Showing papers on "Sintering published in 2006"

Ink‐jet Printing and Microwave Sintering of Conductive Silver Tracks

Abstract: Printing techniques, such as ink-jet printing, are interesting alternatives to conventional photolithography for the production of electronic devices. The advantages of printing include the ease of mass production, low cost, and flexibility. Compared to other printing techniques (e.g., screen printing), ink-jet printing does not offer the same production speed. However, the unprecedented flexibility of ink-jet printing makes it very well suited for rapid prototyping applications. In addition, it allows the use of inviscid fluids, such as dilute polymer solutions or suspensions without added binders. A typical application involves the ink-jet printing of conductive tracks, for example, by using inks based on (in)organic silver or copper precursors. The precursor is reduced to the corresponding metal via a post-printing thermal annealing step. In most cases, however, the ink is a dispersion of noble-metal nanoparticles, usually silver or gold. A sintering step is necessary to render the tracks conductive. The use of nanoparticles reduces the sintering temperature due to their high surface to volume ratio. In the past, two different techniques have been used to sinter printed nanoparticle structures. Conventional radiation– conduction–convection heating is the most commonly used method, wherein the sintering temperatures are typically above 200 °C. Therefore many potentially interesting substrate materials, such as thermoplastic polymers or paper, cannot be used. In fact, one of the very few, if not the only organic substrate that can be used is (expensive) polyimide (PI). The long sintering times required—usually 60 min or more— also imply that the technique is not feasible for fast industrial production. As an alternative, a laser sintering method was developed. The laser follows the conductive tracks and sinters these selectively, without affecting the substrate. However, this method is costly and complex from a technical point of view. Thus, there is a clear need for a fast, simple, and costeffective technique that would allow the sintering of the printed structures by the selective heating of only the printed components. Microwave heating fulfills these requirements. Microwave heating is widely used for the sintering of dielectric materials and in synthetic chemistry. It offers advantages such as uniform, fast, and volumetric heating. Microwave radiation is absorbed due to coupling with charge carriers or rotating dipoles. The absorbed power per unit volume P is,

Size Effects on Carbon-Free LiFePO4 Powders The Key to Superior Energy Density

Abstract: C-free LiFePO 4 crystalline powders were prepared by a synthesis method based on direct precipitation under atmospheric pressure. The particle size distribution is extremely narrow, centered on ca. 140 nm. A soft thermal treatment, typically at 500°C for 3 h under slight reducing conditions was shown to be necessary to obtain satisfactory electrochemical Li + deinsertion/insertion properties. This thermal treatment does not lead to grain growth or sintering of the particles, and does not alter the surface of the particles. The electrochemical performances of the powders obtained by this synthesis method are excellent, in terms of specific capacity (147 mAh g -1 at 5C-rate) as well as in terms of cyclability (no significant capacity fade after more than 400 cycles), without the need of carbon coating.

Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features

Abstract: In the present work, the densification and microstructural evolution during direct laser sintering of metal powders were studied. Various ferrous powders including Fe, Fe–C, Fe–Cu, Fe–C–Cu–P, 316L stainless steel, and M2 high-speed steel were used. The empirical sintering rate data was related to the energy input of the laser beam according to the first order kinetics equation to establish a simple sintering model. The equation calculates the densification of metal powders during direct laser sintering process as a function of operating parameters including laser power, scan rate, layer thickness and scan line spacing. It was found that when melting/solidification approach is the mechanism of sintering, the densification of metals powders ( D ) can be expressed as an exponential function of laser specific energy input ( ψ ) as ln(1 − D ) = − Kψ . The coefficient K is designated as “densification coefficient”; a material dependent parameter that varies with chemical composition, powder particle size, and oxygen content of the powder material. The mechanism of particle bonding and microstructural features of the laser sintered powders are addressed.

Ferroelectric and Piezoelectric Properties of Fine‐Grained Na0.5K0.5NbO3 Lead‐Free Piezoelectric Ceramics Prepared by Spark Plasma Sintering

Abstract: Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO 3 and KNbO 3 have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na 0.5 K 0.5 NbO 3 ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920 C, the density of the Na 0.5 K 0.5 NbO 3 solid solution ceramics was raised to 4.47 g/cm 3 (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na 0.5 K 0.5 NbO 3 ceramics with submicron grains. The crystal phase of the Na 0.5 K 0.5 NbO 3 has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter (d 33 ) of the Na 0.5 K 0.5 NbO 3 ceramics reached 148 pC/N.

Sintering and Electrical Properties of Lead-Free Na0.5K0.5NbO3 Piezoelectric Ceramics

Abstract: Lead-free Na0.5K0.5NbO3 (NKN) piezoelectric ceramics were fairly well densified at a relatively low temperature under atmospheric conditions. A relative density of 96%–99% can be achieved by either using high-energy attrition milling or adding 1 mol% oxide additives. It is suggested that ultra-fine starting powders by active milling or oxygen vacancies and even liquid phases from B-site oxide additives mainly lead to improved sintering. Not only were dielectric properties influenced by oxide additives, such as the Curie temperature (Tc) and dielectric loss (D), but also the ferroelectricity was modified. A relatively large remanent polarization was produced, ranging from 16 μC/cm2 for pure NKN to 23 μC/cm2 for ZnO-added NKN samples. The following dielectric and piezoelectric properties were obtained: relative permittivity ɛT33/ɛ0=570–650, planar mode electromechanical coupling factor, kp=32%–44%, and piezoelectric strain constant, d33=92–117 pC/N.

Effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming

Abstract: Steam methane reforming was studied at 823 K, a total pressure of 20 bar and steam to carbon ratios from 0.08 to 2.4 over conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 catalysts as well as catalysts supported on hydrotalcite derived materials. Catalyst activity, coke formation and deactivation at steam reforming conditions were studied using the tapered element oscillating microbalance (TEOM). Nickel supported on hydrotalcite derived materials had a smaller crystal size and a higher resistance to coke formation than the conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 . The higher resistance to carbon formation could be due to a higher saturation concentration of carbon in the smaller nickel crystals. Sintering experiments were performed at 903 K and 20 bar on the hydrotalcite derived catalysts and compared with an industrial NiO/CaAl 2 O 4 catalyst. The particle growth for the hydrotalcite derived catalysts was larger than for the industrial catalyst, but the hydrotalcite derived catalysts had the smallest size of stabilized Ni crystals.

Processing and properties of monolithic TiB2 based materials

Abstract: Titanium diboride (TiB2) based materials have received wide attention because of their high hardness and elastic modulus, good abrasion resistance, and superior thermal and electrical conductivity. Potential applications include high temperature structural materials, cutting tools, armour, electrodes in metal smelting and wear parts. Despite its useful properties, the application of monolithic TiB2 is limited by poor sinterability, exaggerated grain growth at high temperature and poor oxidation resistance above 1000°C. Pure TiB2 can be densified only at high temperatures (∼2000°C), with an applied pressure generally being necessary during sintering. However, these high sintering temperatures cause abnormal grain growth and microcracks, which are detrimental to the mechanical properties. Various sinter additives are commonly added to obtain dense TiB2 with optimised mechanical properties at lower sintering temperature. The present review surveys the current state of knowledge on development of bulk...

Neck Formation and Self-Adjusting Mechanism of Neck Growth of Conducting Powders in Spark Plasma Sintering

Abstract: By using spherical Cu powders as the conducting sintering material, the microstructures of sintered powder particles at different stages in the process of spark plasma sintering (SPS) have been investigated. Theoretical analyses are proposed to quantify the effects of the pulsed direct current on the neck formation and the neck growth of conducting powders. It is found that there is a considerable inhomogeneous distribution of the temperature increase from the particle-contacting surface to the center of the particle when the pulsed current passes through. The temperature at the particle-contacting surface may reach the boiling point of the material, which results in neck formation at relatively low-sintering temperatures through a process of local melting and rapid solidification. The neck growth depends on the local distribution of the current intensity, which is determined by the competition between the neck cross-sectional area and the electrical resistivity increasing with the temperature. Accordingly, we propose that the coarsening of necks follows a "self-adjusting" mechanism, which is likely to be the essential reason for the homogeneous distributions of neck sizes and sizes of fine grains formed in the neck zones during the SPS process.

Two-Step Sintering of Ceramics with Constant Grain-Size, I. Y2O3

Abstract: Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring's scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.

Low-Temperature Sintered Nanoscale Silver as a Novel Semiconductor Device-Metallized Substrate Interconnect Material

Abstract: A nanoscale silver paste containing 30-nm silver particles that can be sintered at 280degC was made for interconnecting semiconductor devices. Sintering of the paste produced a microstructure containing micrometer-size porosity and a relative density of around 80%. Electrical and thermal conductivities of around 2.6times105 (Omegamiddotcm)-1 and 2.4W/K-cm, respectively, were obtained, which are much higher than those of the solder alloys that are currently used for die attachment and/or flip-chip interconnection of power semiconductor devices. The sintered porous silver had an apparent elastic modulus of about 9GPa, which is substantially lower than that of bulk silver, as well as most solder materials. The lower elastic modulus of the porous silver may be beneficial in achieving a more reliable joint between the device and substrate because of increased compliance that can better accommodate stress arising from thermal expansion mismatch

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Abstract: Zirconium diboride (ZrB 2 ) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B 2 O 3 and ZrO 2 ) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B 4 C were evaluated as additives to facilitate the removal of ZrO 2 Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB 2 (surface area ∼ 1 m 2 /g) or attrition-milled ZrB 2 (surface area ∼ 7.5 m 2 /g) with WC and/or B 4 C as a sintering aid. ZrB 2 containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B 4 C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.

Two‐Step Sintering of Ceramics with Constant Grain‐Size, II: BaTiO3 and Ni–Cu–Zn Ferrite

Abstract: We investigated the preparation of bulk dense nanocrystalline BaTiO 3 and Ni-Cu-Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO 3 with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y 2 O 3 , the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO 3 ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.

Temperature‐Stable Dielectrics Based on Chemically Inhomogeneous BaTiO3

Abstract: The dielectric properties and chemical homogeneity of BaTiO3 ceramics sintered with additions of the pseudophase “CdBi2Nb2O9” were investigated using SEM, TEM, STEM, and EDX. In materials showing the “X7R” dielectric temperature characteristic, the microstructure exhibits the grain core-grain shell structure. The perovskite material in the shell shows a temperature characteristic determined by mixed crystals of BaTiO3 with the complex perovskites Ba(Bi1/2Nb1/2)O3 and Ba(Cd1/3Nb2/3)O3 having an approximate Curie point of -80°C. The chemical inhomogeneity emerges during a process of reactive liquid-phase sintering. Application of too-high sintering temperatures leads to uniform distributions of the additives via solid-state diffusion and to the loss of the X7R characteristic.

Normal Sintering of (K,Na)NbO3‐Based Ceramics: Influence of Sintering Temperature on Densification, Microstructure, and Electrical Properties

Abstract: Normal sintering of Li-doped and Li/Ta-codoped potassium sodium niobate (KNbO 3 -NaNbO 3 , KNN)-based ceramics was investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. It was found that density increased greatly within a narrow temperature range but tended to decrease when the sintering temperature slightly exceeded the optimal one, accompanied by the appearance of abnormal grain growth, which was considered to be due to the intensified volatilization of alkali metal oxides. Piezoelectric and dielectric properties also showed a similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density, because both densification and composition affect the electrical properties. The highest d 33 value of 206 pC/N was obtained for the Li-and Ta-codoped KNN ceramics prepared at 1090°C.

Coercivity Enhancement by the Grain Boundary Diffusion Process to Nd–Fe–B Sintered Magnets

Abstract: Changes in magnetic properties after applying grain boundary diffusion process to Nd-Fe-B sintered magnets with a thickness of several millimeters were investigated, and their microstructures were also observed Coercivities of the magnets coated with heavy rare earths (HRE=Dy and Tb) oxide or fluoride powder and then heat-treated, were enhanced significantly without a considerable reduction in a remanence During the heat treatment, HRE element diffused into the magnet through the grain boundary, and formed HRE-rich shells around the grain boundary of the Nd2Fe14B phase by substituting Nd The coercivity increased by more than 400 kA/m with a thickness of up to 3 mm for the magnet processed with TbF3

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

Abstract: The electrical properties of a series of CaCu3Ti4O12 ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu3Ti4O12 ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N2 at 800°–1000°C but can be recovered to the original value by heat treatment in O2 at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N2 or O2 at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu3Ti4O12 at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.

Design of powder metallurgy titanium alloys and composites

Abstract: Low cost and good performance are two major factors virtually important for Ti alloy development. In this paper, we have studied the effects of alloying elements, thermo-mechanical treatment and particle reinforcement on microstructures and mechanical properties of powder metallurgy (PM) Ti alloys and their composites. Our results indicate that low cost PM Ti alloys and their composites with attractive properties can be fabricated through a single compaction-sintering process, although secondary treatments are required for high performance applications. Three new PM Ti alloys and one TiC/Ti composite of high performance are developed, and new design principles are also proposed. For design of PM Ti alloys, addition of alloying elements has the beneficial effect of enhanced sintering and/or improved mechanical properties. For example, Fe element accelerates the sintering process, Mo and Al are good candidates for solution strengthening, and rare earth elements effectively increase the material ductility by scavenging oxygen from the Ti matrix. For the design of Ti-based composites, in situ formation of strengthening particles and solid solution hardening of the matrix both should be considered simultaneously for alloy development. Cr 3 C 2 is found to be a very suitable additive for processing particle reinforced Ti composites.

Microstructure and piezoelectric properties of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics

Abstract: For 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 (0.95NKN-0.05BT) ceramics sintered at 1040–1075°C, abnormal grain growth occurred but the grain size decreased when the sintering temperature exceeded 1075°C. The dielectric constant (ϵ3T∕ϵ3), electromechanical coupling factor (kp), and piezoelectric constant (d33) were considerably increased with increasing relative density and grain size. Evaporation of Na2O deteriorated the piezoelectric properties by decreasing the resistivity. To minimize Na2O evaporation, specimens were muffled with 0.95NKN-0.05BT powders during the sintering. Improved piezoelectric properties of d33=225pC∕N, kp=36%, and ϵ3T∕ϵ3=1058 were obtained for specimen sintered at 1060°C for 2h with muffling.

New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides

Abstract: A series of La 2 O 3 –ZrO 2 –CeO 2 composite oxides were synthesized by solid-state reaction The final product keeps fluorite structure when the molar ratio Ce/Zr ≥ 07/03, and below this ratio only mixtures of La 2 Zr 2 O 7 (pyrochlore) and La 2 O 3 –CeO 2 (fluorite) exist Averagely speaking, the increase of CeO 2 content gives rise to the increase of thermal expansion coefficient and the reduction of thermal conductivity, but La 2 (Zr 07 Ce 03 ) 2 O 7 has the lowest sintering ability and the lowest thermal conductivity which could be explained by the theory of phonon scattering Based on the large thermal expansion coefficient of La 2 Ce 325 O 95 , the low thermal conductivities and low sintering abilities of La 2 Zr 2 O 7 and La 2 (Zr 07 Ce 03 ) 2 O 7 , double-ceramic-layer thermal barrier coatings were prepared The thermal cycling tests indicate that such a design can largely improve the thermal cycling lives of the coatings Since no single material that has been studied so far satisfies all the requirements for high temperature thermal barrier coatings, double-ceramic-layer coating may be an important development direction of thermal barrier coatings

Fast Densification of Ultra‐High‐Temperature Ceramics by Spark Plasma Sintering

Abstract: This work investigated suitability and efficacy of the sintering technique known as spark plasma sintering to produce ultra-high-temperature-based Hf and Zr borides. Ceramic–matrix composites in the systems HfB2–SiC, ZrB2–MoSi2, and ZrB2–ZrC–SiC were processed by spark plasma sintering and hot pressing. The effects of processing were evaluated comparing the materials microstructure and properties. Compared with hot-pressing technique, spark plasma sintering offers the great advantage to fabricate successfully in short time (i.e., cuts in costs) poorly sinterable powder compositions without the help of any sintering activators.

Solid-State Reactive Sintering of Transparent Polycrystalline Nd:YAG Ceramics

Abstract: Transparent polycrystalline Nd:YAG ceramics were fabricated by solid-state reactive sintering a mixture of commercial Al2O3, Y2O3, and Nd2O3 powders. The powders were mixed in methanol and doped with 0.5 wt% tetraethoxysilane (TEOS), dried, and pressed. Pressed samples were sintered from 1700° to 1850°C in vacuum without calcination. Transparent fully dense samples with average grain sizes of ∼50 μm were obtained at 1800°C for all Nd2O3 levels studied (0, 1, 3, and 5 at.%). The sintering temperature was little affected by Nd concentration, but SiO2 doping lowered the sintering temperature by ∼100°C. Abnormal grain growth was frequently observed in samples sintered at 1850°C. The Nd concentration was determined by energy-dispersive spectroscopy to be uniform throughout the samples. The in-line transmittance was >80% in the 350–900 nm range regardless of the Nd concentration. The best 1 at.% Nd:YAG ceramics (2 mm thick) achieved 84% transmittance, which is equivalent to 0.9 at.% Nd:YAG single crystals grown by the Czochralski method.

Microstructural Evolution of Crystalline‐Oriented (K0.5Na0.5)NbO3 Piezoelectric Ceramics with a Sintering Aid of CuO

Abstract: The microstructural development of crystalline-oriented (K0.5Na0.5)NbO3 (KNN)-based piezoelectric ceramics during sintering was investigated. The addition of CuO as a sintering aid was found to be effective for fabricating highly oriented and dense KNN ceramics. KNN specimens containing 0.5–1.0 mol% CuO sintered at 1100°C for 1 h were found to have relative densities and pseudo-cubic {100} orientation degrees of 95% or higher. In the early stages of sintering, KNN is formed in the reaction between complementary reactants NaNbO3 and KNbO3, after which oriented grain growth proceeds at a relative density of more than 90%. In addition, the results of transmission electron microscopy observation showed that textured KNN ceramics have a unique pectinate-like domain structure with domain walls consisting of {101} planes.

Thermomechanical Reliability of Low-Temperature Sintered Silver Die Attached SiC Power Device Assembly

Abstract: A thermomechanical reliability study was conducted on a low-temperature sintered silver die attached SiC power device assembly. The silver die attachment was formed by sintering nanoscale silver paste in air at 300 degC to form a strong bond between silver- or gold-coated direct-bond-copper substrates and silver-metallized SiC Schottky diodes. Using the 50% drop in the die-shear strength as the failure criterion, an accelerated thermal cycling experiment between 50 degC and 250 degC showed that the silver die attachment can survive more than 4000 cycles, indicating its high thermomechanical reliability at the interested temperature range. Established mainly by scanning electron microscopy/energy-dispersive spectroscopy, the drop of the die-shear strength during the thermal cycling was attributed to the pile-up of creeping dislocations to form microcavities at the grain boundaries of the sintered silver

Sintering of nickel catalysts: Effects of time, atmosphere, temperature, nickel-carrier interactions, and dopants

Abstract: Supported nickel catalysts are widely used in the steam-reforming process for industrial scale production of hydrogen and synthesis gas. This paper provides a study of sintering in nickel-based catalysts (Ni/Al 2 O 3 and Ni/MgAl 2 O 4 ). Specifically the influence of time, temperature, atmosphere, nickel-carrier interactions and dopants on the rate of sintering is considered. To probe the sintering kinetics, all catalysts were analyzed by sulfur chemisorption to determine the Ni surface area. Furthermore selected samples were further analyzed using X-ray diffraction (XRD), mercury porosimetry, BET area measurements, and electron microscopy (EM). The observed sintering rates as a function of time, temperature, and P H 2 O / P H 2 ratio were consistent with recent model predictions [J. Sehested, J.A.P. Gelten, I.N. Remediakis, H. Bengaard, J.K. Norskov, J. Catal. 223 (2004) 432] over a broad range of environmental conditions. However, exposing the catalysts to severe sintering conditions the loss of nickel surface area is faster than model predictions and the deviation is attributed to a change in the sintering mechanism and nickel removal by nickel-carrier interactions. Surprisingly, alumina-supported Ni particles grow to sizes larger than the particle size of the carrier indicating that the pore diameter does not represent an upper limit for Ni particle growth. The effects of potassium promotion and sulfur poisoning on the rates of sintering were also investigated. No significant effects of the dopants were observed after ageing at ambient pressure. However, at high pressures of steam and hydrogen (31 bar and H 2 O:H 2 = 10:1) potassium promotion increased the sintering rate relative to that of the unpromoted catalyst. Sulfur also enhances the rate of sintering at high pressures, but the effect of sulfur is less than for potassium.