Study on the sintering mechanism of KNN-based lead-free piezoelectric ceramics

The sintering process and mechanism of (K, Na, Li) (Nb, Ta, Sb)O₃ (KNLNTS) solid solution were investigated. The results show that the KNLNTS forms via the reaction of A₂CO₃(A: K,Na,Li) and B₂O₅(B: Nb,Ta,Sb) at 400–800 °C. The kinetics for the formation of KNLNTS during solid-state reaction is investigated using an isothermal method. Based on the reaction kinetic isothermal analysis, KNLNTS formation is corroborated as being controlled by diffusion mechanism. It has been found that sintering densification occurs within a narrow temperature range, and the density decreases apparently when the sintering temperature slightly exceeds the optimal one. Abnormal grain growth tends to occur after reaching the maximum density and get intensified with increasing temperature. The effect of sintering temperature and time on the shrinkage of powder compacts during sintering were studied. The sintering process in the KNLNTS solid solutions may be explained by the grain boundary diffusion model inferring from the shrinkage rate of about 0.3.     

Pre-sintering densification phenomenon of YBa2Cu3O7−y in the presence of non-equilibrium Ba-Cu-O compounds

The dynamic sintering behaviour of YBa₂Cu₃O_7-y containing Ba-Cu-O and other nonequilibrium impurity phases was investigated through dilatometry. It revealed the occurrence of subsidiary shrinkage just prior to the solid-state sintering. Densification tests also showed a rapid increase in the apparent density in the temperature range between 920 and 940 °C. X-ray diffraction analysis and optical microscope observations verified the correlation with the Ba-Cu-O phases. The phenomenon was attributed to the effect of rearrangement densification exerted by the transient liquid from Ba-Cu-O and to the reaction sintering densification when Ba-Cu-O reacts with other phases during heating.     

Pressureless sintering of Si3N4

An investigation of the pressureless sintering of Si₃N₄ powder with the addition of 5 wt % MgO revealed that shrinkage by a liquid phase mechanism and bulk decomposition are two countervailing processes. Within the temperature range studied, i.e. between 1500 and 1750° C, high densities can be achieved when sintering is performed either for long periods at low temperatures or short periods at higher temperatures. A model is presented showing that pore growth due to decomposition causes a decrease in the driving force for sintering and causes shrinkage to cease.     

Physical evolution of Na-geopolymer derived from metakaolin up to 1000 °C

The thermal shrinkage and weight loss of a systematic series of geopolymers with nominal composition of NaAlO₂(SiO₂) _z · 5.5H₂O (1.15 ≤ z ≤ 2.15) made by activation of metakaolin with sodium silicate solutions are presented. The thermal behaviour of Na-geopolymers are varied, but may be categorised into four regions of behaviour exhibited by all specimens. This investigation explores the effect of nominal Si/Al on the processes and mechanisms of thermal shrinkage and weight loss throughout constant heating of Na-geopolymer. The overall thermal shrinkage of Na-geopolymer increases with increasing nominal Si/Al, with the onset temperature of structural densification occurring at lower temperature with increasing Si/Al. Thermal shrinkage is observed to result from capillary strain, dehydroxylation and viscous sintering in different temperature regions, and is explored by use of dilatometry, thermogravimetry, nitrogen porosimetry and use of different constant heating rates.     

Thermodilatometric behaviour of pure and doped ZrTiO4-SnO2

Thermodilatometric tests have been performed to investigate the shrinkage of green compacts of pure and ZnO-doped 4ZrO₂-5TiO₂-SnO₂ prepared by dry ball milling, flo-deflocculation and coprecipitation. Experiments have shown that the powder preparation procedure has a significant influence on the sintering process. Optimizing the homogeneity composition, which is enhanced from dry ball milling to coprecipitation, raises the starting sintering temperature. The reduction of the dimension of the starting particles increases the sintering rate and the addition of ZnO favours the shrinkage of the green bodies. Coprecipitated products lead to the highest final density because the evaporation of tin oxide on firing is reduced.     

Investigation in the sintering of Y2O3 powders in the temperature range 1000 to 1400° C

This paper is devoted to a study of the sintering of two Y₂O₃ powders in the temperature range where only minor densification occurs. Two powders have been examined; one powder, Y₂O₃-A, was obtained by decomposition of hydroxide, because earlier examinations showed [11] that use of this powder resulted in the highest densities of samples in the sintering temperature range from 1300 to 1900° C. The second powder, Y₂O₃-D, was purchased externally. In order to ensure that the pores in the Y₂O₃-A compacts closed as late as possible, the heating rates up to the appropriate temperatures (1000 to 1400° C) were varied in the range 0.013 to 6° C sec^–1. The results obtained show that the heating rate in this temperature range, for the powder obtained by decomposition of hydroxide, is of primary importance in the densification of the material, and that cessation of shrinkage was not observed in the period of 240 min.     

Sintering aids for ceria zirconia alloys

Copper and manganese additives for pressureless sintering of Ce-TZP have been studied, the former because its atomic radius is close to that of Ce⁴⁺, the latter because its atomic radius is close to that of Zr⁴⁺. The temperature at which shrinkage begins did not decrease when using CuO or MnO₂, but at a given temperature, the sintering rate was higher. CuO and MnO₂ were also very effective in improving both the tetragonal phase content retained at room temperature and the mechanical properties (hardness, bending strength and toughness).     

Multicomponent toughened ceramic materials obtained by reaction sintering

Fully dense zirconia toughened ceramics with a mullite matrix from the basis of information on the quaternary system ZrO₂-Al₂O₃-SiO₂-CaO, in a temperature range as low as 1425 to 1450° C, have been obtained by reaction sintering of zircon/alumina/calcium carbonate mixtures. The shrinkage, advance of reaction, microstructure and mechanical properties of different compositions are reported. The results are explained in terms of a transitory liquid phase that appears at temperatures lower than 1400° C.     

Sintering study of calcium aluminate

Measurement of the initial sintering shrinkage of CaAl₂O₄ at temperatures of 1300, 1325 and 1350 °C are reported. The particle sizes chosen were –53 + 45, –63 + 53 and –75 + 63 microns and the soaking periods were from 15 to 360 min. A time dependence of the shrinkage has shown that volume diffusion is the dominant mechanism of sintering. At any given time and temperature, the per cent shrinkage was found to be a decreasing function of particle size. The activation energy for the sintering of CaAl₂O₄ was found to be 766.38 KJ mol^–1.     

Sintering and microstructural studies in the system ZrO2 · TiO2 · CeO2

Tetragonal zirconia polycrystals (TZP) in the system ZrO₂ · TiO₂ · CeO₂ have been prepared from titanium and zirconium alkoxides and cerium nitrate precursors. The change in microstructure with sintering temperature in the range 1300 to 1600° C has been characterized. A fully tetragonal structure with theoretical final density has been achieved after liquid-phase sintering in the range 1350 to 1400° C for 2h. Sintering at temperatures above 1450° C resulted in a loss of stabilizer from the matrix, by the formation of zirconium titanate and to a cerium-, titanium-rich liquid. The loss of stabilizer was such that in the temperature range 1500 to 1600° C, extensive transformation to monoclinic zirconia occurred spontaneously on cooling. The tetragonal zirconia formed after sintering at 1350° C was found to be very stable. Thec/a ratio of the tetragonal phase in this system is higher than in any of the binary TZP systems reported in the literature. The stability of the tetragonal phase is believed to be associated with this highc/a ratio.     

Effect of sintering on TiO2-impregnated alumina foams

The effect of sintering on the bulk properties, morphology and phase composition of ultralight Al₂O₃ foams impregnated with TiO₂ was investigated in comparison with pure alumina foam in the temperature range of 900–1600°C in air. Impregnation was carried out by immersion of pre-sintered alumina foam in a sol of titanium isopropoxide-acetylacetone complex. The changes of the foam linear shrinkage, effective density and porosity were studied along with morphological evolution and relationship between these properties was demonstrated. Titania impregnation increased the linear shrinkage (LS) during sintering by a maximum of 5% relative to pure alumina foams. The change of LS and weight loss of TiO₂/Al₂O₃ foams lead to a final density of 0.19 g/cm³ and porosity of 95%. The initial coating was found to develop a mosaic structure due to early shrinkage of the coating. After sintering at 1600°C the coating reacted with the underlying Al₂O₃ surface and became uniformly distributed. Finally, it was shown that the reacting TiO₂ layer formed the tialite (Al₂TiO₅) phase below 1400°C. This Tialite coating remained intact under 1200°C without stabilizers.     

The sintering of combustion-synthesized titanium diboride

A comparative study of the sinterability of combustion-synthesized titanium diboride was conducted over the temperature range of 1800 to 2100° C. During the initia! sintering stage, the densification rate was slightly higher in the combustion-synthesized than in the commercially obtained titanium diboride. For sintering times of > 30 min, however, the shrinkage rates for both types of powders were the same. The activation energy for the late sintering stage was 774 ± 46 kJ mol^–1, consistent with 8 volume diffusion mechanism, end was the same for both combustion-synthesized and commercial powders. The microstructures of sintered specimens with initial particle size below 1O µm exhibited a grain size ranging from 5 to over 40 µm after 30 min of sintering. The addition of 5 wt% NbB₂ to the combustion-synthesized resulted in enhanced shrinkage during the initial sintering stage, but did not affect later stage kinetics. Various amounts of additives of CrB₂, NiB and TiC had no effect on early and late stage sintering kinetics, with the exception of 50 wt% TiC which appreciably inhibited densification.     

Co-firing preparation and properties of piezoelectric ceramic/structural ceramics layered composites

Pb(Zr, Ti)O₃(PZT)-based piezoelectric ceramics and Al₂O₃-based structural ceramics were cast and co-fired to prepare a layered piezoelectric ceramic/structural ceramic composite. Considering the significant differences in sintering characteristics of PZT- and Al₂O_3-based ceramics, control of the sintering temperature and the dependence of the linear shrinkage on the solid content of the tape-casting films were systematically conducted at first. The sintering density and the interface bonding properties of the prepared composites were then investigated. The results of electrical and mechanical properties of the composite ceramics indicate: By using sintering aids, Al₂O₃ ceramic could be fully densified and co-fired with PZT ceramic at 1150 °C. Shrinkage matching during sintering was achieved by adjusting the solid contents to 45 vol.% and 65 vol.% for PZT and alumina tape-casting films. In the layered composites, Al₂O₃ structural ceramic presents an excellent mechanical property with HV hardness of 667, while the PZT functional ceramic presents d₃₃, ε_r and tanδ of 259 pC/N, 965 and 0.37%, respectively.

     

Sintering mechanisms of 0.99 SnO2-0.01 CuO mixtures

The densification kinetics of 0.99 SnO₂-0.01 CuO molar mixtures have been studied between 850 and 1150 °C. Both experimental analyses and theoretical modelling show the role of a liquid phase for sintering temperatures T _s 940 °C: sintering is controlled by a liquid phase after a fast shrinkage due to grain rearrangement. The analytical formulation of the shrinkage behaviour suggests that dissolution at the solid-liquid interface is the limiting process when T _s 1000 °C and diffusion is the limiting step at higher temperatures.     

Effect of iron & boron content on the Spark Plasma Sintering of Ti-B-Fe alloys

The effect of a change in iron and boron contents on the Spark Plasma Sintering behaviour in the Ti-B-Fe alloys has been investigated through dilatometric sintering study using various samples having Ti-rich compositions. These samples are adequately chosen to study the sintering behaviour on the addition of boron and iron on the Ti-rich corner. The rate of sintering of Ti-B-Fe alloys was found to increase with an increase in iron content. The sintering enhancement can be attributed to the high diffusivity of iron in α-Ti and β-Ti phases and due to the formation of the intermetallic phase which results in greater overall shrinkage. The shrinkage was found to decrease with increase in boron content due to a greater amount of TiB₂ which doesn’t participate in sintering in the observed range.     

Enhanced thermal stability in K<Subscript>2</Subscript>O-metakaolin-based geopolymer concretes by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> fillers addition

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K₂O–Al₂O₃–SiO₂ system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.     

Studies on sintering behaviour of Al2O3–ZrO2 oxide composites processed by extended arc thermal plasma and conventional heating

Al₂O₃–ZrO₂ composites were sintered using low cost extended arc thermal plasma reactor and conventional heating. Composites prepared in a wide range of composition were studied in terms of their density, shrinkage, hardness, structure, microstructure and dielectric response. Experimental parameter such as sintering time, sintering temperature and plasma power were optimized to achieve higher sintered end product. Highly dense sintered products were obtained by plasma heating route within short sintering time compared with conventional sintered method. Interesting development pertaining to structure and phase evolution, structure and dielectric response are analyzed. It is found that compositional variation in this composite produces structural phase separation at different sintering conditions, which is more in plasma heating product than conventional heated product. Plasma sintered product always shows less dielectric constant as compared to conventional sintered sample.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO₂-Al₂O₃-SiO₂-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at 1425° C and ~ 1450° C respectively.     

Compositionally graded ceramics based on Ba<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> solid solutions

Homogeneous and compositionally graded barium strontium titanate (Ba_{1 − x} Sr _x TiO₃) ceramics have been prepared in the form of thick films by slip casting. The strontium content was varied across the graded films from 0 to 30 mol %. Microstructural examination and dielectric measurements have shown that, varying the composition and thickness of the layers in graded structures and the sintering conditions, one can control the behavior of their dielectric characteristics and obtain a small temperature coefficient of capacitance in a predetermined temperature range.     

Influence of stoichiometry and impurity on the sintering behaviour of barium titanate ceramics

The influence of stoichiometry, i.e. Ba/Ti ratio, and impurity on the densification of BaTiO₃ were investigated. The BaTiO₃ powders were prepared by conventional calcination of BaCO₃ and TiO₂. The stoichiometric ratios (Ba/Ti) were in the range 0.99 to 1.005. Impurity effects on the sintering behaviour were investigated with different purities of raw powders. The sintering behaviour of BaTiO₃ has been studied extensively but an understanding of stoichiometric effects on densification is still incomplete. An excess of TiO₂ lowered the onset temperature of sintering (initial state of sintering — 3% shrinkage). These results indicate that stoichiometric variation of BaTiO₃ affects the initial state of sintering. The rate of densification for a Ti-rich sample was considerably faster than that for a Ba-rich sample. It was a so-called activated sintering. The TiO₂ excess reacts with BaTiO₃ to form Ba₆Ti₁₇O₄₀, which forms with BaTiO₃ a eutectic melt at 1320 °C. The liquid phase, however, enhanced grain growth, not densification.