Vacuum sintering of highly transparent La1.28Yb1.28Zr2O7.84 ceramic using nanosized raw powders

Highly transparent La_1.28Yb_1.28Zr₂O_7.84 ceramic was prepared by vacuum sintering using nanosized raw powders, which were synthesized by a simple solution combustion method using rare earth nitrate as the raw materials. The as-burnt powders were calcined at 1200?℃ and then ball-milled for 24?h with resultant particle size of about 60?nm. The two phases, cubic pyrochlore and defective fluorite, are uniformly distributed in the ceramic. La_1.28Yb_1.28Zr₂O_7.84 transparent ceramic with the maximum in-line transmittance of 83.9% was successfully prepared at 1850?℃ for 6?h in a vacuum furnace.     

Vacuum sintering of highly transparent La1+xYb1+yZr2O7 ceramics with excess La and Yb contents

In this study, a series of transparent ceramics with chemical composition of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) were successfully prepared by vacuum sintering using combustion synthesized powders. The effects of excess contents on the phase composition, microstructure and in-line transmittance have been studied. The detailed results indicate that the in-line transmittance increases at first and then decreases as La content be elevated. It was also determined that the highest in-line transmittance of La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics is 84.1 % at 1100 nm when the excess amount of co-doped La-Yb is 30 %. Compared with stoichiometric LaYbZr₂O₇ ceramic, the nonstoichiometric La_1+xYb_1+yZr₂O₇ (x, y = 0.1?0.5) ceramics exhibit much higher transparency. In addition, the high excess amount of La, Yb and co-doped La-Yb also shows effects on the phase composition and crystal structure.     

Pyrochlore glass-ceramics fabricated via both sintering and hot isostatic pressing for minor actinide immobilization

Pyrochlore glass-ceramics (GCs) have been investigated with samples fabricated via both sintering and hot isostatic pressing (HIPing) of a mixed oxide precursor. It has been demonstrated that sintering at 1200°C in air is necessary to obtain well-crystallized pyrochlore crystals in a sodium aluminoborosilicate glass through a one-step controlled cooling. The crystallization, structure, and microstructure of Eu₂Ti₂O₇ pyrochlore as the major phases in residual glass were confirmed with X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy. The structures of major Eu₂Ti₂O₇ pyrochlore and minor [Eu_4.67O(SiO₄)₃] apatite in both sintered and HIPed samples were refined using synchrotron XRD data. While the processing atmosphere did not appear to affect the cell parameter of the main pyrochlore phase, very small volume expansion (~0.3%) was observed for the minor apatite phase in the HIPed sample. In addition, static leaching of the HIPed sample confirmed that pyrochlore GCs are chemically durable. Overall, pyrochlore GCs prepared via both sintering and HIPing with the Eu partitioning factor of ~23 between ceramics and the residual glass are suitable waste forms for minor actinides with processing chemicals.     

Thermal diffusivity characterization of europium zirconate,cerate and hafnate

The following paper presents study on synthesis and thermal diffusivity of europium zirconate, cerate and hafnate as a new types of materials with pyrochlore or fluorite type of lattice, dedicated to replacement of conventional 8YSZ (yttria stabilized zirconia) in TBC (thermal barrier coatings) systems for aircraft gas turbines.All materials were synthesized via solid-state reaction (SSR) preceded preparation of powder mixtures (mechanical milling in ethanol). The feedstock powders were characterized by analysis of crystallite size, particle size distribution and phase composition. On the base of DSC measurements, the parameters of high temperature sintering were selected (1350?°C/2?h/15?MPa). Structural characterization of obtained materials inclusive analysis of morphology, chemical and phase composition of sinters was performed. Thermal diffusivity was measured in temperature range 25÷1400?°C by laser flash analysis (LFA). The obtained materials with pyrochlore and fluorite type of lattice exhibit lower thermal diffusivity compared to that of yttria-stabilized zirconia at high temperatures. In particular, the most promising material in view of insulating properties is Eu₂Ce₂O₇, which exhibit much lower thermal diffusivity than Eu₂Zr₂O₇ and Eu₂Hf₂O₇, especially at temperature higher than 1000?°C. However, in temperature range 25–700?°C, europium hafnate and zirconate exhibit lower thermal diffusivity compared to that of europium cerate and 8YSZ.     

Effect of sintering temperature on the microstructure and performance of a ceramic coating obtained by the slurry method

In this paper, SiO₂, Cr₂O₃, Al₂O₃, and MgO were used as ceramic aggregates, and a small amount of Al powder was added. A ceramic coating was prepared on a Q235 steel substrate. The effect of the sintering temperature on the coating microstructure, phase structure and wear resistance was studied by Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) and friction and wear testing. The results show that the tensile strength of the ceramic coating is increased after sintering, the structure becomes dense, and the size of coated micropores is increased to release the internal tensile stress. With the increase of the sintering temperature and tensile stress, the micropores begin to release the excess tensile stress in the form of crack initiation and expansion. The mineralization of MgO, Cr₂O₃, nMgO and mSiO₂ phases can be achieved by sintering the coating at 200?°C; the oxygen in the atmosphere migrates along the micropores in the coating to react with Fe in the steel substrate, forming FeO, and the resulting FeO reacts with the SiO₂ in the coating to form the Fe₂SiO₄ phase. The coating has the best wear resistance after being sintered at 400?°C, and the abrasion resistance of the sample is 6.7 times higher than that of the sample dried at room temperature.     

The microstructure and characteristics of 0.875PZT–0.125PMN ceramics with addition of Pb-based flux

The paper tries to prepare dense piezoceramics by way of reactive liquid phase sintering. Technique concerning a low-temperature sinterable process is developed by incorporating 4PbO.B₂O₃. The host system is a perovskite type piezoceramics, 0.875Pb(Ti,Zr)O₃–0.125Pb(Mg_1/3Nb_2/3)O₃. It is clear that PbO deficiency of PMN-based relaxor can result in an excessive amount of pyrochlore phase which causes poor densification and greatly degraded dielectric properties. Additives, such as the Pb-based flux, 4PbO·B₂O₃, that increase the amount of PbO also reduce the fraction of pyrochlore phase of PMN-based. If small amounts of 4PbO·B₂O₃ glass powder are added to the calcined 0.875PZT–0.125PMN ceramics, the liquid pase is formed during sintering. Hence, the piezoelectric and dielectric properties are enhanced and the sintering temperature can be reduced. Grain growth in ceramics with sintering time and amounts of 4PbO·B₂O₃ dopants was also studied. The grain growth was analyzed from the kinetic grain growth equation: Rⁿ=k×t.     

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.     

Microstructure and electrical properties of (Pb0.87Ba0.1La0.02)(Zr0.68Sn0.24Ti0.08)O3 anti-ferroelectric ceramics fabricated by the hot-press sintering method

(Pb_0.87Ba_0.1La_0.02)(Zr_0.68Sn_0.24Ti_0.08)O₃ anti-ferroelectric ceramics with various amounts of excess PbO have been fabricated by the conventional solid-state reaction process and the hot-press sintering method, and the dependence of microstructure, ferroelectric and dielectric properties on sintering approaches and lead volatilization conditions has been studied. When an appropriate quantity of excess PbO is added, the lead volatilization effect can be compensated and the content of pyrochlore phase is decreased. Hot-pressed anti-ferroelectric ceramics exhibit much higher densities than conventionally sintered samples. The spontaneous polarization and maximum dielectric constant of the AFE ceramics decrease when the hot-press sintering method is used. The dielectric loss can be obviously reduced by using the hot-press sintering method and adding a proper amount of excess PbO. The systemic investigation of the lead volatilization effect and the hot-press sintering method will contribute to the development of high properties of anti-ferroelectric ceramics in practical applications.     

New pathway for the preparation of pyrochlore Nd2Zr2O7 nanoparticles

Pyrochlore Nd₂Zr₂O₇ nanoparticles were prepared by complex-precipitation in aqueous media, followed by calcination in MgO matrix and subsequent dissolution processing. A suite of characterization techniques, including X-ray diffraction, Raman, TEM, SEM, dynamic light scattering, and nitrogen sorption, was employed to investigate the structure and particle size of the synthesized nano materials. Results show that calcination at 1200 °C for 20 h forms Nd₂Zr₂O₇ with pyrochlore structure. The matrix phase (MgO) had no effect on the formation of pyrochlore phase. The MgO phase was readily removed by dissolution at 0.5 M HNO₃ aqueous solution; and the remaining pyrochlore Nd₂Zr₂O₇ nanoparticles had a diameter of approximately 200 nm estimated by TEM and approximately 550 nm determined by light scattering due to slight aggregation. The bulk density of the pelletized powder reached approximately 99% of theoretical value, after uniaxial pressing at 2.0–2.5 MPa and sintering at 1400 °C for 48 h.     

Chemical interactions between 3 mol% yttria-zirconia and Sr-doped lanthanum manganite

The chemical interactions between porous (La_0.8Sr_0.2)MnO₃ (LSM) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate have been investigated over the temperature range of 1300–1500 °C in air. Two distinct reaction layers of fluorite-type cubic zirconia solid solution c-(Zr,Mn,La,Y)O₂ and lanthanum zirconate pyrochlore (La,Sr)₂(Zr,Y)₂O₇ were observed at the interface of LSM/TZ3Y. It has been found that the diffusion/dissolution of Mn ions in TZ3Y leads to the formation of the fluorite-type cubic zirconia solid solution, while the interaction of lanthanum with TZ3Y results in the formation of the lanthanum zirconate pyrochlore phase. Phase studies in the (ZrY)O₂–La₂O₃–Mn₃O₄ system show that the fluorite-type cubic zirconia solid solution phase c-(Zr,Mn,La,Y)O₂, rather than the tetragonal 3 mol% Y₂O₃–ZrO₂ phase, is in equilibrium with LSM perovskite at high temperatures. A ternary phase diagram of the system at the (ZrY)O₂-rich end at 1400 °C in air was proposed based on the experimental results. It is suggested that the fundamental reason for the beneficial effect of A-site non-stoichiometry or Mn excess of LSM in the inhibiting of the lanthanum zirconate formation is due to the fact that Mn₃O₄ does not equilibrate with lanthanum zirconate at high temperatures.     

Flash Sintering of dense alumina ceramic discs with high hardness

MgO-doped-Al₂O₃ ceramic discs were fabricated by flash sintering (FS) and pressureless sintering (PS). The results showed that MgO-doped Al₂O₃ exhibited typical characteristics of flash sintering under an electric field in excess 2500 V/cm. Compared with the PS- fabricated specimen, the flash sintered specimens exhibited sub-micron grains (≤760 nm) and homogeneous microstructures. The relative density of the ?ash sintered MgO-doped Al₂O₃ ceramics increased with current density, reaching 99.91 % when the current density increased to 7 mA/mm². The FS-fabricated sample exhibited higher hardness (21.02 GPa) and fracture toughness (3.46 MPa m^1/2) than PS-fabricated sample.     

Two‐Phase LaLuZr2O7 Transparent Ceramic with High Transparency

Two‐phase LaLuZr₂O₇ transparent ceramic was fabricated via vacuum sintering using powders obtained by a simple sol–gel combustion method. These two phases are pyrochlore and defective fluorite phase with different chemical components. The in‐line transmittance of LaLuZr₂O₇ transparent ceramic is 73.4% at 1100 nm, showing an excellent optical performance. LaLuZr₂O₇ transparent ceramic is a promising candidate for scintillator host due to its high density (6.82 g/cm³).     

Effects of the nature of the doping salt and of the thermal pre-treatment and sintering temperature on Spark Plasma Sintering of transparent alumina

A slurry of α-Al₂O₃ was doped with Mg, Zr and La nitrates or chlorides, in various amounts in the range 150-500 wt ppm and then freeze-dried to produce nanosized doped powder (∼150 nm). The powder was sintered by SPS to yield transparent polycrystalline alpha alumina. The influence of the nature of the doping element and the starting salt, the thermal treatment before sintering and the sintering temperature on the transparency of the ceramics were investigated. The transparency of the ceramics of nanosized Al₂O₃ was shown to depend mainly on the way the powder was prepared, the nature of the doping salt also had an effect. Finally, a high real inline transmittance, reaching 48.1% was achieved after optimization.     

Structural and microstructural effects produced by microwave sintering over La2O3 doped TiB2-Ni composites

In this paper, an agglomerated composite powder of titanium diboride-nickel (TiB₂-Ni) was milled and spray-dried. The effects of the sintering method, sintering temperature, holding time, and La₂O₃-doped on the morphology, phase content and properties of the powder were investigated. The results showed that the microwave-sintered La₂O₃-doped powder had the best properties when sintered at 1273 K for 15 min. La doping reduced the oxygen content, inhibited the formation of the brittle Ni₃B phase, and improved the mechanical properties of the powder. The powder may be a suitable raw material for thermally-sprayed TiB₂-Ni coatings.     

A new method for production of glass-Ln2Ti2O7 pyrochlore (Ln = Gd,Tb, Er,Yb)

Glass-Ln₂Ti₂O₇ pyrochlore (Ln = Gd, Tb, Er, Yb) was fabricated by sintering the pelletized mixture of glass precursor and Ln-Ti composite at 1200 °C. The phase pure pyrochlore was in-situ crystallized in the amorphous glass matrix. The Ln-Ti composite was prepared by a simple soft chemistry route in an aqueous solution to ensure the homogeneity of the product. Thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy techniques were employed to investigate the glass-Ln₂Ti₂O₇ pyrochlore structure formation. The cell parameters for the pyrochlore structures by Le Bail fitting are in good agreement with the published data. The presence of the melting glass matrix facilitates the pyrochlore formation, with particle sizes in the range of 1–3 μm. This new aqueous synthetic method provides a simple pathway to produce glass-Ln₂Ti₂O₇ pyrochlore without using organic solvent and/or milling procedures, making it an attractive potential method for scale-up production.     

Phase equilibria and electrical properties of pyrochlore and zirconolite phases in the Bi2O3-ZnO-Ta2O5 system

The complete subsolidus phase diagram of the system Bi₂O₃-ZnO-Ta₂O₅, including cubic pyrochlore and monoclinic zirconolite phases, has been determined at 950-1050 °C. Through systematic heat treatment and X-ray diffraction of over 100 compositions, the layout of compatibility triangles (both 2-phase and 3-phase) and single phase solid solution areas has been determined. Pyrochlore and zirconolite phases have ideal nominal compositions Bi_1.5Zn_1.0Ta_1.5O₇ and Bi₂(Zn_1/3Ta_2/3)₂O₇ respectively, but both form solid solution areas. The sintering condition of pyrochlore pellets has been optimised to obtain high density ceramics with minimal weight loss: optimised condition is 1100 °C for pellets covered with sacrificial powder. Permittivity, ?′ dielectric loss and temperature coefficient of capacitance, TCC, of single phase materials were measured using impedance spectroscopy; ?′ and TCC show little variation with composition but the losses are higher for Zn-deficient compositions.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.     

High‐Temperature Aging of Plasma Sprayed Quasi‐Eutectoid LaYbZr2O7–Part II: Microstructure & Thermal Conductivity

LaYbZr₂O₇ ceramic thermal barrier coatings (TBC) of meta‐stable structure were prepared by an air plasma spraying process. Their microstructure and associated thermal transport properties evolution during high‐temperature annealing at 1300°C were characterized. The as‐sprayed LaYbZr₂O₇ TBCs underwent a fast crystallization and a quasi‐eutectoid transformation during annealing, resulting in a biphase composite consisting of La‐rich pyrochlore phase and Yb₂Zr₂O₇ fluorite phase with coherent phase boundaries. Due to the diffusion barriers between the two phases as well as the low interface energy of the coherent boundaries, sintering and grain growth of materials was significantly refrained. Therefore, a final thermal dynamically stable microstructure with a grain size of ~300 nm and a total porosity about 5% could be maintained even after long‐term aging at a high temperature of 1300°C. Resulting from this stable microstructure, an ultralow thermal conductivity of 1.3 W·(m·K)^?1 could be obtained even after 216 h high‐temperature aging, which is much lower than that of the state‐of‐art 7 wt% yttria‐stabilized zirconia TBCs. Both the high phase and microstructure stability and the extremely low thermal conductivities could be particularly beneficial for TBC material in gas turbine applications.     

Solid-state reactive sintering of La2-xGdxZr2O7 transparent ceramics and their optical properties

Transparent polycrystalline La_2-xGd_xZr₂O₇ (x = 0.4–2.0) ceramics were fabricated by solid-state reactive sintering using commercially available La₂O₃, Gd₂O₃ and ZrO₂ nanopowders as the raw materials. The phase composition, microstructure evolution, densification and optical transmittance of the resulting ceramics were investigated. XRD and Raman results reveal that both the powders and ceramics are in single-phase pyrochlore structure. With the promotion of Gd content, the disorder degree of pyrochlore structure increases gradually while the cell parameters decrease. The x = 1.0, 1.2 and 1.6 samples exhibit high optical transmittance in the 450 nm-6.0 μm range, and the La_0.4Gd_1.6Zr₂O₇ sample shows the highest transmittance of 83.84%. The transparent La_2-xGd_xZr₂O₇ ceramics with high optical quality are available as scintillator matrice and high temperature window material.     

High‐Temperature Aging of Plasma Sprayed Quasi‐Eutectoid LaYbZr2O7‐Part I: Phase Evolution

Phase and microstructure stability is an important issue for the durability and performance of thermal barrier coating (TBC) materials which have to work at high temperature for long time. In this work, we present a meta‐stable structure LaYbZr₂O₇ by air plasma spraying process, which can convert into thermodynamically stable fine‐grained quasi‐eutectoid structure with enhanced thermal insulation properties even under high‐temperature annealing. In this part, we first report on the phase composition and relationship in the LaYbZr₂O₇ coatings. The as‐sprayed LaYbZr₂O₇ coatings initially exhibited a mixture of amorphous phase and a nonequilibrium fluorite phase. Then it underwent a fast crystallization and a quasi‐eutectoid transformation during the first few hours of annealing at 1300°C. The phase constitution quickly reached an equilibrium state consisting of La‐rich pyrochlore phase and Yb₂Zr₂O₇ fluorite phase after 6 h annealing and kept stable ever since. Coherent phase boundaries were observed between the La‐rich pyrochlore and Yb₂Zr₂O₇ fluorite phase, indicating a lower interface energy, a lower ionic diffusion rate, a higher strength and creeping resistance of this material at high temperature, all of which could be particularly advantageous to a TBC material for high‐temperature gas turbine applications.