Kinetics of densification of powder compacts during the initial stage of sintering with constant rates of heating. A thermal analysis approach. Part I. Theoretical considerations

Densification of powder compacts during sintering (initial stage) under constant heating rate has been studied and methods of analysing the densification kinetic data have been suggested. p]Green compacts can broadly be envisaged to consists of two phases, viz. porosity and solid material. Annihilation of the pores causes densification of the compacts during sintering. So, the kinetics of the initial stage of densification may very well be represented by a densification parameter (α),where

Here, v_s and v_p,respectively, denote the pore volumes of the sintered and the green compact. Continuous and in situ recording of the linear shrinkage (ΔL) of the compacts during the sintering may be done with a dilatometer and using this value of ΔL,the value of α at any temperature (T) may be calculated.These kinetic data (α vs. T)may then be subjected to analysis using the well-known methods of thermal analysis, and the so-called “derived activation energy (E)” of densification may be evaluated therefrom. The exact form of g(α)—governing rate equation—is ascertained by a trial-and-error method.These methods were used to analyse the densification kinetic data of haematite, copper and silver powders (with particles of irregular shapes and sizes). The results of such analyses will appear on other parts of this paper.     

Finite-difference model for the reaction sintering of oxide ceramics by direct oxidation of intermetallic powder compacts

A kinetic model for the reaction sintering of oxide ceramics in the system Al₂O₃–SiO₂–ZrO₂ using mixtures of intermetallic compounds is presented. A 2D finite-difference model is developed to describe the exothermic gas-solid reactions taking place during the firing of ZrAl₃/ZrSi₂ powder compacts. The model accounts for the oxidation kinetics of the powder particles, as well as the consumption and diffusion of gaseous oxygen through the porous matrix. Additionally, possible changes in the pore structure of the green body due to the oxidation reactions and sintering effects are incorporated in the model. The resulting differential equations are coupled with a two-dimensional Fourier heat balance equation leading to a system of nonlinear partial differential equations, which is solved by the numerical method of lines. The influence of different processing parameters like sample composition and heating cycle on the reaction sintering process is investigated and the model-predicted reaction behaviour is compared to experimental results.     

Influence of Gas Type on the Thermal Efficiency of Microwave Plasmas for the Sintering of Metal Powders

Microwave plasmas have enormous potential as a rapid and energy efficient sintering technology. This paper evaluates the influence of both plasma atmosphere and metal powder type on the sintering temperatures achieved and the properties of the sintered powder metal compacts. The sintering is carried out using a 2.45 GHz microwave-plasma process called rapid discharge sintering (RDS). The sintering of three types of metal powder are evaluated in this study: nickel (Ni), copper (Cu) and 316L stainless steel (SS). An in-depth study of the effects of the plasma processing parameters on the sintered powder compacts are investigated. These parameters are correlated with the mechanical performance of the sintered compacts to help understand the effect of the plasma heating process. The substrate materials are sintered in four different gas discharges, namely hydrogen, nitrogen oxygen and argon. Thermocouple, pyrometer and emission spectroscopy measurements were taken to determine the substrate and the discharge temperatures. The morphology and structure were examined using scanning electron microscopy and X-ray diffraction. The density and hardness of the sintered compacts were correlated with the plasma processing conditions. As expected higher densities were obtained with powders with lower sintering temperatures i.e. nickel and copper when compared with stainless steel. Under the power input and pressure conditions used, the highest substrate temperature attained was 1,100°C for Cu powder sintered in a nitrogen atmosphere. In contrast under the same processing conditions but in an argon plasma, the temperature achieved with SS was only 500°C. The effect of the plasma gas type on the sintered powder compact chemistry was also monitored, both hydrogen and nitrogen yielded a reducing effect for the metal in contrast with the oxidising effect observed in an oxygen plasma.     

Effect of additives on sintering of zirconia ceramics

The effect of two types of additives on the sintering processes of zirconium ceramics was investigated. The impact of both polyvinyl alcohol (12 mass%) and bismuth oxide (1 mass%) on compaction of ultradisperse powders of stabilized zirconia produced by plasma-chemical method and on densification of the compacts during sintering was studied. In the initial state, plasma-chemical powders demonstrate hindered compactibility and sinterability. The additives were incorporated through mechanical stirring with intermediate sieving using a sieve. It was found that introduction of polyvinyl alcohol significantly reduces wall friction and hence facilitates the compaction process. However, no effect on densification and sintering kinetics was observed. Incorporation of bismuth oxide microadditives decreases the temperature of the peak compacting rate. In one-step sintering, the compacts are found to markedly expand at the isothermal ageing stage. The expansion is more distinct at increased compaction pressure. It is shown that this expansion is associated with an increase in the volume of pores surrounded by the liquid phase of bismuth oxide. Two-step sintering with an intermediate stage of isothermal ageing at 1200 °C shows a 100 °C decrease in the sintering temperature, no expansion of the samples occurs at the final ageing stage at sintering temperature, and the ceramics produced exhibits good operational parameters. Recommendations are given on the use of low-melting microadditives in sintering ultradisperse powders obtained by plasma-chemical method.     

Sintering kinetic study of the nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal ceramics

Dilatometric shrinkage data was utilized to study the sintering kinetics of the in-house synthesized nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal (TZP) ceramics. The objective was to determine activation energy (Q) of sintering and the sintering mechanism (n) relevant to the initial stage of sintering. The product of activation energy and sintering parameter, i.e., “nQ” was calculated from the shrinkage data acquired from the constant rate of heating experiment. The apparent activation energy of sintering (Q) was calculated using modified-“Dorn” method. Modified Johnson’s equation was used to determine value of “n” using the activation energy obtained from the Dorn method. Stepwise isothermal dilatometry technique was utilized as an independent method to determine the “n” value. The activation energy of sintering was in the range of 400–525 kJ mol^?1 and found to be dependent on the dopant concentration. The value of “n” was found to be ~0.33 for both 3 mol% yttria-doped (3Y-TZP) and yttria-samaria codoped (3(Y,S)-TZP) TZP, whereas for 3 mol% samaria-doped tetragonal zirconia (3S-TZP), the value of “n” was ~0.40. From the obtained “n” values, it may be concluded that grain boundary diffusion (GBD) was the dominant sintering mechanism in 3Y-TZP and 3(Y,S)-TZP, whereas an intermediate of GBD and volume diffusion influences the initial sintering stage in the 3S-TZP.     

Nanosized bismuth titanate (Bi4Ti3O12) system drive through auto-combustion process by using suspension titania (TiO2)

Bismuth titanate (Bi₄Ti₃O₁₂) was developed by means of titanium oxide (TiO₂) suspension in auto-combustion process at 220 °C to get nanosized (20 ± 5 nm) bismuth titanate (Bi₄Ti₃O₁₂) powder. Complete piezoelectric phase (tetragonal) was obtained after calcination at 700 °C. Dilatometery of compacts was performed to find out sintering temperature. On the basis of shrinkage results, compacts were sintered at 750, 800, and 850 °C for 2 h. After sintering single phase was obtained with orthorhombic structure analyzed by X-ray diffraction and also investigated by Rietveld method. High-resolution scanning electron microscopy revealed that fine plate-like structure which is a characteristic of BIT powder can be obtained at 850 °C. Sintering results indicate that density and average grain size increase with the increasing temperature. A maximum of about 90 % of the theoretical density was achieved for the sintered product at 850 °C.     

Sintering studies on Ni–Cu–YSZ SOFC anode cermet processed by mechanical alloying

New 40 vol%[(Cu)–Ni]–YSZ cermet materials processed by mechanical alloying (MA) of the row powders are prepared. The powder compacts are sintered in air, hydrogen and inert (argon) atmospheres at a dilatometer and tubular furnace up to 1,350 °C. Sintering by activated surface concept (SAS) can anticipate and enhance the densification in such powders. Stepwise isothermal dilatometry (SID) sintering kinetics study is performed allowing determining kinetic parameters for Ni–YSZ and Ni–Cu–YSZ pellets. Two-steps sintering processes is indicated while Cu-bearing material features the smallest activation energy for sintering. The allied MA–SAS method is a promising route to prepare SOFC fuel cell anode materials.     

Thermal and electrical properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> synthesized by soft chemistry route

The non-isothermal sintering process of cerium dioxide containing gadolinium sesquioxide powders within a wide range of specific surface area was investigated by dilatometry. Linear shrinkage data of powder compacts were recorded under several constant rates of heating. Dilatometry data were analyzed by two methodologies enabling to preview the relative density for any temperature/time profile, and determination of the apparent activation energy for sintering. Correlation of dilatometry results with microstructure evolution was also carried out. Remarkable differences in sintering powders with different specific surface areas were found. The apparent activation energy for sintering increases with decreasing specific surface area and, in most cases, it does not change significantly in the approximately 70–85% range of relative density.     

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of -Al₂O₃ spinel and mullite, or simple -Al₂O₃ spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min^-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of -Al₂O₃ spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.     

Processing of ZnO-Based Varistors Using Oxide, Alkoxide, Nitrato-Alkoxide and Carboxylato Precursors

We have investigated the preparation of ZnO varistors by different chemical solution routes with the aim of improving the homogeneity of the phases formed and to obtain a better control of microstructure. One conventional oxide mixing route and four solution chemical routes have been used to prepare the precursor materials. In all cases, the same composition (mol%) was used; ZnO (95.9), Bi₂O₃ (1), Sb₂O₃ (1), NiO (1), Co₂O₃ (0.5), MnO (0.5), Cr₂O₃ (0.1). The same sintering procedure was also applied. It was found that the precursor materials consisting of ZnO grains covered by a thin film of the additive oxides yielded smaller ZnO grains. Also the microstructure in the final compacts was improved, compared with that of compacts prepared from oxide mixing routes. The smaller ZnO grain size in the final compacts was attributed to the presence of spinel grains. The spinel grains are formed at lower temperatures and, when they reach a size of 1 μm, they hinder the growth of ZnO grains.     

Investigation of sintering behavior of ZrO<Subscript>2</Subscript> (Y) ceramic green body by means of non-isothermal dilatometry and thermokinetic analysis

Using the method of dilatometry, kinetic characteristics of compacts shrinkage manufactured from ultrafine plasmochemical ZrO₂ (Y) powders and commercial Tosoh’s powders are investigated. The shrinkage curves are constructed with a DIL 402 C high-sensitivity dilatometer in a non-isothermal heating mode at different heating rates (1, 2, 5 and 10 °C min^?1). It is shown that the plasmochemical powders are characterized by a lower sintering efficiency than the Tosoh’s powders. The kinetic results are processed using a Netzsch Thermokinetics license software program developed for those who make use of devices manufactured by the Netzsch-Geratebau GmbH. The kinetic characteristics of compact shrinkage are determined as a function of partial length variation using Friedman’s analysis. Considerable differences are found between the values of apparent shrinkage activation energy for plasmochemical and Tosoh’s powders in the initial and final shrinkage stages. We attribute the mentioned differences in phase composition of the powders and their degree of agglomeration. In the intermediate shrinkage stage, the values of the apparent activation energy obtained for both types of powders have only marginal differences.     

纳晶稀土复合氧化物Dy~1~-~xSr~xCoO~3~-~yⅡ: 固相烧结动力学研究

研究了纳晶氧化物Dy~1~-~xSr~xCoO~3~-~y(x=0.6)的等温烧结动力学,计算了烧结激活能, 结果表明: 在烧结初期, 致密化机制主要为蒸发-凝聚传质; 在烧结中期, 致密化机制转为晶界控制。烧结过程的激活能为5.255×10^4J·mol^-^1。较小的激活能体现了纳米粒子的尺寸效应。快速运动的晶界, 导致晶界与气孔的运动速度不匹配, 造成纳晶氧化物密度低, 最终形成纳米海绵态网络结构。     

Kinetics of isothermal oxidation of WC–20Co hot-pressed compacts in air

The present paper presents an isothermal analysis of the oxidation behavior in which hot-pressed compacts rather than powders are used over the temperature range 700–850 °C. This was done to better simulate the extent of oxidation occurring on use. WC–Co powders were first subjected to non-isothermal kinetic analysis to follow the oxidation mechanism. In the isothermal runs, a thermobalance was used to follow up the mass with time at different constant temperatures. The diameter of compacts was measured as function of time at these temperatures, and a simple model was proposed to relate the diameters to extent of oxidation. Two reactions were found to take place that are controlled by chemical reaction at interface: Oxidation of cobalt and oxidation of WC with the formation of WO₃ and CoWO₄. The activation energies for the two steps of oxidation were calculated and found to equal 157 kJ mol^?1 and 205 kJ mol^?1, respectively. These values are in reasonable agreement with published data for WC–Co powders.     

Two-step sintering behavior of sol–gel derived nanocrystalline corundum abrasive with MgO–CaO–SiO<Subscript>2</Subscript> additions

Nanocrystalline corundum abrasive with mean crystal size of less than 100 nm was synthesised by sol–gel process via two-step sintering technique. A remarkable suppression of grain growth was achieved by controlling sintering temperature and taking advantage of sintering aids (MgO–CaO–SiO₂) during the final stage of a two-step sintering process. The grain size of the high densification samples (>99% theoretical density) produced by two-step sintering method was about seven times less than that of the samples made by the conventional sintering technique. The microstructure of the samples was homogeneous without abnormal grain growth and the obtained corundum abrasive exhibited excellent abradability compared to conventional sintering methods.     

A novel low temperature synthesis technique of sol–gel derived nanocrystalline alumina abrasive

The effect of ball milling process, co-doped seed and two step sintering technique on the properties of sol–gel derived alumina abrasive sintered at low temperature was investigated. The results showed that ball milling time with 10 h can be effective in enhancing the activity of the precursor and the microstructural uniformity of sintered alumina abrasive. A small amount of Al₂O₃–(NH₄)₃AlF₆ co-doped seed addition had potential synergistic effects for reducing α-Al₂O₃ phase transformation temperature and improving the mechanical property of alumina abrasive. A remarkable suppression of grain growth was achieved by controlling sintering temperature with two-step sintering method. Therefore, by using ball milling process, co-doping α-Al₂O₃–(NH₄)₃AlF₆ seed and two-step sintering technique, the sol–gel derived uniform nanocrystalline alumina abrasive is easily achieved at low temperature. Nanocrystalline alumina abrasive prepared at these conditions exhibited excellent mechanical properties and wear resistance compared to fused corundum abrasive and those sol–gel derived corundum abrasive with conventional sintering methods.     

The effect of calcination on morphology of phosphate coating and microstructure of sintered iron phosphated powder

Carbonyl iron powder was coated with phosphate layer using phosphating precipitation method. The phosphated powder was dried at 60 °C for 2 h in air and heat treated by calcination at 400 and 800 °C for 3 h in air. Cylindrical specimens density of ~6.5 g.cm^?3 based on iron phosphated powder calcined at 400 °C were sintered at 820, 900, 1110 °C in N₂ + 10%H₂ atmosphere and 1240 °C in vacuum for 30 min. The morphology and phase composition of the phosphate coating and sintered compacts were studied by scanning electron microscopy, atomic force microscopy (AFM) and X‐ray diffraction (XRD) analysis. Gelatinous morphology of dried phosphate coating (thickness of ~100 nm) containing nanoparticles of iron oxyhydroxides and hydrated iron phosphate was observed. From XRD, diffractogram indicated the presence of goethite α‐FeOOH, lepidocrocite γ‐FeOOH and ludlamite Fe₃(PO₄)₂.4H₂O. The calcined phosphate coating (thickness of ~ 400 nm) contained non‐homogeneous consistency of α‐Fe₂O₃ layer on iron particles, an inter‐layer of amorphous FePO₄ and Fe₃O₄ top layer. The transformation to crystalline FePO₄ structure occurred during calcination at 800 °C with the presence of α‐Fe₂O₃ forming a light top zone (rough morphology). The microstructure of compacts sintered in solid state at temperatures up to 900 °C has retained composite network character. A fundamental change in microstructure due to the liquid phase sintering occurred after sintering at temperatures of 1100 and 1240 °C. It was confirmed that the microstructure complex consists of spheroidized α‐Fe and α‐Fe₂O₃ phases surrounded by solidified liquid phase consisting various phosphate compounds. Copyright © 2013 John Wiley & Sons, Ltd.     

Improvement of magnetic properties by hot rolling of sintered powder alloy in the Fe–Cr–Co system

A method to produce a technically suitable, hard-magnetic material of composition Fe–26% Cr–16% Co–2% Mo–2% W from a powder raw material was proposed. A distinguishing feature of the method is a reduced temperature of sintering of powder compacts (1200°C). The sintered samples are subjected to hot rolling. The magnetic and strength characteristics of the thus obtained magnetic material are on the level of those of alloys produced by powder metallurgy at a sintering temperature of about 1400°C and their cast analogs. Data on the magnetic hysteresis and strength properties of the synthesized material were presented.     

The influence of different additives and the mode of their addition on the sintering behavior and the properties of semiconducting barium titanate ceramics

The influence of different additives (TiO₂, TiO₂/SiO₂, CaO/TiO₂/SiO₂, 2BaO/TiO₂/2SiO₂) and the mode of their incorporation (spray drying: series 1; mixed-oxide method: series 2) on the sintering behavior and the microstructural and electrical properties of n-doped BaTiO₃ ceramics has been investigated. The incorporation of the additives from their aqueous solution by spray drying produces a homogeneous distribution of the additive in the BaTiO₃ matrix powder by coating the BaTiO₃ particles. This homogeneous distribution significantly lowers the activation energy for the densification process compared with that of the powders of series 2. Dynamic and isothermal dilatometric measurements revealed that the sintering process is considered as a classical solid phase sintering followed by recrystallization of the BaTiO₃ matrix particles by a eutectic melt. The densification process is dominated by sliding processes. These sliding processes are caused by the amorphous layer of the additive and by the defect-rich grain boundary layers of the BaTiO₃ grains generated intermediately by diffusion processes and reactions of the matrix material with the additive forming the secondary phases Ba₂TiSi₂O₈ and Ba₄Ti₁₃O₃₀, respectively. In series 1, ceramics with a homogeneous microstructure and useful electrical properties (e.g. low resistivity at room temperature) were already produced at a sintering temperature of 1280°C (with SiO₂-containing additives) due to the homogeneous distribution of the additive. The ceramics of series 2 sintered at the same temperature could only be obtained in poor quality. At higher sintering temperatures the differences between the two series vanished.     

Investigation on the sintering behaviors of low-temperature lignite ashes

To investigate the sintering behaviors of lignite ashes, the 450 °C Xiaolongtan (XLT) and Huolinhe (HLH) lignite ash samples were analyzed by press-drop sintering technique, scanning electron microscope, and X-ray diffraction. The result shows the sintering temperature of XLT ash is lower than that of HLH, as a result of that base/acid (B/A) ratio of XLT is higher than that of HLH. The sintering temperatures of two lignite ashes under reducing atmospheres (H₂ and CO) are lower than those under oxidizing atmospheres (CO₂ and O₂), which result from the effects of different iron states under different atmospheres. The sintering temperatures of two lignite ashes decrease with the increase in pressure. It decreases slightly in the range of low pressure, changes clearly in the range of 0.7–1.0 MPa, and changes slightly again with further increase in pressure. The sintering process of lignite ashes is proposed by the presentation of partial-melting phases, the generations of aggregates, and the densification of aggregates.     

Study of dielectric and ac impedance properties of citrate-gel synthesized Li0.35Zn0.3Fe2.35O4 ferrite

Nano-crystals of Li_0.35Zn_0.3Fe_2.35O₄ ferrite have been synthesized using citrate precursor method. The sample synthesized was sintered at different temperatures in order to vary their crystallite size. The average crystallite size was found in the range 24?C57?nm by varying the temperature from 300 to 1,100?°C. X-ray diffraction measurements confirmed the formation of cubic spinel structure at all the sintering temperatures in this work. The high frequency performance of the ferrite samples were estimated by measuring the frequency dispersion of the dielectric constant, dielectric loss and ac electrical conductivity. The dielectric constant has been observed to show normal behavior with frequency and decreases with the decrease in crystallite size. It is also observed that decrease in dielectric constant depends on sintering temperature because of lithium evaporation at higher temperature. A low value of dielectric constant and dielectric loss has been found, which makes them applicable for high frequency applications by decreasing the skin effect. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of Li_0.35Zn_0.3Fe_2.35O₄ ferrite. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the prepared samples.