Post-sintering Annealing Research Articles

Giant dielectric behaviour of CaCu3Ti4O12 subjected to post-sintering annealing and uniaxial stress

This paper reports the influences of the post-sintering annealing in argonand uniaxial compressive pre-stress on the giant dielectric properties of theCaCu3Ti4O12 ceramics sinteredat 1100 °C in air for 6 and16 h. The CaCu3Ti4O12 ceramicsintered at 1100 °C for6 h exhibited high ε′ of ∼1 × 104 whereasthe CaCu3Ti4O12 ceramicsintered at 1100 °C for 16 h possessed one order of magnitude higher dielectric constant(ε′∼2 × 104).The dielectric behaviour of both samples exhibits Debye-like relaxation, and can be explained basedon a Maxwell–Wagner model. Post-sintering annealing in argon for 5 h leads to a significant increase inε′ forCaCu3Ti4O12 ceramic sinteredat 1100 °C for 16 h but aslight decrease in ε′ for the CaCu3Ti4O12 ceramicsintered at 1100 °C for6 h. The ε′ of the16 h sintered CaCu3Ti4O12 ceramic after annealing in argon increases with increasing temperatures, and exhibits a peak at about150 °C, which is closely related to the oxygen vacancies. The dielectric behaviour of thisargon-annealed sample follows the UDR law. The dielectric properties of theargon-annealed samples change significantly with the applied compressive stress (theabsolute change can reach 25% at a maximum stress of 130 MPa). However, the changes indielectric properties with the stress in the samples subjected to different sintering timesfollow opposing trends. The mechanisms responsible for this difference are discussed.

Oxygen-vacancy-related dielectric anomaly inCaCu3Ti4O12: Post-sintering annealing studies

The influence of post-sintering annealing on the dielectric properties of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ was investigated in the temperature range form $200\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A dielectric peak featuring thermally activated Debye-like behavior was observed around $340\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ $(100\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$. This peak can be eliminated by annealing in oxidizing atmosphere and created by annealing in reducing atmosphere. The dielectric peak was explained in terms of the combinational contributions from the $n$- and $p$-type carriers.

Effect of annealing conditions and concentration of oxygen vacancies on vanadium doped SrBi 2Ta 2O 9

This paper studied the annealing effects on the dielectric characteristics of vanadium doped SrBi 2Ta 2O 9 (SBT). SBT was synthesized at 1000 °C and vanadium doped SBT at 900 °C by solid-state reaction. Crystallization structure and phase purity of the prepared ceramic samples was observed by X-ray diffraction analysis. XRD analysis indicated a single layered perovskite structure without any secondary phases up to 15% of vanadium doping in SBT ceramics. Detailed dielectric study on vanadium doped SBT ceramics indicated that post-sinter annealing enhances the peak dielectric permittivity, which is attributed to the increased homogeneity in the system at atomic scale upon annealing. Annealing for larger time interval suppresses the permittivity growth beyond transition temperature which gives a direct evidence for the existence of lower valance state of vanadium (V +4) in as-sintered SBTV ceramics and also the permittivity growth is related to the oxygen ions or oxygen vacancies created during sintering. UV–Vis spectroscopy was also performed to confirm the lower valance state of the vanadium ions in the ceramics.

Low-voltage varistors from ZnO+CaMnO3 ceramics

Varistors prepared from ZnO with CaMnO3 perovskite as the only forming additive, exhibit voltage-limiting current-voltage characteristics with nonlinearity coefficient α up to 380 at low voltages of 1.8–12V∕mm. High nonlinearity is observed only with a suitable combination of processing parameters. The most crucial of them are (i) initial formulation of ceramics and (ii) the sintering temperature and conditions of post-sinter annealing. An electrically active intergranular phase is formed between ZnO grains with the composition ranging from Ca4Mn6Zn4O17 to Ca4Mn8Zn3O19, which creates the n-p-n heterojunctions. The low-voltage nonlinearity originates as a result of higher concentration of Mn(III)∕Mn(IV) present at the grain boundary layer regions, being charge compensated by zinc vacancies. Under the external electric field, the barrier height is lowered due to the uphill diffusion of holes mediated by the acceptor states. Above the turn-on voltages, the unhindered transport of charge carriers between grains generates high current density associated with large nonlinearity.

Fast firing of lead zirconate titanate ceramics at low temperature

A fast firing technique for densification of PZT (up to 97% of theoretical density) at a low temperature (950 °C) has been found out. A small amount of excess PbO (3–5 wt.%) and a fast firing schedule are required to achieve the desired sintering. The final composition of the sintered ceramics can be kept close to the morphotropic phase boundary by modifying the firing time and post-sintering annealing treatment at 800 °C. The g 33 values of fast fired samples are comparable to those of conventionally sintered samples, though the d 33 values are somewhat lower than those reported for conventionally sintered (at 1250 °C or above) samples.

Gas pressure sintering of SiC sintered with rare-earth-(III)-oxides and their mechanical properties

The sintering behaviour of LPS-SiC and the influence of the size of the rare-earth cations on the secondary phase characteristics were investigated with different rare-earth oxide additions. In all cases, the most important sintering mechanism was found to be the solution-reprecipitation process. This fact was corroborated by TEM and EDS analyses. Post-sintering annealing resulted in devitrification of the secondary phases coupled with anisotropic grain growth due to the phase transformation from β-SiC to α-SiC. Improved fracture toughness of the annealed materials was attributed to crack deflection by the elongated grains. SEM microstructural analyses were performed in order to elucidate structure–property relationships. A comparative study in reference to the additive system Y 2O 3–AlN demonstrates significantly improved high-temperature properties of the Lu 2O 3-containing SiC ceramics.

Origin of the Superlattice Reflections in Pb(Fe2/3W1/3)O3

Two types of superlattice reflections have been observed in pressureless-sintered Pb(Fe 2/3 W 1/3 )O 3 perovskite ceramics from selected-area electron diffraction patterns (SADPs). The presence of commensurate superlattice reflections of the {1/2 1/2 1/2} type found in as-sintered samples persisted after prolonged postsintering annealing at 800°C for 300 h. These reflections, conventionally termed the F-spots, represent the existence of the nanoscale cation-ordered domains of cubic perovskite Pb 1-x (Fe 1/2 W 1/2 )O 3 . However, the satellite-type incommensurate superlattice reflections, appearing as sideband spots to the fundamental reflections and occurring only along , (110), and (100), have gradually reduced their intensities and finally disappeared on postsintering annealing at 800°C. Observations via transmission electron microscopy (TEM) suggested that the black dots revealed in bright-field (BF) images have given rise to the incommensurate superlattice sideband spots. These dots of ∼5-10 nm are in fact a different type of nanocrystalline domain whose structure (and chemical composition) differs from those of the disordered Pb(Fe 2/3 W 1/3 )O 3 matrix and the ordered nanoscale Pb 1-x (Fe 1/2 W 1/2 )O 3 domains. The nanocrystalline precipitates containing the paracrystalline phase are pyrochlore (Pb 2 (Fe,W)O 6.5 ) formed during powder preparation via the solid-state mixed-oxide route. They become dispersed in the Pb(Fe 2/3 W 1/3 )O 3 perovskite matrix grain and have also undergone phase transformation to perovskite progressively during sintering and postsintering annealing. It is suggested that the nanocrystalline pyrochlore precipitates are metastable and disappear when chemical composition within the perovskite grains becomes homogenized from solid-state diffusion on postsintering annealing.

Influence of post-sintering annealing on relaxor behaviour of (Pb 0.75Ba 0.25)(Zr 0.70Ti 0.30)O 3 ceramics

The dielectric and relaxor properties of lead–barium–zirconate–titanate (PBZT) ceramics of composition Ba/Zr/Ti:25/70/30 has been studied as-prepared (unannealed) as well as after annealing at 1000 °C for 20, 40 and 60 min, respectively. After the annealing treatment the Pb deficiency for the studied ceramics was found. The influence of a post-sintering annealing on the values of characteristic parameters describing a relaxor behaviour of the studied ceramics ( ɛ′ max, T m, T f, T B) was determined. The temperature T m shifts towards the lower value and the dielectric constant increases after annealing. The general trend seems to indicate that the annealing broadens the temperature region of the specific relaxor properties occurrence.

Post-Sinter Annealing of Pb0.7La0.2TiO3 Derived from Mechanical Activation

To investigate the relationships between the dielectric behaviors and defect structures in Pb0.7La0.2TiO3, such as A-site vacancies and oxygen vacancies, post-sinter annealing in oxygen and nitrogen were conducted. Upon initial annealing in oxygen, space charge polarization, manifesting with A-site vacancies created by PbO evaporation from the surface region, dominates the dielectric properties of Pb0.7La0.2TiO3. Prolonged annealing in oxygen, for example, for more than 4 hours, led to a steady fall in both dielectric constant and dielectric loss. Annealing in nitrogen resulted in a similar rise in the dielectric properties at the initial stage, followed by a slow down in the increase rate at extended duration of annealing, when the perovskite structure is destabilized in combination with the interactions between oxygen vacancies and A-site vacancies.

Effects of post-sintering annealing on magnetic properties of Nd–Fe–B sintered magnets

Dependence of the coercive force of (Nd, Dy)–Fe–B magnets on annealing time, temperature and the order of annealing step were investigated. The two schemes of heat treatment were used: HT1—cooling at 925°C to 20°C before annealing and HT2—cooling at 925°C to annealing temperature. The annealing temperature dependences of H ci for HT1 and HT2 are substantially different. These dependences reveal three phase's transformations in microstructure of magnets, which corresponded the maxima of H ci around 450°C, 535°C, 590°C. The correlation between annealing behavior of H ci and hypothetic metastable Fe–Nd diagram was found. We supposed that below eutectic point at 640°C (L⇌A+Nd) the series of consecutive eutectoid decompositions of A phase take place. The origin of new Fe-rich phases occurs preferentially at the surface of Nd 2Fe 14B grains. The size, morphology and number of initial precipitates are determined by temperature of annealing. At high temperature the separate large precipitates form “roughness” on the surfaces of Nd 2Fe 14B grains and decrease H ci. At low temperature continuous thin layers make “smoothing” better and increase H ci.

Postsintering annealing induced extrinsic dielectric and piezoelectric responses in lead–zinc–niobate-based ferroelectric ceramics

The effect of postsintering annealing on the dielectric and piezoelectric responses in lead–zinc–niobate (PZN)-based ferroelectric ceramics was systematically studied in this work. Sintered PZN ceramics with the same composition were annealed at temperatures from 700 to 900 °C from 30 min to 24 h in various atmospheres. As a result, significant improvements in the dielectric and piezoelectric responses were observed in this system. After annealing, there was a huge increase of up to 130% in the dielectric constants, especially near the temperature of maximum dielectric constant. While the dielectric peaks became sharper and less diffuse, there was almost no change in frequency dispersion. Similar large improvements in the piezoelectric properties such as the coupling factor and piezoelectric constant were also observed. Optimized parameters, such as d33=642 pC/N, d31=−256 pC/N, kp=48% and εm=28 000, were achieved by annealing at 850 °C in an O2-rich atmosphere for 4–8 h. The observed improvement in the electrical properties can be attributed to an extrinsic contribution induced by domain wall motion. After annealing, the pinning effects caused by oxygen vacancies and internal stress on domain wall motion were largely reduced or eliminated, therefore a significantly reduced coercive field, an increased polarization level, and a greatly increased dielectric constant were observed in the PZN-based ferroelectric ceramics.

Evidence of lower valence state of vanadium on the dielectric relaxation of ferroelectric SrBi2 (V0.1Nb0.9)2O9

Dielectric properties of SrBi2(V0.1Nb0.9)2O9 (SBVN) were investigated in a broad range of temperatures (400–1000 K) and frequencies (1 Hz to 10 MHz). Strong dielectric relaxation at the ferroelectric transition temperature was observed. The effect of post-sinter annealing on the frequency dispersion of SBVN and the possible mechanisms for the observed dielectric relaxation are presented. Electron paramagnetic resonance and x-ray photoelectron spectroscopy investigations provide direct evidence for the existence of lower valence state of vanadium (V4+) in SBVN ceramics.

Dielectric relaxation in SrBi 2(V 0.1Nb 0.9) 2O 9 layered perovskite ceramics

Strontium bismuth niobate vanadates SrBi 2(V 0.1Nb 0.9) 2O 9 (SBVN) were prepared by solid state reaction sintering of high purity powder mixtures of constituent oxides. With the partial substitution of niobium by vanadium (10 at.%), the single phase layered perovskite structure was preserved and the sintering temperature of SBVN was significantly reduced. Dielectric properties of SBVN were investigated in a broad range of temperatures (400–1000 K) and frequencies (1 Hz–10 MHz). Strong relaxor-like dielectric relaxation at the ferroelectric transition temperature was observed. X-ray diffraction and scanning electron microscopy analysis were performed to envisage the influence of vanadium doping on the dielectric properties of SBN. The effect of post-sinter annealing on the frequency dispersion of SBVN and the possible mechanism for the observed dielectric relaxation are also presented.

Hexabarium 17-titanate in pressureless-sintered TiO 2-excess barium titanate

The microstructure of TiO 2-excess BaTiO 3 ceramics sintered by using a CO 2-laser and conventional furnace has been analysed via scanning and transmission electron microscopy. Second phases located at the triple-grain junctions and along the grain boundaries of laser-sintered samples contain monoclinic-Ba 6Ti 17O 40. They exhibit a characteristic lamellar structure arising from the solidification of the BaTiO 3–Ba 6Ti 17O 40 eutectic generated at ≈1332 °C upon fast cooling. This microstructure can be taken as the diagnostic feature to distinguish between Ba 6Ti 17O 40 formed by solid-state reaction and that by eutectic solidification. A self-consistent crystallographic orientation relationship of (001) t//(101) m, (1 1 ̄ 1) t /(001) m , (1 1 ̄ 0) t //( 1 ̄ 03) m , (1 1 ̄ 1 ̄ ) t //( 1 ̄ 01) m and [010] m //[ 1 ̄ 1 ̄ 0] t between tetragonal-BaTiO 3 and monoclinic-Ba 6Ti 17O 40 has been found. The monoclinic Ba 6Ti 17O 40 is stable below the eutectic temperature of 1332 °C after post-sintering annealing at 1000 °C for 100 h.

A correlation between intrinsic coercivity–electrical conductivity–thermal treatment in a Nd 11.9MM 2.9Fe 73.9Co 3.3Ni 1.1B 6.9-type magnet

In this work, the effects of sintering temperature, cooling rate after sintering and post-sintering annealing treatments on the microstructure and magnetic properties of a Nd 11.9MM 2.9Fe 73.9Co 3.3Ni 1.1B 6.9-type magnet have been investigated. It was found that the best magnetic properties were only obtained at a narrow range of sintering temperature at 1333–1353 K. Porosity was found to be minimized with increasing sintering temperature up to 1333 K. However, some grain growth occurred by increasing the sintering temperature from 1333 to 1353 K. Further increase of sintering temperature resulted in a significant decrease in the magnetic properties due to appearance of α-iron in the microstructure of the magnets and/or grain growth. The dependence of intrinsic coercivity ( H cJ) on the cooling rate (after sintering) and post-sintering annealing treatments has been related to electrical conductivity (EC) of the magnets; thus an increase in the EC resulted in a significant improvement in the H cJ values. This could be due to reduction of residual stresses within the specimens during slow cooling after sintering/post-sintering annealing treatments. The compatibility between the variations of the H cJ and EC with the cooling rates (after sintering) or a post-sintering annealing treatment at around 873 K confirms this mater.

Oxygen-vacancy-related dielectric anomalies in La:SrTiO3

The influence of post-sintering annealing on the dielectric relaxation behavior was studied in (Sr1−1.5xLax)TiO3 in the temperature range of 1.5–300 K. The results show that the dielectric permittivity peaks with frequency dispersion can be gradually and absolutely eliminated by annealing in an oxidizing atmosphere, and recovered by annealing in a reducing atmosphere; this implies that these anomalies are closely related to oxygen vacancies.

Stability of α-Sialons in Low Temperature Annealing

Phase and microstructural change due to post-sintering annealing of Nd, Dy, Y and Yb-α-sialon ceramics have been investigated for the composition of α-sialon-rare earth aluminum garnet (RAG) systems. For R= Dy, Y and Yb, only α-sialon was observed as the crystalline phase after sintering at 1750°C. After subsequent post-sintering annealing at 1450°C for 72 h, no phase transformation from α-to β-sialon was observed, although the crystallization of garnet phase at grain boundaries was detected. However, for R=Nd, α-,β-sialon and melilite phase (M’) were observed after sintering at 1750°C. The β-sialon and melilite phases increased with low-temperature heat treatment, implying that the transformation from α- to β-sialon occurred. It was concluded that the α-sialon phase stabilized by small ions like Y, Dy or Yb has high thermal stability and does not transform to β-sialon when there is no chemical reaction between α-sialon grains and the grain boundary phase.

Variable structural ordering in lead scandium tantalate-lead titanate materials

A phase region of variable order-disorder [VOD] has been identified in this study for compositions in the range [x- 0.0–0.075] (x: mole fraction PbTiO3). Materials from this region retain some degree of long-range ordering of 5-site cations as evidenced by x-ray and electron diffraction which is enhanced with post-sintering annealing. Annealed ceramics exhibit sharp dielectric responses [K(T] and near-normal first-order ferroelectric behavior while as-fired specimens with lesser degrees of long-range ordering show the more diffuse and dispersive characteristics of a relaxor ferroelectric.

Fast firing of lead magnesium niobate at low temperature

A fast-firing technique for the sintering of lead magnesium niobate relaxor ceramics at relatively low temperature has been described. In this process, the samples containing excess PbO (up to 5 wt. %) are directly introduced into a furnace maintained at a temperature of 950 °C and kept there for 15–80 min, followed by a postsintering annealing treatment at 800 °C for 10 h. The importance of fast heating as well as annealing treatment has been justified. The sintered samples are near-phase-pure perovskite materials showing high bulk densities (>94%), uniform and dense microstructure, and satisfactory dielectric properties (Kmax > 13,000). The technique is simple and economic, does not require any controlled atmosphere, and minimizes hazards from lead volatilization.

Influence of Bi3+ ions in enhancing the magnitude of positive temperature coefficients of resistance in n-BaTiO3 ceramics

Bi3+ ions substituting at Ba-sites in a limited concentration range with another donor dopant occupying the Ti-sites in polycrystalline BaTiO3 enhanced the positive temperature coefficient of resistance (PTCR) by over seven orders of magnitude. These ceramics did not require normal post sinter annealing or a change to an oxygen atmosphere during annealing. These ceramics had low porosities coupled with better stabilities to large applied electric fields and chemically reducing atmospheres. Bi3+ ions limited the grain growth to less than 8 mum in size, they enhanced the concentration of acceptor-type trap centres at the grain-boundary-layer regions and maintained complete tetragonality at low grain sizes in BaTiO3 ceramics.