Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Sintering,micro-structure and Li+ conductivity of Li7−xLa3Zr2−xNbxO12/MgO (x = 0.2–0.7) Li-Garnet composite ceramics

Nb-doped Li₇La₃Zr₂O₁₂ (Nb-LLZO) is one of the promising electrolyte candidates in the Li-Garnet family due to its high Li-ion conductivity. The sintered Nb-LLZO ceramics, however, often exhibit abnormal grain growth with high porosity and poor mechanical properties. For advantaged electrochemical and mechanical properties, a uniform and dense microstructure is desired. In this research, MgO has been added as a secondary phase to inhibit abnormal grain growth in Nb-LLZO. The sintering process of the Nb-LLZO/MgO composite ceramics has been studied for different Nb doping levels (0.2–0.7 pfu) at sintering conditions of 1250?°C for 1–360?min. The ceramic density, microstructure, and Li-ion conductivity are reported. The composite ceramics have shown a very fast sintering speed. At 1250?°C, the 0.4Nb-LLZO/MgO composite can be well-sintered in 1?min. For sintering at 1250?°C for 40?min, ceramic samples showing relative density of 97%, conductivity of 6?×?10^?4 S?cm^?1 at 25?°C, and activation energy of 0.40?eV are obtained.     

Pump laser induced photodarkening in ZrO2-doped Yb:Y2O3 laser ceramics

5 at.% Yb:Y₂O₃ transparent ceramics were fabricated using vacuum sintering plus HIP. The ceramics doped with 1 at.% ZrO₂ as the sintering additive were densified at 1700 °C in vacuum followed by HIPing at 1775 °C, while those without sintering additives were densified at 1520 °C in vacuum followed by HIPing at 1450 °C. After sintering, both ceramics had relatively high in-line transmittance. However, during laser experiments, the ZrO₂-doped Yb:Y₂O₃ (Zr-YbY) ceramics were photodarkened when irradiated by 940 nm pump light. The discoloration might be attributed to the formation of Zr³⁺ color centers during lasing. In contrast, no photodarkening effect was detected in the pure Yb:Y₂O₃ ceramics without sintering additives (P-YbY). The P-YbY ceramics exhibited much higher lasing efficiency (17%) than the Zr-YbY ceramics (9%). To our best knowledge, it is the first time that the photodarkening effect was detected in rare-earth doped sesquioxide laser ceramics.     

Structural, thermal and mechanical properties of transparent Yb:(Y0.97Zr0.03)2O3 ceramic

Yb doped (Y_0.97Zr_0.03)₂O₃ transparent ceramics were fabricated by solid state reaction and vacuum sintering. The microstructure, thermal and mechanical properties of Y₂O₃ ceramic, as well as the effect of Yb doping concentration on these properties were investigated in detail. The lattice parameter and unit cell volume decrease with the increasing of Yb content, whereas thermal expansive coefficient increases. With Yb content increasing from 0 to 8 at.%, the mean grain size increases from 15.82 μm to 26.54 μm, and the thermal conductivity at room temperature (RT) decreases from 11.97 to 6.39 W/m/K. The microhardness decreases with Yb content, and the microhardness and fracture toughness of (Y_0.97Zr_0.03)₂O₃ transparent ceramic is 11.11 GPa and 1.29 MPa m^1/2, respectively.     

High Transparency Nd: Y2O3 Ceramics Prepared with La2O3 and ZrO2 Additives

High transparency Nd: Y₂O₃ ceramics were prepared by vacuum sintering with La₂O₃ and ZrO₂ sintering additives. The optimum in‐line transmittance of the sintered Nd: Y₂O₃ is 80.98% at the wavelength of 1100 nm, for which the content of La₂O₃ and ZrO₂ are 10.0 and 3.0 at.%, respectively. This specimen demonstrates homogeneous microstructure with the average grain size of 8.3 μm. The mechanism of sintering with La₂O₃ and ZrO₂ aids and the optical properties was discussed. The absorption, emission cross section, and fluorescence lifetime have been estimated as 1.62 × 10^?20 cm², 5.13 × 10^?20 cm², and 232 μs, respectively. Vickers hardness and the fracture toughness were measured of 9.18 GPa and 1.03 Mpa·m^1/2, respectively. All the results indicate that Nd: Y₂O₃ transparent ceramic is a promising candidate for laser material.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

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.     

Enhanced mechanical properties of 3 Y2O3 stabilized tetragonal ZrO2 incorporating tourmaline particles

The current study reports on the improvement of mechanical properties of 3?mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) by introduction of tourmaline through ball milling and subsequent densification by pressureless sintering at 800, 1200, 1300, 1400?°C. Findings demonstrate that no matter which sintering temperature the 3Y-TZP ceramic containing 2?wt% tourmaline reach a maximum value in flexural strength and fracture toughness as compared to other composite ceramics. As the tourmaline content is 2?wt% and the sintering temperature is 1300?°C, the flexural strength and fracture toughness of the composite ceramics are the highest, increases of 36.2% and 36.6% over plain 3Y-TZP ceramic respectively. The unique microstructure was systematically investigated through X-ray diffraction, scanning electron microscopy, energy dispersive spectrum, and flourier transform-infrared. The strengthening and toughening mechanism of tourmaline in 3Y-TZP ceramic were also discussed.     

High densification and Li-ion conductivity of Al-free Li7-xLa3Zr2-xTaxO12 garnet solid electrolyte prepared by using ultrafine powders

Ta-doped Li₇La₃Zr₂O₁₂ (Ta-LLZO) is considered as a promising solid electrolyte due to high Li-ion conductivity and good chemical stability against electrode materials. In this work, Ta-LLZO was prepared by a conventional solid-state reaction. Ultrafine powders were obtained by ball-milling to improve the surface activity. Ta-LLZO is sintered in ZrO₂ crucibles to avoid introducing Al into the samples. The particle size distribution, phase structure, morphology, ionic conductivity, electronic conductivity, density and electrochemical performance of semi-solid battery were characterized by laser diffraction particle size analyzer, X-ray diffraction, scanning electron microscope, AC-impedance, DC polarization, Archimedes method and a battery testing system, respectively. The results show that the ball milling to reduce the particle size is an effective way to solve the problem of relatively low density and Li-ion conductivity for Al-free Li_7-xLa₃Zr_2-xTa_xO₁₂. For Al-free Li_7-xLa₃Zr_2-xTa_xO₁₂, the increase of x (0.2?≤?x?≤?0.4) promotes the grain growth and sintering densification, but the increase of x (0.4?<?x?≤?0.6) has an adverse effect. Li_6.7La₃Zr_1.7Ta_0.3O₁₂ sintered at 1180?°C for 12?h shows the relative density of 92% and the highest Li-ion conductivity of 1.03?×?10^?3 S/cm at ³0?°C with the activation energy of about 0.37?eV, while Li_6.6La₃Zr_1.6Ta_0.4O₁₂ sintered at 1180?°C for 12?h shows the highest relative density of 96% and the Li-ion conductivity of 6.68?×?10^?4 S/cm at 30?°C with the activation energy of about 0.46?eV. The electronic conductivity of Al-free Li_7-xLa₃Zr_2-xTa_xO₁₂ is 10^?9 S/cm orders of magnitude. The semi-solid battery shows the first discharge capacity of 104.6 mAh/g and 92.5% capacity retention after 20 cycles.     

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.     

Fabrication,microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders

Ytterbium doped lutetium oxide (Yb:Lu₂O₃) transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) of the powders synthesized by the co-precipitation method. The effects of calcination temperature on the composition and morphology of the powders were investigated. Fine and well dispersed 5?at% Yb:Lu₂O₃ powders with the mean particle size of 67?nm were obtained when calcined at 1100?°C for 4?h. Using the synthesized powders as starting material, we fabricated 5?at% Yb:Lu₂O₃ ceramics by pre-sintering at different temperatures combined with HIP post-treatment. The influence of pre-sintering temperature on the densities, microstructures and optical quality of the 5?at% Yb:Lu₂O₃ ceramics was studied. The ceramic sample pre-sintered at 1500?°C for 2?h with HIP post-treating at 1700?°C for 8?h has the highest in-line transmittance of 78.2% at 1100?nm and the average grain size of 2.6?µm. In addition, the absorption and emission cross sections of the 5?at% Yb:Lu₂O₃ ceramics were also calculated.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

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.     

Thermoelectric properties of Bi2O3-added WO3 ceramics

Tungsten trioxide (WO₃) ceramics were prepared by firing Bi₂O₃-added WO₃ compacts with atomic ratios of Bi/W?=?0.00, 0.01, 0.03, or 0.05, in which Bi₂O₃ was mixed as a sintering agent. Dense ceramics consisting of remarkably grown WO₃ grains were obtained for Bi-containing samples with Bi/W?=?0.01, 0.03, and 0.05. The grain growth was enhanced by the liquid phase of Bi₂W₂O₉ formed among the WO₃ grains while firing. The XRD patterns did not show evidence for Bi inclusion into the WO₃ lattice, but the SEM-EDX showed an intensive distribution of Bi into the grain boundaries. Electrical conductivity σ and Seebeck coefficient S were measured in a temperature range of 373–1073?K. The temperature dependences indicated that the Bi₂O₃-added WO₃ ceramics were n-type semiconductors. It was considered that the electron carriers were generated from oxygen vacancies included into the WO₃ grains. The thermoelectric power factors S²σ for the ceramics ranged from 1.5?×?10^?7 W?m^?1 K^?2 to 2.8?×?10^?5 W?m^?1 K^?2, and the highest value occurred at 970?K for the ceramic with Bi/W?=?0.01.     

Effect of Zn addition on the structure and electrochemical properties of co-doped BaCe0.6Zr0.2Ln0.2O3-δ (Ln=Y,Gd, Yb) proton conductors

In this work, BaCe_0.6Zr_0.2Y_0.2-xYb_xO_3-δ and BaCe_0.6Zr_0.2Gd_0.2-xYb_xO_3-δ (x?=?0–0.20), proton conducting materials are prepared by the freeze-drying precursor method. The sintering conditions were optimized by adding Zn(NO₃)₂·6H₂O as sintering additive. The materials are thoroughly characterized by different structural and microstructural techniques, including X-ray diffraction, scanning and transmission electron microscopy, and thermogravimetric-differential thermal analysis. The addition of Zn favours the phase formation and densification at lower sintering temperatures; however, it leads to the segregation of a Zn-rich secondary phase, with general formula BaLn₂ZnO₅ (Ln?Y, Gd and Yb), which is identified and quantified for the first time. All samples with Zn as sintering aid exhibit cubic structure; however, the samples without Zn crystallize with orthorhombic or cubic structure, depending on the composition and thermal treatment. The electrical properties are studied by impedance spectroscopy. A deep analysis of the bulk and grain boundary contributions to the conductivity has revealed that the bulk conductivity remains almost unchanged along both series over Yb-doping; however, the grain boundary resistance decreases. The highest conductivity values are found for the intermediate members of both series, BaCe_0.6Zr_0.2Y_0.1Yb_0.1O_3-δ and BaCe_0.6Zr_0.2Gd_0.1Yb₁O_3-δ, with 33 and 28?mS?cm^?1 at 750?°C, respectively.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Phase evolution,structure, and electrochemical performance of Al-, Ga- and Ta- substituted Li7La3Zr2O12 ceramic electrolytes by a modified wet chemical route

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is one of the most promising solid-state electrolytes (SSEs) for advanced solid-state lithium batteries (SSLBs). In this work, Li_6.25Al_0.25La₃Zr₂O₁₂, Li_6.4Ga_0.2La₃Zr₂O₁₂, and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ceramics are prepared by a modified wet chemical route. The composition of the black mixtures derived from the precursors is ascertained. The phase evolution and structural properties from the ceramic mother powders to the final ceramic electrolytes are discussed in detail. The characteristic of cubic LLZO with the space group I-43d arises in the Li_6.4Ga_0.2La₃Zr₂O₁₂ ceramic electrolyte pellet after the secondary higher-temperature (1200 °C) sintering. The Rietveld refinement reveals the roles of Al³⁺ substitution at the Li⁺ sites and Ta⁵⁺ substitution at the Zr⁴⁺ sites to adjust crystal structure. In addition, the electrochemical performance of the ceramic pellets is also investigated. Remarkably, the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ceramic electrolyte has the most outstanding electrochemical performance, showing the high ionic conductivity of 6.88 × 10^?4 S cm^?1 (25 °C), the low activation energy of 0.42 eV and an extremely low electronic conductivity of 1.77 × 10^?8 S cm^?1 (25 °C). Overall, it is supposed that this work may help to achieve high-quality modified LLZO ceramic electrolytes, especially using the wet chemical strategy.     

Mechanical and thermal expansion studies on Ca0.5Sr0.5Zr4-xTixP6O24 ceramics

Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ (x?=?0?0.2) ceramics belonging to the NZP family were prepared and dense ceramics with no microcracks were obtained. All of the ceramic samples were still composed of the typical NZP structure with a small amount of Ti⁴⁺ substitution for Zr⁴⁺. The mechanical and thermal expansion properties of the ceramics were characterized and the result showed that the flexural strength monotonically increased to 66.5?MPa. The thermal expansion coefficient varied from 1.8 to 3.4?×?10^?6/°C with Ti⁴⁺ content increasing. Thus, it was clear that the substitution of Ti⁴⁺ for Zr⁴⁺ had obvious effects on the sinterability, mechanical and thermal expansion properties of Ca_0.5Sr_0.5Zr_4-xTi_xP₆O₂₄ ceramics, which were discussed in detail.     

Effects of Yb3+ doping on phase structure,thermal conductivity and fracture toughness of (Nd1-xYbx)2Zr2O7

To investigate the effects of Yb³⁺ doping on phase structure, thermal conductivity and fracture toughness of bulk Nd₂Zr₂O₇, a series of (Nd_1-xYb_x)₂Zr₂O₇ (x?=?0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics were synthesized using a solid-state reaction sintering method at 1600?°C for 10?h. The phase structures were sensitive to the Yb³⁺ content. With increasing doping concentration, a pyrochlore-fluorite transformation of (Nd_1-xYb_x)₂Zr₂O₇ ceramics occurred. Meanwhile, the ordering degree of crystal structure decreased. The substitution mechanism of Yb³⁺ doping was confirmed by analyzing the lattice parameter variation and chemical bond of bulk ceramics. The thermal conductivities of (Nd_1-xYb_x)₂Zr₂O₇ ceramics decreased first and then increased with the increase of Yb³⁺ content. The lowest thermal conductivity of approximately 1.2?W?m^?1 K^?1 at 800?°C was attained at x?=?0.4, around 20% lower than that of pure Nd₂Zr₂O₇. Besides, the fracture toughness reached a maximum value of ~1.59?MPa?m^1/2 at x?=?0.8 but decreased with further increasing Yb³⁺ doping concentration. The mechanism for the change of fracture toughness was discussed to result from the lattice distortion and structure disorder caused by Yb³⁺ doping.