Fabrication and Luminescent Properties of Nd3+-Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

The fabrication of transparent Nd³⁺ ion-doped Lu₂O₃ ceramics is investigated by pressureless sintering under a flowing H₂ atmosphere. The starting Nd-doped Lu₂O₃ nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH₄OH+NH₄HCO₃ mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd³⁺:Lu₂O₃ powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m²/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd³⁺:Lu₂O₃ ceramics are obtained after being sintered under a dry H₂ atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu₂O₃ ceramic at 807 nm reached 14 cm⁻¹, while the emission cross section at 1079 nm was 6.5 × 10⁻²⁰ cm².     

Microstructure and Photoluminescence Properties of Mg-Doped BaTiO3:Pr3+ Phosphor

The photoluminescence of Mg-doped BaTiO₃:Pr³⁺ (Pr³⁺: 0.1 mol%) ceramics was investigated by changing the doping concentration of Mg and the sintering temperature. The results indicated that the intensity of red emission due to the ¹ D ₂→³ H ₄ transition of Pr³⁺ exhibited significant dependence on both the Mg doping content and the sintering temperature; the strongest red emission intensity was observed for 2.0 mol% Mg-doped ceramics sintered at 1050°C. An interpretation of the results obtained was made in terms of the changes in the crystal structure and microstructure of the ceramics.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Low Temperature Sintering of Zn3Nb2O8 Ceramics from Fine Powders

A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn₃Nb₂O₈ powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn₃Nb₂O₈ from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb₂O₆. Maximum sintering activity was observed for single-phase crystalline Zn₃Nb₂O₈ powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V₂O₅ sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T _s)≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Controlled Oxygen Partial Pressure Sintering of (Pb,La)(Zr,Ti)O3 Ceramics

Transparent PLZT(7/60/40) ceramics with large piezoelectric coefficients were obtained using a two-step sintering process with controlled oxygen partial pressure. Specifically, low-oxygen-pressure and low-temperature sintering were used in the first step, followed by a high-oxygen-pressure, high-temperature sintering cycle. High-density ceramics with small grain sizes of about 3 µm were prepared. As a result, k _p= 0.71, k ₃₃= 0.78, d ₃₃= 850 × 10^-12 C/N, and a transparency of 15% (λ= 610 nm, thickness of 1 mm) have been achieved; 20% improvement of d ₃₃ was gained compared to conventional processed PLZT ceramics ( d ₃₃= 710 × 10^-12 C/N).     

Transparent Cr4+-Doped YAG Ceramics for Tunable Lasers

Transparent Cr⁴⁺:YAG (Y₃Al_SO₁₂) ceramics doped with Ca and Mg as counterions and SiO₂ as a sintering aid were fabricated by a solid-state reaction method using high-purity powders of Al₂O₃, Y₂O₃, and Cr₂O₃. The mixed powder compacts were sintered at 1750°C for 10 h in oxygen, or 1750°C for 10 h under vacuum, and then annealed at 1400°C for 10 h in oxygen. Cr-doped YAG ceramics sintered in oxygen had a brown color and characteristic absorption by Cr⁴⁺ ions, whereas these YAG ceramics sintered under different conditions (vacuum + oxygen) had a green color and absorption at ∼590 and 430 nm by Cr³⁺ ions. The absorption behavior of YAG ceramics sintered in oxygen was almost equivalent to that of Cr⁴⁺:YAG single crystals fabricated by the Czochralski method.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

Microwave Characteristics of (Pb,Ca)(Fe,Nb,Sn)O3 Dielectric Materials

The dielectric properties of (Pb_{1− x} Ca _x ){(Fe_1/2Nb_1/2)_{1– y} Sn _y }O₃ solid solutions, where 0.4 lessthan equal tox ≤ 0.6, y = 0.05, 0.1, have been investigated at microwave frequencies. The replacement of Fe³⁺/Nb⁵⁺ by Sn⁴⁺ at the B–site of the perov-skite structure considerably improves the loss quality factor Q and does not remarkably affect the dielectric constant epsilon_r and the temperature coefficient of resonant frequency tau_f. The tau_f value of nearly 0 ppm/°C can be realized for x= 0.55. New high-quality dielectric ceramics having epsilon_r of 85.3-89.9,Qf values of 7510-8600 GHz, and τ_f of 0-9 ppm/°C were obtained at 1150°C for 3 h sintering in air. The influence of the sintering atmosphere on dielectric properties was also investigated.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Effect of CuO on the Sintering Temperature and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

When a small amount of CuO was added to (Na_0.5K_0.5)NbO₃ (NKN) ceramics sintered at 960°C for 2 h, a dense microstructure with increased grains was developed, probably due to liquid-phase sintering. The Curie temperature slightly increased when CuO exceeded 1.5 mol%. The Cu²⁺ ion was considered to have replaced the Nb⁵⁺ ion and acted as a hardener, which increased the E _c and Q _m values of the NKN ceramics. High piezoelectric properties of k _p=0.37, Q _m=844, and ɛ₃ ^T /ɛ₀=229 were obtained from the specimen containing 1.5 mol% of CuO sintered at 960°C for 2 h.     

Phase Structure, Microstructure, and Electrical Properties of Sb-Modified (K, Na, Li) (Nb, Ta) O3 Piezoelectric Ceramics

Lead-free (K_0.44Na_0.52Li_0.04)(Nb_{0.80− x} Ta_0.20Sb _x )O₃ piezoelectric ceramics were prepared by the ordinary sintering method. The much higher Pauling electronegativity of Sb compared with Nb makes the ceramics more covalent. By increasing x from 0.00 to 0.06, the phase structure of the ceramics changed from the tetragonal to the pseudocubic phase, and both the bands in the Raman scattering spectra shifted to lower frequency numbers. The grain growth of the ceramics was improved by substituting Sb⁵⁺ for Nb⁵⁺. By increasing x , the dielectric properties were optimized and the variation of dielectric constants before and after poling became smaller. Only the tetragonal–cubic phase transition was observed above room temperature in all the ɛ_r– T curves. The degree of diffuseness increased from 1.29 at x =0.00 to 1.96 at x =0.06, indicating that the ceramics at x =0.06 changed to an approximate ideal relaxor ferroelectric. The temperature dependences of f _r and k _p became better by increasing x properly. Significantly, the ceramics with x between 0.00 and 0.04 had high density and outstanding electrical properties ( d ₃₃=241–272pC/N, k _p=0.42–0.52, ɛ_r=1258–1591, tan δ=0.015–0.025, T _c=280°–355°C, E _c=10.62–12.60 kV/cm, and P _r=16.19–20.13 μC/cm²). Besides, the underlying mechanism for variations of the electrical properties due to Sb⁵⁺ substitution was explained in this work.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Preparation and Luminescence Spectra of Calcium- and Rare-Earth (R = Eu, Tb, and Pr)-Codoped α-SiAlON Ceramics

Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ= 550–590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu²⁺ from the lowest crystal field component of 4 f ⁶5 d to ⁸S_7/2 (4 f ⁷) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the ⁵D₄→⁷F _j ( j = 3, 4, 5, 6) transitions of Tb³⁺ and ³P₀→³H _j ( j = 3, 4, 5) transitions of Pr³⁺, respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors.     

Effect of Rare-Earth Ions on Electrical Conductivity of BaTiO3 Ceramics

Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO₃ ceramics doped with 0 to 1.2 at.% rare-earth ions. BaTiO₃ doped with low levels of rare-earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare-earth ions occupy Ba²⁺ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare-earth ions occupy both Ba²⁺ and Ti⁴⁺ sites, and, consequently, BaTiO₃ becomes insulative because of charge compensation.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

Defect Structure of PLZT Doped with Mn, Fe, and Al

Practically pore-free Mn-, Fe-, and Al-doped PLZT ceramics were prepared using isostatic hot-pressing. The incorporation of the dopants in the perovskite lattice of PLZT (Pb_0.9La_0.1 |Zr_0.5Ti_o.5O₃₊δ) ceramics was analyzed from measurements of the density, lattice constants, and weight loss during sintering. It was deduced that Mn, Fe, and Al are incorporated as trivalent ions at (Zr, Ti)⁴⁺ sites and that charge compensation is effected by elimination of cation vacancies present in the undoped PLZT.