Showing papers on "Sintering published in 2011"

Electric Current Activation of Sintering: A Review of the Pulsed Electric Current Sintering Process

Abstract: The phenomenal increase during the past decade in research utilizing pulsed electric current to activate sintering is attributed generally to the intrinsic advantages of the method relative to conventional sintering methods and to the observations of the enhanced properties of materials consolidated by this method. This review focuses on the fundamental aspects of the process, discussing the reported observations and simulation studies in terms of the basic aspects of the process and identifying the intrinsic benefits of the use of the parameters of current (and pulsing), pressure, and heating rate.

Field assisted and flash sintering of alumina and its relationship to conductivity and MgO-doping

Abstract: We show that flash-sintering in MgO-doped alumina is accompanied by a sharp increase in electrical conductivity. Experiments that measure conductivity in fully dense specimens, prepared by conventional sintering, prove that this is not a cause-and-effect relationship, but instead that the concomitant increase in the sintering rate and the conductivity share a common mechanism. The underlying mechanism, however, is mystifying since electrical conductivity is controlled by the transport of the fastest moving charged species, while sintering, which requires molecular transport or chemical diffusion, is limited by the slow moving charged species. Joule heating of the specimen during flash sintering cannot account for the anomalously high sintering rates. The sintering behavior of MgO-doped alumina is compared to that of nominally pure-alumina: the differences provide insight into the underlying mechanism for flash-sintering. We show that the pre-exponential in the Arrhenius equation for conductivity is enhanced in the non-linear regime, while the activation energy remains unchanged. The nucleation of Frenkel pairs is proposed as a mechanism to explain the coupling between flash-sintering and the non-linear increase in the conductivity.

Conductive Inks with a “Built-In” Mechanism That Enables Sintering at Room Temperature

Abstract: At present there is no metallic ink that enables formation of conductive patterns at room temperature by a single printing step. Printing conductive features by metallic nanoparticle-based inks must be followed by sintering while heating to elevated temperatures, thus preventing their utilization on most plastic substrates used in plastic electronics. In this report we present a new silver nanoparticle-based conductive ink, having a built-in sintering mechanism, which is triggered during drying of the printed pattern. The nanoparticles that are stabilized by a polymer undergo self-sintering spontaneously, due to the presence of a destabilizing agent, which comes into action only during drying of the printed pattern. The destabilizing agent, which contains Cl− ions, causes detachment of the anchoring groups of the stabilizer from the nanoparticles’ surface and thus enables their coalescence and sintering. It was found that the new metallic ink leads to very high conductivities, by a single printing step: u...

Enhanced piezoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free ceramics by optimizing calcination and sintering temperature

Abstract: Lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCTZ) piezoelectric ceramics were prepared by conventional oxide-mixed method at various calcination and sintering temperatures. Both calcination and sintering temperatures have a significant effect on the density and grain size, which are closely related with piezoelectric and other properties of ceramics. The calcination temperature has a great influence on the grain boundary, which also plays an important role in piezoelectric properties. With increased calcination and sintering temperature, the ferroelectric and piezoelectric properties have enhanced significantly. The BCTZ ceramics calcined at 1300 °C and sintered at 1540 °C exhibit optimal electrical properties: d33 = 650 pC/N, d31 = 74 pC/N, kp = 0.53, kt = 0.38, k31 = 0.309, s 11 E = 14.0 × 10 − 12 m 2 /N , ɛr = 4500, Pr = 11.69 μC/cm2, which is a promising lead-free piezoelectric candidate.

Reactive Sintering of Copper Nanoparticles Using Intense Pulsed Light for Printed Electronics

Abstract: Most commercial copper nanoparticles are covered with an oxide shell and cannot be sintered into conducting lines/films by conventional thermal sintering. To address this issue, past efforts have utilized complex reduction schemes and sophisticated chambers to prevent oxidation, thereby rendering the process cost ineffective. To alleviate these problems, we demonstrate a reactive sintering process using intense pulsed light (IPL) in the present study. The IPL process successfully removed the oxide shells of copper nanoparticles, leaving a conductive, pure copper film in a short period of time (2 ms) under ambient conditions. The in situ copper oxide reduction mechanism was studied using several different experiments and analyses. We observed instant copper oxide reduction and sintering through poly(N-vinylpyrrolidone) functionalization of copper nanoparticles, followed by IPL irradiation. This phenomenon may be explained by oxide reduction either via an intermediate acid created by ultraviolet (UV) light irradiation or by hydroxyl (-OH) end groups, which act like long-chain alcohol reductants.

Relating rates of catalyst sintering to the disappearance of individual nanoparticles during Ostwald ripening.

Abstract: Sintering of nanoparticles (NPs) of Ni supported on MgAl2O4 was monitored in situ using transmission electron microscopy (TEM) during exposure to an equimolar mixture of H2 and H2O at a pressure of 3.6 mbar at 750 °C, conditions relevant to methane steam reforming. The TEM images revealed an increase in the mean particle size due to disappearance of smaller, immobile NPs and the resultant growth of the larger NPs. A new approach for predicting the long-term sintering of NPs is presented wherein microscopic observations of the ripening of individual NPs (over a span of a few seconds) are used to extract energetic parameters that allow a description of the collective behavior of the entire population of NPs (over several tens of minutes).

Flash‐Sintering of Cubic Yttria‐Stabilized Zirconia at 750°C for Possible Use in SOFC Manufacturing

Abstract: We show that cubic 8 mol% yttria (8YSZ) can be sintered at 750°C with the application of DC electrical fields; in comparison the lowest sintering temperature for 3YSZ was 850°C. Furthermore, cubic zirconia exhibits the onset of flash sintering at 30 V/ cm, whereas 3YSZ begins flash sintering at 60 V/cm. However, the volume specific power dissipation for the onset of flash sintering remains similar at ∼10 mW/mm 3 . The easier sintering of 8YSZ is ascribed to its higher ionic conductivity.

Effects of sintering temperature and particle size on the translucency of zirconium dioxide dental ceramic.

Abstract: The aim of this study was to evaluate the effects of sintering temperature and particle size on the translucency of yttrium stabilized tetragonal zirconia polycrystals (Y-TZP) dental ceramic. Eighty disc-shaped and cylindrical specimens were fabricated from zirconia powers of particle size 40 and 90 nm. These specimens were sintered densely at the final sintering temperature 1350, 1400, 1450 and 1500°C, respectively. The visible light transmittance, sintered density and microstructure of the sintered block were examined. The results showed that the sintered densities and transmittances increased with the temperature from 1,350 to 1,500°C. Y-TZP could gain nearly full density and about 17–18% transmittance at the final sintering temperature of 1,450–1,500°C. The 40-nm powders had higher sintered density and transmittance than the 90-nm. The translucency of Y-TZP dental ceramic could be improved by controlling the final sintering temperature and primary particle size.

Alumina Over-coating on Pd Nanoparticle Catalysts by Atomic Layer Deposition: Enhanced Stability and Reactivity

Abstract: ALD Alumina was utilized as a protective layer to inhibit the sintering of supported nano-sized ALD Pd catalysts in the methanol decomposition reaction carried out at elevated temperatures. The protective ALD alumina layers were synthesized on Pd nanoparticles (1-2 nm) supported on high surface area alumina substrates. Up to a certain over-coat thickness, the alumina protective layers preserved or even slightly enhanced the catalytic activity and prevented sintering of the Pd nanoparticles up to 500 C.

Sintering of Inkjet-Printed Silver Nanoparticles at Room Temperature Using Intense Pulsed Light

Abstract: Intense pulsed light (IPL) was used to sinter printed silver nanoink patterns consisting of 20-nm to 40-nm silver nanoparticles dispersed in diethylene glycol (DEG). Three consecutive pulses at 50 J/cm2 in less than 30 ms was sufficient to adequately sinter silver nanoink patterns for printed electronics without degradation of the substrates. This is an exceptionally short time compared with that of the conventional thermal sintering process. On the sintered conductive silver patterns, neck-like junctions between nanoparticles were observed using scanning electron microscopy (SEM). The melting temperature, 194.1°C, of silver nanoparticles was found using differential scanning calorimetry (DSC). Also, x-ray diffraction (XRD) was used to find the grain size of the printed silver nanoink patterns. The IPL-sintered silver pattern had a grain size of 86.3 ± 7.2 nm. From this work, it was found that the IPL-sintered silver pattern had a low resistivity of 49 ± 3 nΩ m, which is low enough to be used for printed electronics.

Silver Nanoparticle Paste for Low-Temperature Bonding of Copper

Abstract: Silver nanoparticle (NP) paste was fabricated and used to bond copper wire to copper foil at low temperatures down to 160°C. The silver NP paste was developed by increasing the concentration of 50 nm silver NP sol from 0.001 vol.% to 0.1 vol.% by centrifugation. The 0.001 vol.% silver NP sol was fabricated in water by reducing silver nitrate (AgNO3) using sodium citrate dihydrate (Na3C6H5O7·2H2O). The bond was formed by solid-state sintering among the individual silver NPs and solid-state bonding of these silver NPs onto both copper wire and foil. Metallurgical bonds between silver NPs and copper were confirmed by transmission electron microscopy (TEM). The silver NPs were coated with an organic shell to prevent sintering at room temperature (RT). It was found that the organic shell decomposed at 160°C, the lowest temperature at which a bond could be formed. Shear tests showed that the joint strength increased as the bonding temperature increased, due to enhanced sintering of silver NPs at higher temperatures. Unlike low-temperature soldering techniques, bonds formed by our method have been proved to withstand temperatures above the bonding temperature.

Effect of SiO2 on Densification and Microstructure Development in Nd:YAG Transparent Ceramics

Abstract: This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3−3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035–0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si4+ substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°–1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd3+ concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained ( 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt% SiO2-doped Nd:YAG when sintered at 1600°C for 8 h.

Sintering Rate and Mechanism of TiO2 Nanoparticles by Molecular Dynamics

Abstract: Titania is the dominant white pigment and photocatalytic material, a key component of sunscreens and has promising applications in photovoltaics and sensors of organic vapors. The growth of TiO2 nanoparticles by sintering, the critical step during their large scale manufacture and processing, is elucidated and quantified by molecular dynamics. Highly mobile ions from the particle surface fill in the initially concave space between nanoparticles (surface diffusion) forming the final, fully-coalesced, spherical-like particle with minimal displacement of inner Ti and O ions (grain boundary diffusion) revealing also the significance and sequence of these two sintering mechanisms of TiO2. A sintering rate for TiO2 nanoparticles is extracted that is much faster than that in the literature but nicely converges to it for increasing particle size.

Slurry-based selective laser sintering of polymer-coated ceramic powders to fabricate high strength alumina parts

Abstract: Instead of conventional powder-based selective laser sintering, a novel slurry-based process to fabricate high strength ceramic parts is proposed. A slurry which was composed of alumina powder coated with water-insoluble semi-crystalline polyvinyl alcohol (PVA) as a structure material, water-soluble PVA as an organic binder, ammonium polymethacrylate (DARVAN C–N) as a dispersant, and deionized water as a solution, could be prepared with colloidal processing. A rigid green block could be built with a self-made rapid prototyping apparatus. The polymers contained in the scanned region were melted to connect the alumina powders, but transformed to be water-insoluble. However, the un-scanned region remained water-soluble. Due to dissolving of the polymers in water, the un-scanned region could collapse to obtain the green part. After binder removing and sintering, an alumina ceramic part could be obtained. An average flexural strength of 363.5 MPa and a relative density of 98% were achieved.

The vitreous phase of porcelain stoneware: Composition, evolution during sintering and physical properties

Abstract: High performance ceramic tiles (ISO 13006 Group BIa, water absorption < 05%) are composed of porcelain stoneware: a compact and light-colored material containing a large amount of vitreous phase, which governs sintering behavior and affects geometrical, mechanical and functional properties of finished products Ninety-three porcelain stoneware tiles were analyzed for bulk chemistry (XRF) and quantitative phase composition (XRD-Rietveld) in order to calculate both chemical composition and physical properties of the vitreous phase; their evolution during the sintering process was followed by lab simulation of industrial firing and quenching in the 1100–1200 °C range Porcelain stoneware tiles contain 40% to 75% wt of a vitreous phase having a quartz-feldspathic composition with an alumina excess coming from clay minerals breakdown Vitreous phase formation by feldspars melting is a fast phenomenon, starting from ~ 1050 °C, that is mostly accomplished before viscous flow begins densification, which goes on involving a slow-rate quartz dissolution Sintering kinetics is expected to be controlled by viscosity and surface tension of the liquid phase, which appear to depend essentially on the alumina content (hence on the mullite stability) along with the Na/K and Na/Ca ratios At any rate, a microstructural control on sintering is claimed as the rheological behavior of the viscous phase (ie the matrix containing both liquid phase and fine-grained crystals of quartz and mullite) is substantially different from that of the liquid phase only

Plasma synthesis of nanostructures for improved thermoelectric properties

Abstract: The utilization of silicon-based materials for thermoelectrics is studied with respect to the synthesis and processing of doped silicon nanoparticles from gas phase plasma synthesis. It is found that plasma synthesis enables the formation of spherical, highly crystalline and soft-agglomerated materials. We discuss the requirements for the formation of dense sintered bodies, while keeping the crystallite size small. Small particles a few tens of nanometres and below that are easily achievable from plasma synthesis, and a weak surface oxidation, both lead to a pronounced sinter activity about 350 K below the temperature usually needed for the successful densification of silicon. The thermoelectric properties of our sintered materials are comparable to the best results found for nanocrystalline silicon prepared by methods other than plasma synthesis.