechanical properties of the TBC are allowed to vary because of sintering. (a) One mechanism relates to exfoliation of an internal separation in the TBC due to a through thickness heat flux. (b) Another is concerned with edge-related delamination within a thermal gradient. (c) The third is a consequence of sintering-induced stresses. The results of these analyses, when used in combination with available properties for the TBC, strongly suggest that the second mechanism (b) predominates in all reasonable scenarios. Consequences for the avoidance of this failure mode are discussed.

Oxidation bonding of porous silicon carbide ceramics

Abstract: A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al2O3 and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al2O3 content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al2O3, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.

First-principles simulations of metal-ceramic interface adhesion: Co/WC versus Co/TiC

Abstract: With the purpose to understand the fundamental difference between two industrially important classes of hard materials, WC-Co cemented carbides and TiC-Co cermets, we do a comparative study of Co/WC and Co/TiC interface adhesion. Using first-principles density-functional plane-wave pseudopotential calculations, we analyze the energetics and electronic structure of the Co/WC interface, and compare with previously reported results for Co/TiC. Values for the work of separation and the interface energy are provided for a set of model Co(001)/WC(001) interfaces with fcc Co and cubic WC, which is a relevant case in sintering of Ti(W)C-Co cermets. The known fact of better wetting for Co on WC than on TiC is confirmed and explained in terms of the electronic structure. Both Co/WC and Co/TiC are characterized by strong covalent Co-C interface bonds. The stronger adhesion in the Co/WC case originates from a larger contribution of the Co-W metal-metal bonding rather than from the Co-C bonds.

Mechanisms of ceramic coating deposition in solution-precursor plasma spray

Abstract: The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y2O3-stabilized ZrO2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.

On the formation of very large WC crystals during sintering of ultrafine WC–Co alloys

Abstract: During liquid phase sintering of very fine-grained WC powders (average particle size in the range 100–200 nm) very large WC crystals form with sizes of > 20 μm Such grains typically exhibit the shape of plates On tracing back the formation of these crystals during sintering it was observed that already on heating to 1150 °C small platelets were present in the still porous sintering body At 1250 °C, ie before eutectic liquid formed, they had grown to a plate length of up to 7 μm with aspect ratios higher than 1:10 This early rapid WC crystal growth was observed preferentially in the outer areas of the sintering body where the sample is in closer contact with the sintering atmosphere SEM imaging and TEM studies reveal the presence of twin boundaries in the plate-like WC crystals, rendering the possibility of a defect-assisted growth of the WC Intermediate formation of η-carbides (M12C, M6C) and their subsequent transformation might be an explanation for this early occurrence of extraordinarily large grains