Three-step Sintering Research Articles

The efficacy of Bi0.46Y0.04K0.5ZrO3 doped three-step sintering to accomplish high piezoelectric and ferroelectric properties in new lead-free NaNbO3-based materials

The novel (1-x) NaNbO3-xBi0.46Y0·04K0·5ZrO3(x = 0.06, 0.09, 0.12 & 0.15), a polycrystalline lead-free ceramic composite made by the traditional solid-state technique, has been extensively studied for its multi-application in the three-step sintering process. The Rietveld refinement of the diffraction pattern shows a perovskite structure with a Pmc21 space group between 0.06 and 0.15. Scherrer's formula and Williamson-Hall plots are used to calculate crystallite size and strain. The Raman spectra have absorbed many vibrational spectral lines and moved to a lower frequency between 100 and 800 cm−1. FE-SEM and TEM images indicate well-defined and diverse grout morphology. The diffused absorbance spectra determine the band structure values of the prepared samples, which are found to be 3.16 eV–3.36 eV in the UV–Vis spectrum. Photoluminescence spectra exhibit the emission bands in the green (522 nm), yellow (575 nm), and red (693 nm) wavelength ranges. Dielectric data have lower symmetry at temperatures ranging from 103 to 106 Hz. Increasing the doping concentration of the material not only strengthens the ferroelectric and piezoelectric coefficient properties, but also enhances the voltage constant g33(49–88Vm/N) and the transduction coefficient d33× g33(22135 ×10−15Jm3). These improvements make the material suitable for applications requiring efficient energy conversion and sensing capabilities.

CHARACTERIZATION AND PERFORMANCE IMPROVEMENT OF SiC-REINFORCED Cu-MATRIX-BASED COMPOSITES AS ELECTRODE FOR EDM MACHINING

Pure copper, copper-based alloys, brass, graphite and steel are the most common tool materials for the electrical discharge machining (EDM) process. The electrode material, which exhibits good conductivity to heat and electricity and possesses better mechanical and thermal properties, is preferred for EDM applications. The major problem with conventional electrodes like copper and graphite is their low wear resistance capacity. This study is on the fabrication of Cu–SiC composites as an electrode of EDM with improved wear characteristics for machining of hardened D2 steel which is widely used in die formation. Powder metallurgy route was used to fabricate the samples which was further followed by three-step sintering process. Copper metal was used as a matrix element which was reinforced with SiC in volume fractions of 10%, 15% and 20%. Based on the desirable properties for the EDM tool, the best composition of Cu–SiC composite tool tip was suggested and was further used for machining of D2 steel. The performance of newly developed Cu–SiC composite electrode in terms of surface roughness (SR), material removal rate (MRR) and tool wear rate (TWR) was explored, and it was compared with the pure copper electrode. Pulse on-time, pulse off-time and the input current were selected as the input process parameters. Result reveals that the TWR and SR were decreased by 12–18% and 10–12%, whereas the MRR was increased by 9–28% for Cu–SiC composite tool as compared to the pure Cu electrode. Adequacy of the results was checked by statistical analysis. The surface texture of tool and machined surface was analyzed using scanning electron microscopy (SEM). SEM micrograph revealed that surface cracks on composite tool tip were lesser than pure Cu tool tip, whereas the work surface was rough while machining with the copper tool surface. Therefore, the result indicates that the newly developed Cu–SiC composite tool can be used for machining of hard materials with EDM.

Tunable ferroelectric properties in (K0.5Na0.5)NbO3 ceramics fabricated by a simple modification of the conventional sintering technique

ABSTRACT In this work, a modified sintering method, i.e. the three-step sintering technology, has been applied to fabricate the K0.5Na0.5NbO3 ceramics. The impact of the detailed sintering conditions on the sinterability, dielectric and ferroelectric properties of K0.5Na0.5NbO3 is systematically investigated. Attributed to the three-step sintering, the highest density of 4.07 g cm−3 can be observed, which is strongly improved compared with that of the single-step sintering technique and is increased up to 90% of the theoretical density. Furthermore, through the analysis on the tunability of the dielectric and ferroelectric performances, it is found that the maxima of permittivity and polarisation can be obtained in K0.5Na0.5NbO3 sintering at 1160–1220–1120°C. These results illustrate that both the dielectric and ferroelectric behaviours can be effectively modulated through the simple modification of sintering technique, which would provide a novel route to achieve tunable electrical properties in ferroelectrics.

Enhanced properties of KNLN–BZ lead-free piezoelectric ceramics via three-step sintering

Enhanced properties of KNLN–BZ lead-free piezoelectric ceramics via three-step sintering

Enhanced Piezoelectric Properties of Pb0.976K0.012Bi0.012[(Zr0.53 Ti0.47)0.99 Nb0.01]O3 Ceramic Tape Casting by Three Step Sintering

A problem of fabricating piezoelectric tape ceramic is poor density after sintering. Three-step sintering (3ss) techniques were prepared to obtain tape casting ceramic with density and piezoelectric properties better than conventional sintering (CS). Pb0.976K0.012 Bi0.012[(Zr0.53 Ti0.47)0.99 Nb0.01]O3 tape ceramic was synthesized using tape casting accompanied with the sintering techniques. The green tapes were sintered on three temperature steps that are 870, 970, and 1070 °C, respectively, and compared with two temperature steps that are 870, and 1070 °C. The optimum density, dielectric properties, polarization electric field loop, piezoelectric coefficient (d33), was attained by three step sintering. The enhanced remanent polarization of 44 μC cm−1 and the recoverable energy storage density was characterized on P-E loop. In addition, d33 value of 226 pC N −1, and kp of 0.36 was enhanced to attain for the tape ceramic on 3SS.

Low-temperature sintering of KNN-based lead free ceramics

The technology of low-temperature sintering is significantly essential for the preparation of ceramic multilayer actuator. CuO-(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3–CaZrO3 lead-free piezoelectric ceramics were synthesized via a novel three-step sintering approach at low temperature. Through a three-step sintering approach, the sintering temperature in the KNN based lead-free ceramics system was effectively reduced to 700 °C. All piezoceramics possess a typical perovskite structure without a secondary phase. At a sintering temperature of 700 °C, the good piezoelectric property of d33= 238 pC/N was obtained.

Effects of CuO on KNN-based ceramics

The sintering aid, CuO was added to (K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-CaZrO3 piezoceramics, and which were prepared by three-step sintering processing to industrialize the lead-free piezoelectric ceramic products. Effects of CuO on crystal structure and properties were investigated. Through a three-step sintering method, the sintering temperature of the KNN based lead-free ceramics system was effectively reduced to 700 °C. All piezoceramics possess a typical perovskite structure without a secondary phase. At a sintering temperature of 700 °C, the good piezoelectric property of d33 = 238 pC/N was obtained.

Effect of three step sintering on piezoelectric properties of KNN-based lead free ceramics

Four different KNN-based piezoceramics were prepared using a three-step sintering method. The crystal structures prepared by this three-step sintering were compared to those from conventional sintering and the different tolerance factors for the dopant effects on the piezoelectric properties were evaluated. The piezoelectric properties for the 0.96 (K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04CaZrO3 ternary system prepared with the three-step sintering method were obtained: the d33 = 420 pC/N, strain levels of 0.18% at 40 kV/cm, and the piezoelectric constant decreased with an increase in the tolerance factor.

Porosity reduction in inkjet-printed copper film by progressive sintering on nanoparticles

This work demonstrates a progressive three-step sintering for inkjet-printed copper (Cu) nanoparticles. For the inkjet-printed Cu thin films, the proposed processes of sequential low-pressure drying, near-infrared sintering, and intense pulsed light (IPL) reduction ensure high nanoparticle compactness, complete Cu sintering, and low oxygen content, respectively. Experiments showed that the highest Cu conductivity resulted from a combination of 65.7% improvement on surface roughness, 37.3% porosity reduction, and 91.7% oxygen elimination provided by the proposed method. Enhanced adhesion between the Cu and the substrate confirmed by mechanical examinations indicated other benefits of this progressive sintering. In addition, the cracks that resulted from different thermal expansions in the Cu thin film and the substrate during IPL reduction were quantified, and the results explained the degraded electrical performance of the Cu thin films that had been processed beyond the optimal condition.

Relation between the microstructure and the electromagnetic properties of BaTiO3/Ni0.5Zn0.5Fe2O4 ceramic composite

The microstructure–property relation in ferroelectric/ferromagnetic composite is investigated in detail, exemplified by typical sol–gel-derived 0.3BTO/0.7NZFO ceramic composite. The effect of microstructural factors including intergrain connectivity, grain size and interfaces on the dielectric and magnetic properties of the composite prepared by conventional ceramic method and three-step sintering method is discussed both experimentally and theoretically. It reveals that the dielectric behavior of the composite is controlled by a hybrid dielectric process that combines the contribution of Debye-like dipoles and Maxwell–Wagner (M–W or interfacial) polarization. Enhanced dielectric, magnetic and conductive behaviors appear in the composite with better intergrain connectivity and larger grain size derived by sol–gel route and three-step sintering method. The effective permittivity contributed by Debye-like dipoles exhibits a value of ~130,000 in three-step sintered composite, which is almost the same as that in conventionally sintered one, but that contributed by M–W response is much smaller in the former. Compared with conventionally prepared samples, the relaxation time (τ) is 3.476 × 10−6 s, about one order of magnitude smaller, and the dc electrical conductivity is 3.890 × 10−3 S/m, one order of magnitude higher in three-step sintered composite. The minimum dielectric loss reveals almost the same (~0.2) for all samples, but shows distinguishable difference in low-frequency region. Meanwhile, an initial permeability of 84, twice as large as that of conventionally prepared composite and 56 % the value of single-phased NZFO ferrite (~150), and a saturation magnetization of 63.5 emu/g, 32 % higher than that of conventional one and approximately 84 % the value of single-phased NZFO ferrite (~76 emu/g), appear simultaneously in three-step sintered composite with larger grain size and better intergrain connectivity. It is clear that the discovery is helpful for establishing a more explicit view on the physics of multi-functional composite materials, while the composite with optimized microstructure is beneficial to be used as a high-performance material.

Microstructure and Mechanical Properties of Titanium Processed by Spark Plasma Sintering (SPS)

Spark plasma sintering (SPS) is emerging as an important processing technique in which powders can be consolidated rapidly with simultaneous application of pressure and current. In the present study, commercially pure titanium was processed by SPS using a three-step sintering process that resulted in 100% densification. The three-step process was arrived at through a comparison with single-step or continuous sintering cycle. Light and scanning electron microscopy showed acicular α-Ti grains with a maximum aspect ratio up to eight and average grain size of 64 µm. Hardness and room temperature uniaxial compressive properties were evaluated and the elastic modulus was determined by nanoindentation and ultrasonic methods. The average hardness was found to be 325 HV0.3. The 0.2% proof stress was 728 MPa and the nominal ultimate compression strength was found to be 1,560 MPa. These values were substantially higher than reported values for CP Ti. Compression samples failed diagonally at angle of about 45° indicating failure along the planes of maximum shear stress.

Strength and fatigue properties of three-step sintered dense nanocrystal hydroxyapatite bioceramics

Dense hydroxyapatite (HA) ceramic is a promising material for hard tissue repair due to its unique physical properties and biologic properties. However, the brittleness and low compressive strength of traditional HA ceramics limited their applications, because previous sintering methods produced HA ceramics with crystal sizes greater than nanometer range. In this study, nano-sized HA powder was employed to fabricate dense nanocrystal HA ceramic by high pressure molding, and followed by a three-step sintering process. The phase composition, microstructure, crystal dimension and crystal shape of the sintered ceramic were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mechanical properties of the HA ceramic were tested, and cytocompatibility was evaluated. The phase of the sintered ceramic was pure HA, and the crystal size was about 200 nm. The compressive strength and elastic modulus of the HA ceramic were comparable to human cortical bone, especially the good fatigue strength overcame brittleness of traditional sintered HA ceramics. Cell attachment experiment also demonstrated that the ceramics had a good cytocompatibility.

Mechanistic Modeling of Cobalt Based Catalyst Sintering in a Fixed Bed Reactor under Different Conditions of Fischer–Tropsch Synthesis

A three-step sintering mechanism is proposed for Co-based catalysts under Fischer–Tropsch reaction conditions. This mechanism includes an intermediate formation of oxide layer on cobalt metal nanoparticles in the presence of water. The partially reversibly oxidized surface accelerates sintering by both reducing the surface energy and enhancing the diffusion rates of cobalt particles. The proposed mechanism is then employed for a fixed-bed unsteady state reactor. The effect of particle growth on the catalytic activity was analyzed within a diverse range of operating conditions (syngas ratio = 1.5–4, water co-feed ratio = 0–6, inert co-feed ratio = 0–6). It is found that, at the same gas space velocity, sintering proceeds faster at higher H2/CO ratios. At the same initial conversion, a low H2/CO syngas ratio increases sintering severity, i.e., catalyst deactivation due to the crystallite growth, as it brings about higher relative water partial pressure. Dilution of syngas with different amounts of inert gas does not affect the cobalt sintering rate. Cobalt sintering proceeds more rapidly if water is co-fed during the reaction.

Fabrication and Characterizations of YSZ Electrolyte Films for SOFC

Yttria stabilized zirconia (YSZ) has been widely used as electrolyte in solid oxide fuel cell (SOFC). The effect of fabrication process on the properties of YSZ electrolyte thick film is discussed in the paper. With YSZ nano-powders of about 20-60nm as raw material, YSZ green adobe was fabricated by tape calendering process. Three-step sintering process was performed firstly holding at 1000°C for 2h, then raising to 1300~1400°C, then decreasing to 1200~1300°C within 30 minutes, and finally calcining at 1200~1300°C for 5~20 hrs. Dense YSZs with relative density of 96-99% are obtained; the grain size of YSZ was reduced to 0.5-3µm. During the process of grain growth, there are both grain boundary diffusion and grain boundary migration. The feasibility of densification without grain growth relies on the suppression of grain boundary migration while keeping grain boundary diffusion active at a temperature as low as 1200~1300°C. Whereas the electric conductivities of the YSZs are even higher than that obtained in conventional single step sintering process. The process is applied to the anode-supported SOFCs co-fired at 1250~1300°C, and the cathode-supported SOFCs co-fired at 1200~1250°C.

Three-step sintering of constrained yttria stabilised zirconia layers and its effect on microstructure and gas permeance

Three-step sintering has been applied to constrained yttria (3 mol%) stabilised zirconia (YSZ) layers. The sintering schedules use a coarsening stage (step one) prior to reaching the peak temperature (step two). Once the peak temperature has been reached the temperature is lowered and an isothermal dwell takes place (step three). As the YSZ layers form part of a solid oxide fuel cell, and thus need to prevent the fuel and air from mixing, gas flow across the layers was measured and gas permeance values calculated. Several of the three-step sintering schedules produced layers with lower gas permeance values compared with a conventional sintering schedule that is used currently. Further, a three-step schedule produced one of the lowest gas flow rates and minimised the maximum temperature exposure thus offering several potential benefits in terms of processing options.

Fabrication and properties of anode-supported solid oxide fuel cell

The anode substrates in the planar anode-supported solid oxide fuel cells (SOFC) were prepared by tape calendaring process, as well as the thin electrolyte films for increasing the power density of SOFCs at reduced operation temperature were fabricated by slurry-coating method. The minimization of co-firing temperature for anode and electrolyte layers was investigated by a novel three-step sintering technique in this paper. In order to obtain large three phase boundaries (TBP) at the electrolyte/anode interface, active anode layer was screen printed onto the anode substrate. Dense yttria stabilized zirconia (YSZ) thin films with a 10–15 µm thickness on porous anode substrates were successfully fabricated by slurry-coating in an YSZ suspension. These three layers were co-fired together at 1300–1350 °C. The fuel cells with a diameter of 25 mm fabricated by this co-firing technique show relatively high power density. The open circuit voltage (OCV) of 1.10 V was observed at 800 °C, the maximum power densities are 0.28, 0.51 and 0.95 W cm − 2 , at 700, 750 and 800 °C, respectively.

Properties of YSZ Electrolyte Sintered at 1250°C

With the YSZ nano-powders as raw materials, the thin electrolyte film was prepared by tape calendaring, then fired by a three-step sintering method. The YSZ film sintered at 1250°C is dense with a grain size of 0.4~3μm.The electrical conductivity, up to 0.2s/cm at 1000°C, and active energy, 0.99~1ev, are nearly the same for the samples sintered at 1250°C for 10, 15 and 20 h.

Research on sintering process of YSZ electrolyte

Yttria stabilized zirconia (YSZ) has widely been used as electrolyte in solid oxide fuel cell (SOFC). The microstructure of YSZ related to the fabrication process was discussed in the paper. With YSZ nano-powders about 40–100 nm as raw material, the YSZ adobe was manufactured by tape calendering process. The named three-step sintering process was performed at 1000 °C for 2 h, then raised the temperature with normal rate and as soon as up to 1400 °C, the furnace was controlled at 1250–1300 °C for 10–20 h. The high dense YSZs with the relative density of 96%-99% were obtained; the grain size of YSZ could be reduced to 0.5–3 μm. The above result is benefited to co-fired in the electrode-supported SOFCs.