Early Stages Of Sintering Research Articles

Organometallic precursors, an alternative route for the doping of silicon nitride powders?

Abstract Two silicon nitride powders prepared by diimide precipitation and direct nitridation were doped with 5 vol.% yttria using (i) a commercial Y2O3 powder and (ii) a soluble Y-organometallic compound as additive. The aim of this work was a direct comparison between those two doping methods. Homogeneity of additive distribution was characterized by Fourier transform infra-red spectroscopy (FT-IR), photo electron spectroscopy (XPS) and transmission electron microscopy (TEM). Depending on the doping technique used, no major difference in sintering behavior and microstructure evolution were monitored. However, chemical doping results in (i) an enhanced densification rate in the early stage of sintering, (ii) a slight shift of the densification rate maximum (ds/dt) to higher temperatures, and (iii) in a slightly finer final microstructure. Note that doping via organometallic precursors can only be successfully applied, when the powder particle surface reveals the presence of silanol groups, as in case of diimide starting powders. Electron microscopy, Organometallic dopants, Sintering, Si3N4

Early-stage sintering mechanisms of Fe-doped CeO2

Cerium oxide is a very useful base material used as catalyst supports, ion conductors and gas sensors. It is also well known that ceria-based materials are difficult to densify below 1500°C. However, a small amount of Fe doping obviously promotes the densification rate and reduces the sintering temperatures. In this study, the early stage sintering mechanisms of undoped and Fe-doped CeO2 were investigated, based on two sintering models. We confirm that pure CeO2 exhibits volume-diffusion controlled sintering, while a viscous flow mechanism dominates the early stage sintering of Fe-doped CeO2.

Electron Microscopy Study of Reaction Sintered HLW Glass

ABSTRACTReaction sintering under pressure can be used to vitrify radioactive waste. We used defense waste simulate with high concentrations of refractory oxide ZrO2 (∼36 wt%). Silicate glasses with 30 to 50 wt% of waste loading were prepared by hot isostatic pressing (800 °C, 28 MPa). Amorphous silica (50 to 70 wt%) was added to calcined waste simulants. The reaction sintered final products were characterized by scanning and analytical electron microscopy. Our results showed that a continuous glass phase formed. Waste components such as Na2O, CaO, MnO2, Fe2O3, and Al2O3 dissolved in the glass phase. ZrO2 dissolved as well but reached its solubility limit. Nanometer size crystals of baddeleyite (ZrO2) suggest that their presence is a result of super-saturation of the glass phase rather than incomplete dissolution. The sodium concentration increases in the early stages of reaction sintering and decreases towards the end and so does solubility of ZrO2 and other oxides. The microstructure of reaction sintered glasses is determined by a) the concentrations of refractory oxides, b) the concentration of reaction sintering agents, c) the particle size of glass formers, and d) sintering conditions, e.g., temperature and time. Evidence is provided by respective analytical data and micrographs.

Powder metallurgical processing of Ni–Ti–Zr alloys undergoing martensitic transformation: part I

Ni–Ti–Zr materials with Zr 12–25 at.% and Ni 42–50 at.% have been produced by powder metallurgy. Suitable temperatures for sintering in Ar-atmosphere Ni–Ti–Zr compacts are within the range 900–1000 °C. Sintering at temperatures above 1000 °C causes melting of the compacts with high Zr content. The presence of ZrC, ZrO 2, Zr 3O, TiO 2 and TiO of different modifications, complex oxides such as Ni 5TiO 7, Ti 0.5Zr 0.5O 0.2 and equilibrium phases after sintering at temperatures above 1000 °C in alloys with low Zr-content was derived from X-ray diffractometry. During sintering at temperatures below 1000 °C the phases belonging to the binary Ti–Ni and Ti–Zr systems were formed. Long-term sintering and slow furnace cooling allowed the precipitation of Ni 4Ti 3 and Ni 2Ti. The process of sintering is controlled by the diffusion of Ni in Ti and Zr particles during the early stages of sintering. Slow diffusion of Zr atoms in Ti 2Ni, Ti–Ni and diffusion of Ti atoms in Zr 2Ni, Ni–Zr controls the later stages of sintering.

Low-temperature fully dense and electrical properties of doped-ZnO varistors by a polymerized complex method

The preparation of homogeneous and submicrometer doped-ZnO ceramic powders by using a metal-organic polymeric method is described. After calcining and granulating, green compacts with a uniform packing powder and a narrow pore size distribution were achieved. Dense ceramic bodies (>99% theoretical) by normal liquid-phase sintering at temperatures of 850–940°C for 1–5 h were fabricated. It is believed that the low pore-coordination-number allowed a uniform filling of the small pores by the formed liquid in the early stages of sintering and, as a consequence, a high shrinkage rate and rapid densification in a short temperature interval (825–850°C) took place. At those sintering temperatures a small grain growth was produced, and the grain size was maintained below 1 μm. Preliminary electrical results obtained on the doped-ZnO ceramics so fabricated showed nonlinearity coefficients α⩾70 and a breakdown voltage Vb (1 mA/cm2)⩾1500 V/mm.

Sintering Behavior and Electrical Properties of Nanosized Doped‐ZnO Powders Produced by Metallorganic Polymeric Processing

Homogeneous and nanosized (28 nm crystallite size) doped‐ZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore‐size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquid‐phase sintering at 850° and 940°C for 1–5 h. Apparently, the low pore‐coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°–850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 μm. Ceramics so‐fabricated showed a nonlinear coefficient, α, of ≥70, and a breakdown voltage, Vb (1 mA/cm2), of ≥1500 V/mm. The high electrical performance of the doped‐ZnO dense ceramics was attributed to liquid‐phase recession on cooling, which enhanced the ZnO‐ZnO direct contacts and the potential barrier effect.

Modeling of Damage Development During Sintering of Ceramics

Sintering of powder-based ceramic composites is often accompanied by developments of cracks or large pores. As a rule, damage development is active only at early stage of sintering when junctures between particles are not strong enough. A macroscopic model of sintering taking into account broken junctures between particles is put forward. The model follows both densification and damage development during sintering.

Microstructural Development in SnO2‐Doped ZnO–Bi2O3 Ceramics

The effects of adding small quantities of SnO2 to the basic ZnO–Bi2O3 varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO2, cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO2 lead to an increase in number of grains with inversion boundaries and to a more fine‐grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO2 addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.

Microstructural development of silicon nitride with aligned β-Si 3N 4 whiskers

Silicon nitride was sintered with 3 wt.% silicon nitride whiskers that were aligned using tape casting. Sintering was carried out at temperatures between 1550 and 1850°C for 1 h. The α to β phase transformation was complete at 1750°C. XRD results also showed that the amount of β-phase grains in the matrix increased faster than growth rate of the whisker grains at the early stage of sintering. The intensities of the peaks diffracted from the whisker grains increased faster than those diffracted from the matrix β-phase grains after the α to β phase transformation was complete. Both XRD results and the etched microstructures indicated that the whisker grains grew preferentially in the length direction.

Influence of O and Co on the early stages of sintering of WC–Co: a surface study by AES and STM

The influence of oxygen on the sintering behavior of WC–Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650°C results in a 2×2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM tip. Annealing at 850°C removes excess Co and leaves only the 2×2 surface. Oxygen exposure of the 2×2 surface results in a “clustered” cobalt oxide overlayer which on annealing at 750°C breaks up and restores the 2×2 structure as the metallic Co wets the surface.

Improved densification of carbonyl iron compacts by the addition of fine alumina powders

An investigation of the effect of alumina particles on the sintering behavior of a carbonyl iron powder compact was carried out in this study. Two different-sized alumina, 0.05 and 0.4 µm, were added to the iron compact at amounts up to 1.2 wt pct. When 0.4 µm alumina particles were added, no sintering enhancement was observed. But, in contrast to previous results reported in literature, the addition of 0.1 to 0.2 wt pct of 0.05 µm alumina particles was found to improve the densification. With 0.1 wt pct, the sintered density increased from 7.25 to 7.40 g/cm3 after the compact was sintered at 1350 °C for 1 hour in hydrogen. Dilatometric curves showed that alumina impeded the early-stage sintering of iron in the α phase, but improved densification in the γ phase at high temperatures. These results, along with microstructural analysis, suggested that alumina particles exhibit dual roles; their physical presence blocks the diffusion of iron atoms, thus causing inhibition of sintering, while their grainboundary pinning effect prevents exaggerated grain growth of iron and helps densification. It follows that, depending upon the amount and size of the alumina powders, either an increase or decrease in the final sintered density can be obtained.

Densification and shape distortion in liquid-phase sintering

Densification and dimensional control are important aspects of liquid-phase sintering. The capillary force and the solid bonding affect both densification and shape preservation. Capillarity, which is orientated isotropically, causes uniform shrinkage and holds grains together to preserve the component shape in the early stage of sintering. On the other hand, solid bonding resists viscous flow and inhibits densification and shape distortion. The capillary force decreases with densification and approaches zero as pores are eliminated. Thus, shape retention eventually requires solid-grain bonding. The solid-grain bonding provides compact rigidity, which is represented by compact strength. Shape distortion occurs when the compact loses its strength. For every situation, there is a critical compact strength above which no shape distortion occurs. Distortion in liquid-phase sintering indicates that the compact strength passed below a critical level.

Early-stage sintering in a powder compact of polyhedral particles II. Experimental analysis with a highly sinterable Al 2O 3

A model of early-stage sintering for polyhedral particles is examined with a highly sinterable Al 2O 3 powder. Combining of the initial and intermediate stages based on the model concerned describes quantitatively its sintering data such as power dependence of time and reduction in a specific surface area. Valid estimations of a model and/or mechanisms of sintering necessitate the accurate measurement of both shrinkage versus sintering time and specific surface area versus shrinkage.

Particle contacts in a porous fibrous copper material at early sintering stages

Electrical conductivity measurements have been used in quantitative state evaluation for particle contacts in fibrous copper pressings at early stages of sintering under vacuum. The results agree with ultrasonic data and stretching tests. Contact formation in such a material begins after the specimen has been heated to 600–800°C, i.e., after the complete relaxation of the residual stresses accumulated in the pressure working.

Influence of Bi 2O 3/TiO 2, Sb 2O 3 and Cr 2O 3 doping on low-voltage varistor ceramics

The influence of the amount of Bi 2O 3 and TiO 2 additions at a TiO 2/Bi 2O 3 ratio of 1, as well as Sb 2O 3 and/or Cr 2O 3 doping, on the microstructural development and electrical properties of varistor ceramics in the ZnO–Bi 2O 3–TiO 2–Co 3O 4–Mn 2O 3 system was investigated. In samples with a low level of Bi 2O 3 and TiO 2 (0·3 mol%) and therefore small amount of liquid phase, exaggerated growth of the ZnO grains results in high microstructural inhomogeneity. Co-doping with Sb 2O 3 significantly changes the phase composition of TiO 2 doped low-voltage varistor ceramics. The Bi 3Zn 2Sb 3O 11 type pyrochlore phase forms at the expense of the γ-Bi 2O 3 and Bi 4Ti 3O 12 phases and decreases the amount of liquid phase in the early stages of sintering. Already small amounts of Sb 2O 3 and/or Cr 2O 3 added to a TiO 2 doped low-voltage varistor ceramics limit ZnO grain growth and increase the threshold voltage V T of the samples.

The effect of excess (Pb,Bi)2212 stoichiometry on the transport Jc of (Pb,Bi)2223 superconducting tapes

The effects on the phase formation and magnitude of the transport J c of the addition of up to 15% excess (Pb,Bi)2212 stoichiometry to Pb 0.34Bi 1.84Sr 1.91Ca 2.03Cu 3.06O x has been studied. The excess (Pb,Bi)2212 was added at the solution stage of precursor preparation to ensure good mixing of constituents. The addition of the excess (Pb,Bi)2212 stoichiometry has been found to change the kinetics of formation of the (Pb,Bi)2223 phase in the early stages of sintering, t<80 h. Transport J c of >2×10 4 A cm −2 have been achieved for tapes with 10% excess (Pb,Bi)2212. The J c– B characteristics ( B≤0.5 T, B ab) of tapes with 10% excess (Pb,Bi)2212 was found to be superior to the Endo composition tapes.

Early-stage sintering in a powder compact of polyhedral particles I. Models

An early-stage sintering model for polyhedral particles suggests combining the initial and intermediate stages. Kinetic equations based on the present model indicate a 1/3-power dependence of time for sintering controlled by volume diffusion. If sintering, on the other hand, is controlled by grain-boundary diffusion, the aforesaid combination decreases the power dependence of time from 1/4 to about 1/6 with sintering. Furthermore, the present model describes the variation of specific surface area by densification.

Densification and Microstructure of the PIMed W-15wt%Cu Nanocomposite Powder.

The densification process and related microstructure evolution during sintering of the powder injection molded (PIMed) W-15wt%Cu nanocomposite powder have been investigated. The densification process of the PIMed composite powder was analyzed in terms of dilatometry, shrinkage anisotropy and microstructure. Especially, the sintering behavior of the W-Cu composite powder compact was in-situ analyzed during heat up, which might be closely related to gas evolution in the early stage of sintering. It was found that residual carbon reduces oxides, that hydrogen is unable to reduce, forming CO gas at 700-1000°C. Such carbon effect resulted in enhancement of the solid state sintering as well as formation of inhomogeneous microstructure of the PIMed W-Cu nanocomposite powder. Based on the measurement of gas evolution, the most effective removal of residual impurities and the best consolidation were achieved without promoting solid state sintering.

La 2Zr 2O 7 formation and oxygen reduction kinetics of the La 0.85Sr 0.15Mn yO 3, O 2(g)|YSZ system

Nucleation and growth of lanthanum zirconate (LZO) between (001) single crystals of 9.5 mol% Y 20 3-stabilized ZrO 2 (YSZ) and porous La 0.85Sr 0.15Mn y O 3± δ ( y=0.95…1.10) perovskites were investigated. The perovskites were screen-printed on the solid electrolyte and sintered in air at 1373 K for short periods. High resolution transmission electron microscopy, electrochemical impedance spectroscopy, and atomic force microscopy were employed for interface characterization. The manganese concentration in the perovskite affects the onset of nucleation and the growth rate of lanthanum zirconate. Excess lanthanum oxide within the perovskite reacts immediately with YSZ to form dense LZO layers. Layer growth kinetics is controlled by bulk diffusion of cations. Stoichiometric perovskite leads to the reductive formation of cubic LZO islands at the cathode/electrolyte interface after a few minutes of sintering. The A-site deficient perovskite is characterized by the reductive dissolution of Mn into YSZ in the early sintering stage and by a pronounced surface diffusion of cations. After the critical Mn concentration in the perovskite is reached, reductive nucleation of LZO originates. Island growth is controlled by surface diffusion of cations and the supply of lanthanum. The electrochemical properties of the cathode/YSZ interfaces are strongly influenced by the formation of LZO at the triple phase boundary. Charge transfer, dissociation of adsorbed molecular oxygen ions, and surface diffusion of atomic oxygen are discriminated from electrochemical impedance data.

A model for the sintering of spherical particles of different sizes by solid state diffusion

In this paper the numerical scheme developed by Pan and Cocks ( Acta metall. 43, 1395–1406, 1995) is used to simulate the co-sintering process of two spherical particles of different sizes by coupled grain-boundary and surface diffusion. The numerical analysis reveals many interesting features of the co-sintering process. For example, it is found that the shrinkage between the two particles is not affected significantly by the size difference of the two particles as long as the difference is less than 50%. Based on the numerical results, empirical formulae for the characteristic time of the co-sintering process and for the shrinkage rate between the two particles are established. The empirical formulae can be used to develop constitutive laws for early-stage sintering of powder compacts which take into account the effect of particle size distribution. To demonstrate this, a densification rate equation for compacts with bimodal particle size distributions is derived.