F. Capel

Low-temperature sintering and microstructural development of nanocrystalline Y-TZP powders

Abstract Powders of yttria-doped tetragonal zirconia (3 mol%) with a narrow pore size distribution and ultrafine particle size (~ 9 nm) have been prepared by the mixed organic + inorganic precursors coprecipitation method. The compaction behaviour of almost agglomerate-free calcined powders was studied, and their sintering behaviour using both isothermal and non-isothermal techniques was evaluated. Fully dense nanoscale ceramics with an average grain size below 95 nm were obtained after sintering at 1200 °C for 20 min or at 1150 °C for 4 h. Several stages were identified in the whole densification process: it was found that a particle rearrangement process assisted by sintering pressure is the densification mechanism in the earlier sintering step (up to 800 °C), and grain boundary diffusion was the dominant mechanism for densification up to 1180 °C. Activation energy values of 130 ± 20 and 300 ± 40 kJ mol−1, respectively, were calculated for these densification mechanisms. The almost complete absence of agglomerates in the calcined powders and the homogeneous pore structure of the green compacts are the two main factors leading to lowtemperature fully densified Y-TZP bodies.

Heating-rate effect on the BaTiO3 formation by thermal decomposition of metal citrate polymeric precursors

Abstract Barium titanate nano-sized powders had been prepared by a slightly modified Pechini method. DTA/TG thermal analysis indicated that thermal decomposition of the precursor proceeds through four major step processes: (a) dehydration reaction, (b) combustion reactions, (c) intermediate phases formation, and (d) decarbonation of the intermediate to give BaTiO 3 . X-ray diffractometry (XRD) and Raman spectroscopy results indicated that, depending on the heating-rate, the BaTiO 3 formation took place via a predominant solid-state reaction between nano-sized BaCO 3 and amorphous TiO 2 (TiO 2− x ) when crystallized by low heating-rate (1.5°C min −1 ). Although a small amount of a quasi-amorphous intermediate phase was also present. BaTiO 3 crystallization by rapid heating-rate (5°C min −1 ) took place through a quasi-amorphous intermediate phase formation as the main rate-controlling factor for the crystallization process. Although room temperature XRD results seems to indicate the formation of pseudo-cubic BaTiO 3 as the final reaction product, the Raman spectra indicated as more probable the formation of a mixture of an oxygen-deficient hexagonal and tetragonal BaTiO 3 phases below 700°C. Above that temperature, the tetragonal BaTiO 3 was the only phase present.

Nanostructured and near defect-free ceramics by low-temperature pressureless sintering of nanosized Y-TZP powders

Nanosized (∼6 nm) Y-TZP (3 mol% Y2O3) powders have been produced by chemical co-precipitation (Y-inorganic + Zr-organic precursors) and thorough isopropanol-washing step, after calcining in air at 450 °C. The nanocrystalline Y-TZP powders consisted of spherical soft agglomerates (∼100 nm in size) which were easily broken down during compaction resulting in a very uniform green microstructure with a narrow pore size distribution (average pore size less than 6.5 nm) and no detectable compacting defects. In spite of the relatively low green density (43% theoretical), Y-TZP powder compacts sintered to near theoretical density in the very low-temperature range of 1000 °C for 80–100 h to 1070 °C for 2 h, maintaining a grain size in the nanoscale (< 100 nm) and the sintered bodies were nearly defect-free. Hardly any grain growth took place up to 1000 °C; it was very rapid above this temperature.

Mixed (Oxygen Ion and n‐Type) Conductivity and Structural Characterization of Titania‐Doped Stabilized Tetragonal Zirconia

By using X-ray diffraction lattice parameter measurements and Raman spectroscopy studies, both the solid solubility limit of titania in yttria tetragonal zirconia polycrystalline solid solutions (Y-TZP, 3 mol % Y 2 O 3 ) and the TiO 2 -YTZP tetragonal solid solution field in the ZrO 2 -Y 2 O 3 -TiO 2 system have been established. Valence state, site symmetry, and changes in local structures of Ti ions in Y-TZP with 5 and 10 mol % TiO 2 are studied for the first time using EXAFS (extended X-ray absorption fine structure), XANES (X-ray absorption near edge structure), and X-ray photoelectron spectroscopy. The total electrical conductivity in air of the TiO 2 -Y-TZP tetragonal solid solution decreases with increasing titania content. XANES results show that as the TiO 2 dissolves into the tetragonal zirconia Y-TZP matrix, a displacement of Ti 4 + ions from the center of symmetry seem to take place which leads to a non-random substitution of Ti 4+ ions on Zr 4+ lattice sites. Ti-O bond distances derived from EXAFS results indicate that Ti 4+ ion can be in a square-pyramidal arrangement, i.e., fivefold oxygen-coordinated. As a consequence, two kind of cation-oxygen vacancy associations (Zr-Vo and Ti-Vo) with different diffusion dynamics are created. This results in a decrease of the global concentration of moving oxygen vacancies and therefore, a decrease of ionic conductivity. Electronic conductivity, n-type, only appeared at oxygen partial pressure lower than 10 -15 atm and above 800°C in Y-TZP containing 10 mol % titania. Such an n-type electronic conduction was attributed to a hopping of electrons between Ti 4+ and Ti 3+ cations by a small polaron hopping mechanism.

Theoretically dense and nanostructured ceramics by pressureless sintering of nanosized Y-TZP powders

Abstract Nanosized yttria-doped tetragonal polycrystalline zirconia (Y-TZP) powder was prepared by the coprecipitation method using isopropanol solutions of organic and inorganic precursors. A isopropanol-washing step led to a powder mainly constituted by very soft agglomerates with 6 nm crystallite size. Such a weakly agglomerated powder compacted uniformly (average diameter pore size = 6 nm) with a homogeneous shrinkage to full density. The small particle size of the Y-TZP powder, the small pores in the green compacts, and its homogeneous pore size distribution led to theoretically dense and nanostructured ( 1200°C) and with a grain size higher than 0,25 mm.

Low-temperature fully dense and electrical properties of doped-ZnO varistors by a polymerized complex method

Abstract The preparation of homogeneous and submicrometer doped-ZnO ceramic powders by using a metal-organic polymeric method is described. After calcining and granulating, green compacts with a uniform packing powder and a narrow pore size distribution were achieved. Dense ceramic bodies (>99% theoretical) by normal liquid-phase sintering at temperatures of 850–940°C for 1–5 h were fabricated. It is believed that the low pore-coordination-number allowed a uniform filling of the small pores by the formed liquid in the early stages of sintering and, as a consequence, a high shrinkage rate and rapid densification in a short temperature interval (825–850°C) took place. At those sintering temperatures a small grain growth was produced, and the grain size was maintained below 1 μm. Preliminary electrical results obtained on the doped-ZnO ceramics so fabricated showed nonlinearity coefficients α ⩾70 and a breakdown voltage V b (1 mA/cm 2 )⩾1500 V/mm.

Large electromechanical anisotropic modified lead titanate ceramics: Part 1 Processing

Modified lead titanate (PT) oxalates were prepared by mixing an ethanol solution with (calcium, lanthanum, neodymium, samarium or gadolinium) and titanium with an ethanol solution of oxalic acid containing lead, and (cobalt and tungsten) or manganese by blending. The resulting complex oxalate, after washing with ethanol for several times, was an amorphous powder with a very fine particle size. The amorphous coprecipitate gave, on calcining, a high sinterable modified lead titanate powder of compositions (Pb0.76Ca0.24) ((Co0.5W0.5)004Ti0.96)O3 or (Pb0.88La0.08) (Ti0.98Mn0.02)O3 which, on sintering, produced highly densified bodies (⩾98% theoretical density) at relatively low temperatures (1100 and 1150°C for Ca-PT and Ln-PT, respectively). Linear thermal expansion measurements, microstructural development of the modified lead titanates, and the influence of grain size onc/a ratio have also been studied.

Processing and dielectric properties of the mixed-layer bismuth titanate niobate Bi7Ti4NbO21 by the metal-organic precursor synthesis method

Abstract The preparation of homogeneous Bi 7 Ti 4 NbO 21 single phase nanosized ceramic powders at temperatures as low as 400–500°C using a metal-citrate complexes method, based on the Pechini-type reaction route, is described. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, Raman spectroscopy and SEM, led to the formation of a well defined orthorhombic Bi 7 Ti 4 NbO 21 compound with lattice parameters a =0.544, b =0.540 and c =2.905 ± 0.0005 nm. Reaction formation takes place through an intermediate binary phase with a stoichiometry close to Bi 20 TiO 32 which forms between 300 and 375°C. The metal-organic precursor synthesis method, allows the control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of the nanosized bismuth titanate niobate, Bi 7 Ti 4 NbO 21 , ceramic powders, at temperatures much lower than usually needed in the conventional route. Sintering of the as-prepared ceramic powders led to near full density samples in the temperature range 1100–1130°C. The mixed-layer structure Bi 7 Ti 4 NbO 21 ceramics showed two phase transitions at about 650 and 850 ± 5°C, and a piezoelectric modulus, d 33 , as high as 20×10 −12 C/N.

BaTiO 3 formation by thermal decomposition of a (BaTi)-citrate polyester resin in air

Barium titanate nanosized powders were prepared by a slightly modified Pechini method. The obtained polymerized resin was used as the precursor for BaTiO 3 powder production. DTA TG thermal analysis indicated that thermal decomposition of the precursors proceeds through four major step processes: (i) dehydration reaction; (ii) combustion reactions; (iii) intermediate phases formation; (iv) decarbonation of the intermediate to give BaTiO3. X-ray diffractometry (XRD) and Raman spectroscopy results indicated that, depending on the heating rate, the BaTiO 3 formation took place via a predominant solid-state reaction between nanosized BaCO 3 and amorphous TiO 2 (TiO 2−x ) when crystallized by a low-heating rate (1.5 °C/min), although a small amount of a quasi-amorphous intermediate phase was also present. BaTiO 3 crystallization by rapid heating rate (5 °C/min) took place through a quasi-amorphous intermediate phase formation as the main rate-controlling factor for the crystallization process. The fact that the low heating rate minimizes the intermediate phase content indicates the strong influence of the thermal heating on the kinetics of the involved transformation or in the mechanism. Although XRD results seem to indicate the formation of pseudocubic BaTiO 3 as the final reaction product, the Raman spectra indicated as more probable the formation of a mixture of an oxygen-deficient hexagonal and tetragonal BaTiO3 phases below 700 °C. Above that temperature the tetragonal BaTiO 3 was the only phase present. As-prepared BaTiO 3 strongly agglomerated powders were relatively sinter active, leading to dense ceramic bodies (≥95% of the theoretical value). Microstructural (grain size approximately 1 mm) and room-temperature dielectric properties (e τ ≈ 2000 and tan δ ≤ 2%) at 10 kHz indicated that the obtained powders have to be optimized.

Structure-electrical properties relationships in TiO2-doped stabilized tetragonal zirconia ceramics

Abstract The tetragonal zirconia solid solution field in the ternary system ZrO2–Y2O3–TiO2 has been established, and the influence of TiO2 addition on the electrical properties of the formed ternary Ti-YTZP solid solutions has also been studied. The unit cell parameters determined from the X-ray diffraction patterns of the sintered samples, and the data obtained from X-ray absorption (XANES and EXAFS) were used to provide information on the environment of Ti atoms. The electrical conductivity of the Ti-YTZP tetragonal solid solutions decreases with increasing titania concentration. EXAFS and XANES results show that as the TiO2 dissolves into the tetragonal zirconia YTZP matrix, a displacement of Ti4+ ions from the center of symmetry seems to take place which leads to a state of non-random substitution of Ti4+ ions on Zr4+ lattice sites. Ti–O bond distances derived from EXAFS indicate that Ti ions can be in a square-pyramidal arrangememt, i.e. five-fold oxygen coordinated. As consequence, two kinds of cation–oxygen vacancy associations with different diffusion dynamics are created. This results in a decrease in the global concentration of moving oxygen vacancies and, therefore, in a decreasing of the ionic conductivity. ©