Effect of La2O3 addition and sintering mode on the mechanical properties and microstructural evolution on an 8YSZ ceramic alloy

In this research, the influence of La₂O₃ addition on the microstructure, phase stability and mechanical properties of 8?mol% yttria stabilized zirconia (8YSZ) was studied. 8YSZ with La₂O₃ (9, 12 and 15?wt%) ceramics were fabricated by microwave and conventional sintering at 1400?°C/ 20?min and 1400?°C/ 5?h, respectively. Irrespective of the sintering technique, the relative sintered density was found to decrease with increasing amount of La₂O₃. The grain growth of 8YSZ was enhanced significantly by the addition of La₂O₃. The XRD results demonstrated that addition of La₂O₃ up to 15?wt% did not disrupt the cubic 8YSZ phase regardless of sintering technique; additionally evolution of pyrochlore phase, La₂Zr₂O₇ was observed in all sintered specimens. Vickers hardness of 8YSZ ceramic compacts were also found to decrease with increasing amount of La₂O₃.     

Role of oxygen pressure on the stability of lanthanum strontium manganite–yttria stabilized zirconia composite

We have investigated the structural and chemical stability of La_0.8Sr_0.2MnO₃ (LSM)–8 mol.% yttria stabilized zirconia (YSZ) composite. LSM and YSZ powders were mixed and sintered at 1400 °C for 10 h in controlled atmosphere (PO₂ = 0.21 to 10^?6 atm). The unit cell volume of LSM increases during exposure to reduced oxygen partial pressure while it remains unchanged for YSZ. During reduction in the oxygen partial pressure from 0.21 atm to 10^?6 atm, the solubility of manganese in YSZ increases from ～10 at.% to ～15 at.%. Lower oxygen partial pressure also results in the grain growth and formation of La₂Zr₂O₇ and MnO_x (Mn₃O₄) compounds lowering the stability of the LSM–YSZ composite. On subsequent sintering in 0.21 atm PO₂, the La₂Zr₂O₇ and MnO_x compounds tend to disappear indicating the reversibility of the interaction. The reversibility of LSM–YSZ reaction has been independently confirmed using La₂Zr₂O₇ and MnO_x.     

DLP 3D printing of scandia-stabilized zirconia ceramics

The present article aims to explore the printability of scandia-stabilized zirconia ceramic parts using desktop and low-cost DLP 3D printer. The acrylate-based homogeneous slurries with zirconia powder stabilized by 6 mol.% of Sc₂O₃ (6ScSZ) and 10 mol.% of Sc₂O₃ and 1 mol.% of Y₂O₃ (10Sc1YSZ) were prepared with appropriate rheological and UV-curing properties. In comparison with yttria-stabilized zirconia, slurries filled with 6ScSZ and 10Sc1YSZ powders reviled lower viscosity at the same solid content. The cure depth of the suspensions was suitable to print the objects with 50 μm of layer thickness, good interlayers connection, and surface finishing. No critical defects in ceramics such as cracks or delamination were observed. Both ceramics have the Vickers microhardness value of 11 GPa and the high ionic conductivity up to 0.2 S/сm at 900 °C demonstrating that the DLP is a promising method of fabricating scandia-stabilized zirconia parts as electrolyte material for SOFC application.     

Formation and properties of solid solutions of zirconium dioxide with oxides of rare earth elements

Conclusions Interaction of zirconium dioxide with oxides of cerium, yttrium and lanthanum in solid phases occurs at 1400°C with the formation of solid solutions with the cubic structure.Sintering of the specimens may result at 1700–1750°C with a 3-h soak. At 1400°C and a 6-h soak the porosity of the specimens was 30–40%.Complete stabilization of the zirconia is attained by heating to 1700–1750°C with additions of 20 mol.% CeO₂, 15% Y₂O₃ or 25% La₂O₃. An addition of ceria and yttria displaces the effects of polymorphic inversion of the zirconia to the lower temperature region.New highly refractory materials may be obtained from solid solutions of ZrO₂-20% CeO₂, ZrO₂-80% CeO₂, ZrO₂-15% Y₂O₃, ZrO₂-80% Y₂O₃ and ZrO₂-25% La₂O₃ and firing to 1750°C. Some of them have a low coefficient of thermal expansion compared with ZrO₂, stabilized with calcium oxide and magnesium oxide, and apparently better thermal-shock resistance. The advantage in regard to resistance during prolonged heating at 1200°C is possessed by the solution ZrO₂-Y₂O₃. The region of the most effective use of goods made from solid solutions of ZrO₂ with CeO₂, Y₂O₃ and La₂O₃ as highly refractory materials should be determined by extra studies.The possibility of reducing CeO₂ (fusing temperature about 2700°C) to Ce₂O₃ (fusing temperature about 1700°C) limits the use of cerium-containing materials as refractories in chiefly oxidizing conditions.     

Sintering of LaCoO3 based ceramics

The densification, microstructure and phase evolution of near stoichiometric, Co-excess and Co-deficient perovskite La_1−xM_xCoO_3−δ (M=Ca, Sr; x=0, 0.2) powders have been investigated by electron microscopy and powder X-ray diffraction. Sub-micron powders were prepared from nitrate precursors using the glycin-nitrate and the EDTA methods. The sintering temperature was observed to decrease with Ca or Sr substitution. Dense materials with grain size in the order of 3–5 μm have been obtained at 1200°C for near stoichiometric powders. Considerable grain growth was observed at higher sintering temperatures. The presence of other crystalline phases in addition to the perovskite due to Co-excess/-deficiency considerably affects the microstructure and acts as grain growth inhibitors by grain boundary pinning. The volume fraction of secondary phases is particularly large in the case of Co-deficient LaCoO₃ due to the formation of La₄Co₃O₁₀. In non-stoichiometric La_0.8Ca_0.2CoO₃, a liquid phase consisting mainly of CaO and CoO was observed at 1400°C causing exaggerated grain growth. Considerable pore coarsening was observed in Co-excess La_0.8Ca_0.2CoO₃ at 1350°C. The present investigation demonstrates the importance of controlling the stoichiometry of LaCoO₃ based ceramics in order to obtain dense materials with well defined microstructure.     

Fabrication and creep properties of eutectic-composition Al2O3/YAG/YSZ sintered composites

Three-phase alumina/YAG/yttria-stabilized cubic zirconia (YSZ) composites were fabricated by a solid-state reaction route starting from commercial powders of Al₂O₃, Y₂O₃ and monoclinic ZrO₂. The final phases Al₂O₃, YAG and YSZ were obtained after calcination of the powder mixtures at 1400 °C. Dense bulk composites were obtained after sintering, with a homogeneous microstructure of fine and equiaxed grains with sizes of 1 μm. Compressive mechanical tests were performed at 1300–1450 °C in air at constant load and at constant initial strain rate. A brittle-to-ductile transition was found with increasing temperature. Grain boundary sliding is the main deformation mechanism in the ductile regime, characterized by a stress exponent of 2 and by the absence of dislocation activity and changes in grain morphology. Alumina seems to be the rate-controlling phase owing to the improvement in creep resistance by the presence of yttrium and zirconium of the other two phases.     

Formation peculiarities and optical properties of highly-doped (Y0.86La0.09Yb0.05)2O3 transparent ceramics

Formation peculiarities of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y₂O₃∙0.18La₂O₃∙0.01Yb₂O₃ powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 °C has been studied by XRD. It has been shown that Yb₂O₃ phase dissolves in Y₂O₃ matrix in the calcination temperature range of 1300–1400 °C. Complete dissolution of La₂O₃ in Y₂O₃ matrix occurs at temperatures above 1400 °C. La³⁺ ions enter in Y₂O₃ and Yb₂O₃ crystal structures simultaneously in the 1200–1300 °C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y₂O₃ and Yb₂O₃ cubic phases have been discussed. Finally, (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been obtained by solid-state vacuum sintering at 1650–1750 °C. Ceramics synthesized at a temperature of 1750 °C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.     

The effect of titanium excess and deficiency on the microstructure and dielectric properties of lanthanum doped Ba0.55Sr0.45TiO3 with colossal permittivity

The temperature dependent dielectric properties of (Ba_0.54875Sr_0.44875La_0.0025)Ti_(1+x)O₃ with both an excess and a deficiency of 0.25 mol.% TiO₂ were investigated. The samples were prepared by the mixed oxide method and sintered in a conventional oven at temperatures ranging from 1400 °C to 1475 °C. The cubic perovskite structure was confirmed with XRD at room temperature. The sample with an excess of 0.25 mol.% Ti exhibited reduced grain growth while abnormal grain growth was observed for samples without Ti modification. Samples exhibited colossal permittivity for all modified compositions. With a 0.25 mol.% deficiency of Ti a permittivity over 65,000 and a tan δ under 0.05 were measured over a temperature range of ?68 °C to 150 °C and a frequency range between 50 kHz and 1 MHz. This paper shows that by fine tuning the composition, materials with new, exciting and widely adjustable dielectric properties can be achieved.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La₂Ti₂O₇ (LTO) ceramics have been enhanced through the substitution of La³⁺ ions by Pr³⁺ ions. Almost all doped Pr³⁺ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La_1.9Pr_0.1Ti₂O₇ ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO₆ oxygen octahedrons’ distortions in grains is 1.6 eV for La_1.8Pr_0.2Ti₂O₇ ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d₃₃ = 3.0 pC/N of La_2-xPr_xTi₂O₇ ceramics maintains stable when the Pr³⁺ doping content x ranging from 0.1 to 0.3.     

Development of a Novel Ceramic Support Layer for Planar Solid Oxide Cells

The conventional solid oxide cell is based on a Ni–YSZ support layer, placed on the fuel side of the cell, also known as the anode supported SOFC. An alternative design, based on a support of porous 3YSZ (3 mol.% Y₂O₃–doped ZrO₂), placed on the oxygen electrode side of the cell, is proposed. Electronic conductivity in the 3YSZ support is obtained post sintering by infiltrating LSC (La_0.6Sr_0.4Co_1.05O₃). The potential advantages of the proposed design is a strong cell, due to the base of a strong ceramic material (3YSZ is a partially stabilized zirconia), and that the LSC infiltration of the support can be done simultaneously with forming the oxygen electrode, since some of the best performing oxygen electrodes are based on infiltrated LSC. The potential of the proposed structure was investigated by testing the mechanical and electrical properties of the support layer. Comparable strength properties to the conventional Ni/YSZ support were seen, and sufficient and fairly stable conductivity of LSC infiltrated 3YSZ was observed. The conductivity of 8–15 S cm^–1 at 850 °C seen for over 600 h, corresponds to a serial resistance of less than 3.5 m Ω cm² of a 300 μm thick support layer.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 by successive freeze-drying and self-ignition of a hydroxypropylmethyl cellulose solution

The present paper reports the synthesis of La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 perovskite powders by a method combining freeze-drying and self-ignition of an aqueous solution of metallic nitrates containing hydroxypropylmethyl cellulose. The precursor powder obtained after self-ignition was submitted to various thermal treatments and the resulting powders were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption–desorption isotherm analysis, mercury porosimetry and laser granulometry. It turns out that this synthesis method yields single-phase powders with good homogeneity and sinterability properties. The precursor powder treated at 1200 °C presents a coral-like structure which collapses under application of low uniaxial pressure, resulting in a narrow grain size distribution suitable for sintering (98.8% relative density for a pellet sintered at 1400 °C during 1 h). The fact that no milling step is necessary is an additional advantage of this method, which shows promising prospects for the synthesis of other multicationic oxides.     

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Undoped and 3 mol% yttrium doped ZrO₂–Al₂O₃ composite powders with partially stabilized ZrO₂ (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO₃)₃, ZrOCl₂ and Y(NO₃)₃. The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K_c). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K_c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al₂O₃ composite. The sintered density and critical stress intensity factor (K_c) achieved were compararble to that achieved earlier by hot press and SPS.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Processing and characteristics of transparent Gd3TaO7 polycrystalline ceramics

Transparent polycrystalline Gd₃TaO₇ ceramics were successfully developed. A sol‐gel process was used to synthesize Gd₃TaO₇ powder with a uniform composition and an estimated average particle size of 100 nm. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol‐gel powders. Crystallization was confirmed by X‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1400°C to achieve >96% theoretical density with closed pore structure followed by a hot isostatic pressing at 1400°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 7 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Thermal conductivity was measured to be >1.6 W/mK at room temperature, decreasing to ~1.3 W/mK by 500°C. Dielectric properties remain stable with relative permittivity values just above 200 and loss tangents <0.005 up to 350°C.     

Influence of α-Al2O3 addition on sintering and grain growth behaviour of 8 mol% Y2O3-stabilised cubic zirconia (c-ZrO2)

The effect of α-Al₂O₃ addition on sintering and grain growth behaviour of high purity 8 mol% yttria-stabilised cubic zirconia (c-ZrO₂) was investigated. For these purposes, 1 wt.% α-Al₂O₃ was selected as a dopant in c-ZrO₂. The slip-cast specimens were sintered at different temperatures between 1150 and 1400 °C. It was seen that doped c-ZrO₂ had a faster sintering rate and lower sintering temperature than undoped c-ZrO₂. In particular, doped c-ZrO₂ achieved a density of 95% of its theoretical value at 1275 °C, while undoped c-ZrO₂ reached the same value at 1325 °C. The different sinterability of doped c-ZrO₂ and undoped c-ZrO₂ can be attributed to their different behaviour of grain growth. For grain growth measurements, the specimens sintered at 1400 °C were annealed at 1400, 1500 and 1600 °C for 10, 30 and 66 h. It was seen that grain growth rate could be controlled by the deliberate addition of 1 wt.% grain boundary phase of α-Al₂O₃. A grain growth exponent of 2 and activation energy for grain growth of 298 kJ/mol were obtained for undoped c-ZrO₂. The α-Al₂O₃ containing specimens had a grain growth exponent of 3 and activation energy of 361 kJ/mol. The slow grain growth in doped c-ZrO₂ is attributed to solute ions segregation in grain boundary region. The addition of the grain boundary phase results in limiting matter transfer along the grain boundary resulting in slower grain growth.     

Surface modification of 3Y-TZP with cerium oxide

Surface modification with cerium oxide of tetragonal zirconia polycrystals stabilized with 3 mol.% yttria (3Y-TZP) was carried out in order to improve the resistance to low temperature degradation. Specimens were coated with pressed CeO₂ powder and then annealed at 1400 °C and 1500 °C for periods of up to 10 h. Similar treatments were performed in specimens coated with a sub-micron CeO₂ layer by means of magnetron sputtering. Cerium penetration in the surface modified specimens is about 10 μm into the bulk and the grain size increases in the surface layer affected by cerium diffusion. The indentation bulk fracture toughness and Vickers hardness are not affected by the surface modification treatments. Berkovich nanoindentation was performed to observe the contact hardness and elastic modulus at the surface, showing no significant difference after surface modification. Surface modification with ceria induces a large increase in the resistance to hydrothermal ageing without impairing mechanical properties.     

The Influence of Cation Additives on Grain‐Boundary Mobility in Yttrium Aluminum Garnet (YAG)

Pure yttrium aluminum garnet (YAG) nano‐powders, doped with 1.4 at.% of La₂O₃, ZrO₂, MgO, Nb₂O₅, and SiO_2, were vacuum sintered to full density and subjected to grain growth at 1700°C, for up to 15 h. The YAG intrinsic grain‐boundary (GB) mobility, determined from the pure fully dense YAG specimens, was 2.9 × 10^?16 [m³·N^?1·s^?1]. All dopants, except for La₂O₃, increased the GB mobility compared to pure YAG (La₂O₃ didn't cause any significant change in YAG's GB mobility). All GBs were found to be free of secondary phases or intergranular films. These findings differ from numerous publications where dopants were found to either inhibit grain growth by solute drag, or to enhance grain growth due to liquid phase sintering. It was found that the GB mobility systematically increased with the decrease in the dopant cation radius. Moreover, it seems that vacancy population plays an important role in determining the GB mobility of YAG.     

Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.     

Microstructure and thermal expansion of Al2TiO5–MgTi2O5 solid solutions obtained by reaction sintering

Solid solutions Mg_0.1Al_1.8Ti_1.1O₅ and Mg_0.5AlTi_1.5O₅ were obtained by reaction sintering of mixtures of the binary oxides at 1350–1600 °C using different precursor powders. For the composition Mg_0.1Al_1.8Ti_1.1O₅, ceramics sintered at 1400–1500 °C have high relative density (⩾90%), reduced grain size (2–6 μm), low thermal expansion (−0.8 to 0.3×10⁻⁶ K⁻¹ in the range 200–1000 °C) and reproducible expansion behaviour. At higher temperature, grain size rapidly increases owing to anisotropic and exaggerated grain growth (EGG) resulting in severe microcracking. Microstructure evolution is affected by the nature of the starting oxides, in particular for what concerns the onset temperature of EGG, the size and the fraction of abnormal grains. For the composition Mg_0.5AlTi_1.5O₅, EGG already takes place at 1350 °C and materials with grain size < 5 μm are difficult to obtain by conventional reaction sintering. Large grained samples (>10 μm) of both compositions show a reduced hysteresis and complex thermal expansion behaviour. In particular, heating to 1000 °C results in a significant increase in specimen size on return to room temperature. Repeated thermal cycling leads to an increase of the hysteresis.