Synthesis, spark plasma sintering and electrical conduction mechanism in BaTiO3-Cu composites

BaTiO₃-Cu composite powders were prepared via an alkoxide-mediated synthesis approach. As-synthesized BaTiO₃ nanoparticles were as small as 40 nm and coated partially larger Cu particles of approximately 1 μm in size. Thermogravimetric analysis (TGA) and dilatometry revealed a gradual increase in weight loss and retarded shrinkage with the increase of Cu addition. BaTiO₃-Cu composites were successfully densified by spark plasma sintering (SPS). The microstructures show an average grain-size for BaTiO₃ of around 100 nm and a crystallite size of about 1 μm for the Cu inclusions. The AC conductivity of the BaTiO₃-Cu composites increased with increasing Cu content or with temperature. The dominant electrical conduction mechanism in SPSed BaTiO₃-Cu composites changed from migration of oxygen vacancies to band conduction of trapped electrons in oxygen vacancies with the increase of Cu content.     

High permeability Ni-Cu-Zn ferrites through additive-free low-temperature sintering of nanocrystalline powders

Nanocrystalline Ni-Cu-Zn ferrite powders Ni_0.20Cu_0.20Zn_0.62Fe_1.98O_3.99 were prepared by thermal decomposition of an oxalate precursor. The particle size is 6 nm and 350 nm, respectively, for powders obtained through calcinations at 350 °C or 750 °C. The shrinkage behavior significantly changes with particle size; the temperature of maximum shrinkage rate is T_MSR = 700 °C for particles of 6 nm size and increases to T_MSR = 880 °C for particles 350 nm in size. Dense samples with a permeability of μ = 780 are obtained by sintering at 900 °C for 2 h. Mixtures of nanocrystalline and sub-micron powders allow tailoring of the shrinkage behavior. A maximum permeability of μ = 840 is obtained after sintering of a 1:1-mixture at 900 °C. This demonstrates the potential of nanocrystalline ferrites for co-firing without additives at 900 °C and integration of ferrite inductors into LTCC modules.     

Effects of Initial Powder Size on the Densification of Barium Titanate Ceramics Prepared by Microwave‐Assisted Sintering

The effects of initial powder size on microwave‐assisted sintering (MWS) were investigated. BaTiO₃ powders with an average particle size of 50, 100, and 500 nm were prepared and sintered with MWS and conventional heating‐based sintering (CS). Samples of the 50 ‐ and 100‐nm‐sized BaTiO₃ powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50‐nm‐sized BaTiO₃ powder resulted in a relative mass density of more than 90% when sintered at 1050°C, whereas the same density was achieved at 1200°C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500‐nm‐sized BaTiO₃ powder was used. The sinterability of the BaTiO₃ ceramics prepared through the MWS of mechanically milled, 50‐nm‐sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO₃ ceramics prepared through the MWS of the 100‐ and 500‐nm‐sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of ~50‐nm‐sized powders, but is negligible for 500‐nm‐sized powders.     

Effect of 1 wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

Densification of nanocrystalline cubic yttria (nc-Y₂O₃) powder, with 18 nm crystal size and 1 wt% LiF as a sintering additive was investigated. Specimens were fabricated by spark plasma sintering at 100 MPa, within the temperature range of 700-1500 °C. Sintering at 700 °C for 5 and 20 min resulted in 95% and 99.7% dense specimens, with an average grain size of 84 and 130 nm, respectively. nc-Y₂O₃ without additive was only 65% dense at 700 °C for 5 min. The presence of LiF at low sintering temperatures facilitated rapid densification by particle sliding and jamming release. Sintering at high temperatures resulted in segregation of LiF to the grain boundaries and its entrapment as globular phase within the fast growing Y₂O₃ grains. The sintering enhancement advantage of LiF was lost at high SPS temperatures.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

Percolative BaTiO3–Ni composite nanopowders from alkoxide-mediated synthesis

BaTiO₃–Ni nanopowders have been synthesized via an alkoxide-mediated synthesis route through the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide in the presence of submicron sized, spherical Ni particles originating from a commercial Ni paste, that was introduced during the preparation procedure. X-ray diffraction (XRD) patterns indicate that nanocomposite powders of the phases BaTiO₃ and Ni could be successfully prepared and tailor-made composition control was confirmed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the synthesized BaTiO₃ nanoparticles were aggregates of nanosized primary particles as small as 40 nm in diameter. The average Ni particle size was estimated to be about 200 nm. Dilatometric measurements on green compacts of these powders revealed that the shrinkage of BaTiO₃–Ni composites is retarded compared to both, pure BaTiO₃ and Ni. Thermogravimetric analysis (TGA) shows weight losses due to the decomposition of organic binder from Ni paste, the release of water from the surface and of hydroxyl ions from inside the lattice of the BaTiO₃ nanoparticles. With the addition of nickel, the dielectric constant increased slightly due to the percolation effect.     

Sintering behavior of nano alumina powder shaped by pressure filtration

In the present study, the sintering behavior of a commercial nano alumina powder with an initial particle size of 100 nm was investigated. The shrinkage response of the powder formed by pressure filtration (PF) during non-isothermal sintering was measured in a laser assisted dilatometer at three different heating rates of 2, 10 and 25 °C min⁻¹ up to 1400 °C. In order to calculate the activation energy of sintering, constant rate of heating (CRH) was employed and the activation energy was found to be 608 ± 20 kJ mol⁻¹ for iso-density method. The heating rate was demonstrated to have a vital role on densification behavior and final grain size. The mean grain size of the full dense specimens decreased from 875 to 443 nm when the heating rate increased from 2 to 25 °C min⁻¹.     

Phase, microstructure evolution and sintering of Sr-doped TiB2 precursors

Precursors for the preparation of bulk Sr-doped TiB₂ composites were synthesized by modified Pechini method. The high temperature behaviour of homogeneous Sr-Ti-B-C-O gels was investigated in the range 1200-1650 °C. DTA-TG analysis of the precursor powder shows two steps of the carbothermal reduction with endothermic peaks at temperatures of 1335 °C and 1500 °C. The influence of strontium content (2, 5, 10, 20 and 50 mol.%) on the phase composition and morphology of powders at 1650 °C was studied.Due to the shift of TiB₂ diffractions and the detection of strontium in TiB₂ grains by EDX analysis the formation of Ti_1 − xSr_xB₂ solid solution is assumed in the Sr-doped powders. Finally, Sr-doped TiB₂ composites were inductive hot-pressed from the as prepared powders at 1900 °C for 7 min. The formation of SrTiO₃ phase in the powders is serving as a sintering aid during the preparation of bulk Sr-Ti-B composites. The exaggerated grain growth (grain size up to ∼60 μm) occurs during the sintering with increasing content of strontium in the precursor.     

Crystalline morphology and photoluminescent properties of YInGe2O7:Eu phosphors prepared from microwave and conventional sintering

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇:Eu³⁺ powders using microwave assisted sintering. For comparison, the properties of YInGe₂O₇:Eu³⁺ powders sintered at 1200 °C in conventional furnace for 10 h were also investigated. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ without second phase or phases of starting materials as YInGe₂O₇:50 mol% Eu powders sintered at 1200 °C in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave assisted sintering. In the PL studies, both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 620 nm under excitation of 393 nm with similar luminescent intensity. The results show that microwave processing has the potential to reduce the time and required energy input for the production of YInGe₂O₇:Eu³⁺ phosphors without sacrificing the photoluminescence.     

Densification and mechanical properties of boron carbide with micro-hole array by micro-powder injection molding

For the purpose of obtaining fully dense B₄C with micro hole array of high quality and precision, synthesis of B₄C powders were carried out by micro powder injection molding. Five kinds of additive systems were used and their influences on mechanical properties were investigated. The relative density, Vickers hardness, bending strength, and fracture toughness of the B₄C ceramics with BS10AY additive sintered at 2000 °C for 2 h could reach 97.56%, 3580.4 HV, 355.3 MPa and 5.87 MPa m^1/2, respectively. The temperature was generally 100–200 °C lower than reported temperatures which was attributed to the additives. The improvement in mechanical properties was attributed to grain refinement. A mixture of intragranular and transgranular fractures occurred due to the fine microstructures and the additive systems in the B₄C ceramics after sintering. Micro-hole array with the diameter of 450 μm and the length-diameter ratio of more than 8 were obtained. The inhomogeneous filling of feedstock from the substrate to the thin wall between two adjacent micro holes caused the inhomogeneous shrinkage of the substrate and micro holes.     

Synthesis of nanocrystalline ytterbium-doped yttria by citrate-gel combustion method and fabrication of ceramic materials

Nanosized ytterbium doped yttria powders were prepared by citrate-gel combustion techniques. As-synthesized precursor and calcined powders were characterized for their crystalline structure, particle size and morphologies. Nanocrystalline Yb³⁺:Y₂O₃ powders with pure cubic yttria crystal structure were obtained by calcination of as-prepared precursors at 1100 °C for 3 h. Powders obtained were well dispersed with an average particle size of 60 nm. By using the obtained powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were produced by vacuum sintering at 1800 °C for 12 h. The emission spectrum of the sintered ceramics under the excitation wavelength of 905 nm illustrates that there are three fluorescence peaks locating at 976 nm, 1030 nm and 1075 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of ytterbium ion.     

Phase structure and nano-domain in high performance of BaTiO3 piezoelectric ceramics

BaTiO₃ ceramics were prepared by conventional sintering technique with a special emphasis on the effects of sintering temperature (1100-1230 °C) on the crystalline structure and piezoelectric properties. XRD patterns indicated that the crystallographic structure changed from tetragonal phase to orthorhombic one with raising sintering temperature from 1160 °C to 1180 °C. Domains were shaped in a stripe and a herringbone in orthorhombic samples for BaTiO₃ ceramics. The domain width and domain density increased with raising sintering temperature. The BaTiO₃ ceramic sintered at 1190 °C showed the excellent electrical properties, d₃₃ = 355 pC/N, k_p = 40%, P_r = 10.2 μC/cm², respectively, which are originated to the contributions of both the crystallographic structure transition and nano-domain.     

In situ preparation of Si3N4–TiN composite by pyrolysis of polytitanosilazane (PTSZ)

Si₃N₄–TiN composite powders were obtained by in situ pyrolysis of polytitanosilazane. Dense Si₃N₄–TiN composites were prepared by hot-pressing at 1800 °C under 20 MPa for 2 h without sintering additive. Crystallization of amorphous PTSZ powders occurred between 1400 and 1500 °C with major phases, α-Si₃N₄, β-Si₃N₄, and small amount of phase TiN. Mechanical properties and microstructure of Si₃N₄–TiN composites were characterized. The results showed that the mechanical strength was 620 MPa, the fracture toughness was 7.8 MPa m^1/2 and the Vickers hardness was 8.5 GPa. SEM analysis indicated that Si₃N₄–TiN composite possessed excellent fracture toughness because TiN grains produced by in situ pyrolysis were well dispersed in Si₃N₄ matrix.     

The effects of nanoparticle addition on the sintering and properties of bimodal AlN

The effects of nanoparticle addition on the pressureless sintering of injection molded and debound aluminum nitride (AlN) samples were studied. Variations in the densification, microstructure, and properties owing to the increased powder content and reduced particle size are discussed. The results indicate the formation of liquid phase at 1500 °C in the bimodal micro (μ)–nano (n) AlN samples, a temperature that is at least 100 °C lower than typically reported values in the literature. Consequently, a densification ≥ 99% was achieved by pressureless sintering at a relatively lower temperature of 1650 °C with ∼14% isometric shrinkage. Additionally, thermal and mechanical properties of the sintered bimodal AlN samples are presented and compared with sintering studies on conventional monomodal μ-AlN systems reported in the literature.     

Impurity control in pressureless reactive synthesis of pure Ti3SiC2 bulk from elemental powders

The impurity control in pressureless reactive synthesis of pure Ti₃SiC₂ from elemental powders is reported. Ti₃SiC₂ bulk samples were prepared by sintering compacts of ball-mixed elemental powders at 1500 °C for 2 h in lidded alumina crucibles under Ar atmosphere. Undesirable TiC impurity was successfully eliminated from the synthesized product. Product with desired phase constituent can be fabricated by preparing samples according to phase diagram data. Keeping away from the phase fields that involve TiC is a vital way to obtain pure Ti₃SiC₂ without containing the undesirable TiC. The key for successful impurity control in the sintering process is the conservation of mass in the reactants.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Synthesis of lanthanum beta-alumina powders by the polymeric precursor technique

La-β-Al₂O₃ (LaAl₁₁O₁₈) powders were synthesized by the polymeric precursor technique using lanthanum nitrate and aluminum nitrate. The transformations during thermal treatment of the precursor solution with ethylene glycol and citric acid were evaluated by thermal analysis. Fourier transform infrared spectroscopy analysis was performed after calcinations of the polymeric resin for determination of residual carbon. The specific surface area was evaluated by the BET method. Fine powders with ∼121 m²/g specific surface area and 20 nm average particle size were obtained and observed by scanning and transmission electron microscopy. Nearly single phase LaAl₁₁O₁₈ was obtained after pressing and sintering these powders at 1600 °C with small additions of MgO. The sintered pellets were characterized by X-ray diffraction and scanning electron microscopy. Impedance spectroscopy measurements carried out in the 1000–1200 °C range show the electrolytic behavior of the La-β-Al₂O₃ pellets, suggesting their application as solid electrolytes in high temperature potentiometric oxygen sensors.     

Properties of BaTiO3 synthesized from barium titanyl oxalate

In order to enhance the tetragonality of BaTiO₃ derived from barium titanyl oxalate (BTO), various treatments were carried out by considering the thermal decomposition mechanism of BTO in air. A multi-step heat treatment process and the addition of carbon black, as a particle growth inhibitor, were effective in increasing the tetragonality, whilst maintaining a particle size smaller than 200 nm. The synthesized BaTiO₃ powder with a mean particle size of 177 nm showed a tetragonality and K-factor of 1.0064 and approximately 3, respectively.     

Effect of the addition ZrO2–Al2O3 on nanocrystalline hydroxyapatite bending strength and fracture toughness

Nanocrystalline hydroxyapatite powder has been synthesized from a Ca(NO₃)₂·4H₂O and (NH₄)₂HPO₄ solution by the precipitation method. In the next step we prepared ZrO₂–Al₂O₃ powder. After preparation, the powder was dried at 80 °C and calcined at 1200 °C for 1 h. Various amounts (HAP–15 wt% ZA, HAP–30 wt% ZA) of powder were mixed with the hydroxyapatite by ball milling. The powder mixtures were pressed and sintered at 1000 °C, 1100 °C and 1200 °C for 1 h. In order to study the structural evolution, X-ray diffraction (XRD) was used. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to estimate the particle size of the powder and observe fracture surfaces. Results show that the bending strength of pressed nanocrystalline HAP was improved significantly by the addition 15 wt% of ZrO₂–Al₂O₃ powders at 1200 °C, but the fracture toughness was not changed, however when 30 wt% of ZA powders were added to nanocrystalline HAP, the bending strength and fracture toughness of the specimens decreased at all sintering temperature.