Microstructure and electrical properties of Mn/Y codoped Ba0.67Sr0.33TiO3 ceramics

Pure and Mn/Y codoped Ba_0.67Sr_0.33TiO₃ (BST) ceramics were fabricated via the citrate–nitrate combustion technique, and the microstructure and electrical properties of BST ceramics were mainly investigated. The Mn/Y codoping concentration has a strong influence on the microstructure and electrical properties of BST ceramics. All BST ceramics possess a pure polycrystalline structure. The density, dielectric loss, leakage current, and ferroelectric properties are improved by codoping 0.5 mol% Mn and 1.0 mol% Y to BST. The relative density of 0.5 mol% Mn/1.0 mol% Y-codoped BST (BST0510) ceramics reaches 97.5% of the theoretical value. BST0510 ceramics have the lowest dielectric loss (tanδ < 0.0073 at 1 kHz) among all BST ceramics. BST0510 ceramics also demonstrate a low leakage current density (1.23 × 10^?7 A/cm²) at an applied field of 10 kV/cm, and excellent ferroelectric properties with a remanent polarization of 2P_r = 15.327 μC/cm² and a coercive field of 2E_c = 3.456 kV/cm. Therefore, the Mn and Y with optimum content help improve the electrical properties of BST materials.     

Strong photoluminescence and good electrical properties in Eu-modified SrBi2Nb2O9 multifunctional ceramics

Bismuth layer-structured ferroelectric (BLSFs) ceramics of Sr_1−xEu_x Bi₂Nb₂O₉ (SBT-xEu, x=0.000, 0.002, 0.004, 0.006) were prepared by a conventional solid-state reaction method. All the samples have a bismuth oxide layered structure with a dense microstructure. The ferroelectric, piezoelectric, dielectric and optical properties of the ceramics were investigated. After Eu³⁺ doping, samples show a bright red photoluminescence upon blue light excitation of the 400–500 nm. Upon the excitation of 465 nm light, the materials have two intense emission bands peaking around 593 nm (yellow) and 616 nm (red). Meanwhile, good electrical properties with large piezoelectric constant d₃₃ of 14 pC/N and large remnant polarization 2P_r of 11.97 μC/cm² are obtained at x=0.006. Moreover, this material has a high Curie temperature (T_c=429 °C) and high resistivity, which makes the material resistant to thermal depolarization up to its Curie temperature. This feature indicates that the SBN-xEu ceramics have a latent use in high temperature applications.     

Effect of composition and grain size on dielectric,ferroelectric and induced strain behavior of PLZT/ZrO2 composites

Lead lanthanum zirconate titanate ceramics (PLZT) are well known for their excellent dielectric, piezoelectric and ferroelectric properties. In this study, PLZT 9/70/30, 9/65/35 and 9/60/40 ceramics were prepared by vibro-milling mixed-oxide method. All compositions of powders were uniaxial pressed in pellets and sintered at the temperatures of 1200–1275 °C with various soaking times of 2, 4 and 6 h. The X-ray diffraction (XRD) patterns confirmed that all the PLZT samples had perovskite structure with ZrO₂ as a second phase and PLZT/ZrO₂ composite structure was formed. Dielectric behavior at the frequency of 1 kHz showed broad peak indicating relaxor ferroelectric behavior and the difference of the temperature at maximum dielectric at different frequencies increased when Zr:Ti ratio increased. Polarization with electric field (P-E loop) at room temperature showed that when Zr:Ti ratio increased, the coercive field decreased resulting from crystal structure change from tetragonal to rhombohedral. Induced strain with electric field depended on microstructure where the value of S_max/E_max tended to decrease with increasing grain size. It can be concluded that dielectric and ferroelectric behavior predominantly depended on composition of PLZT ceramics and induced strain behavior predominantly depended on grain size of PLZT ceramics.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

Effect of Zr substitution on structural,magnetic, and optical properties of Bi0.9Dy0.1Fe1−xZrxO3 multiferroic ceramics prepared by rapid liquid phase sintering method

Zr substituted Bi_0.9Dy_0.1Fe_1−xZr_xO₃ (x=0.03, 0.06 and 0.10) multiferroic ceramics were synthesized by rapid liquid phase sintering technique to improve its multiferroic properties. Rietveld structural refinement of XRD patterns and Raman spectra revealed a partial structural phase transition from rhombohedral (R3c) to biphasic structure (R3c+P4mm) on codoping. The substitution of larger ionic radii and higher valence Zr⁴⁺ ions at Fe-site leads to decrease in the grain size as a result of charge compensation at Fe site. The weak ferromagnetic behavior were observed in all samples along with maximum M_r value of 0.159 emu/g for x=0.03 concentration, which is also endorsed by second order Raman modes. The distortion in FeO₆ octahedra due to Zr substitution leads to splitting of electronic bands of 3.2 eV into multiplets, which in turn reduced the optical band gap value in the range of 2.06–2.10 eV for all samples.     

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y₂O₃, α-Al₂O₃ and Tb₄O₇ powders as raw materials. Samples sintered at 1750 °C for 20 h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29 µm. For the 5 at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544 nm originated from the energy levels transition of ⁵D₄→⁷F₅ of Tb³⁺ ions, and the intensity of the emission peak reaches a maximum value when the Tb³⁺ concentration is 5 at%. The in-line transmittance of the 5 at% Tb:YAG ceramics is 73.4% at the wavelength of 544 nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.     

Reactive spark plasma sintering of Ti3SnC2, Zr3SnC2 and Hf3SnC2 using Fe,Co or Ni additives

This work studied the effect of adding 10 at% Fe, Co or Ni to M-Sn-C mixtures with M = Ti, Zr or Hf on MAX phases synthesis by reactive spark plasma sintering. Adding Fe, Co or Ni assisted the formation of 312 MAX phases, i.e., Ti₃SnC₂, Zr₃SnC₂ and Hf₃SnC₂, while their 211 counterparts Ti₂SnC, Zr₂SnC and Hf₂SnC formed in the undoped M-Sn-C mixtures. The lattice parameters of the newly synthesized Zr₃SnC₂ and Hf₃SnC₂ MAX phases were determined by X-ray diffraction. Binary MC carbides were present in all ceramics, whereas the formation of intermetallics was largely determined by the selected additive. The effect of adding Fe, Co or Ni on the MAX phase crystal structure and the microstructure of the produced ceramics was investigated in greater detail for the case of M = Zr. A mechanism is herein proposed for the formation of M₃SnC₂ MAX phases.     

In situ preparation of CoFe2O4–Pb(ZrTi)O3 multiferroic composites by gel-combustion technique

Diphase magnetoelectric composites of CoFe₂O₄–Pb(ZrTi)O₃ were prepared by citrate–nitrate combustion technique by using Pb(Zr,Ti)O₃ template powders obtained by the mixed oxide method. Pure diphase powder composites with a good crystallinity were obtained after calcination. The composition and purity were maintained after sintering at temperature of 1100 °C/2 h, which ensured limited reactions at interfaces, while by sintering at 1250 °C/2 h, some small amounts of secondary phases identified as nonstoichiometric ZrO_2?x resulted. The method allowed to produce diphase ceramics with homogeneous microstructures and a very good mixing of the two phases. The dielectric and magnetic investigation at room temperature confirmed the formation of composite ceramics with both dielectric and magnetic properties at room temperature, with permittivity and magnetization resulted as sum properties from the parent Pb(Zr,Ti)O₃ and ferrite phases.     

Fatigue-resistant,temperature-insensitive strain behavior and strong red photoluminescence in Pr-modified 0.92(Bi0.5Na0.5)TiO3–0.08(Ba0.90Ca0.10)(Ti0.92Sn0.08)O3 lead-free ceramics

An electric-field-induced large strain and strong photoluminescence was achieved by introducing trivalent Pr³⁺ as the activator into 0.92(Bi_0.5Na_0.5)TiO₃ − 0.08(Ba_0.90Ca_0.10)(Ti_0.92Sn_0.08)O₃ (BNT−8BCST) ceramics. Around a critical composition of 0.4 mol% Pr³⁺, a large strain of ∼0.39% with a relatively small hysteresis compared with existing lead-free Bi-perovskite ceramics was obtained. In particular, the strain is very resistant to field cycling and thermal shock, giving the materials attractive for its exceptionally good fatigue resistance and high temperature stability. Besides the excellent electrical properties, Pr³⁺-modified BNT−8BCST host exhibits a strong photoluminescence with a bright red emission at 610 nm assigned to ¹D₂ → ³H₄ transitions of the Pr³⁺ ions upon a blue light excitation of 400–500 nm. The photoluminescence can be enhanced through poling treatment of the samples. Moreover, samples have a superior water resistance property which almost maintaining the same photoluminescence intensity after 40 h water immersion time. These results suggest the material may have potential application as a multifunctional device such as “on-off” actuator and electric field-controlled photoluminescence devices by integrating its excellent luminescence and electrical properties.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Preparation and photoluminescence properties of Eu3+-doped ZrO2 nanotube arrays

Zr–Eu alloy containing 3 at% Eu was prepared by a powder metallurgical method and Eu³⁺-doped ZrO₂ nanotube arrays were prepared by anodising the Zr–Eu alloy. The properties of Eu³⁺-doped ZrO₂ nanotube arrays were studied in contrast to undoped ZrO₂ nanotube arrays under different annealing temperatures. Results showed that the Eu³⁺ ions could not only stabilise the tetragonal phase of zirconium oxide, but also make the crystallite sizes smaller. Annealing temperature exerted a significant influence on the absorbance value, as well as the intensity and position of the photoluminescence peaks. When the excitation wavelength was either 248 nm or 270 nm, the sample annealed at 600 °C displayed the strongest emission peak; while under excitation at 232 nm, the sample annealed at 400 °C exhibited the strongest emission peak.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

Preparation and properties of a novel precursor-derived Zr-C-B-N composite ceramic via zirconocene and borazine

A novel preceramic polymer polyzirconocenyborazane (PZCBN) was synthesized by the polymerization of Bis(cyclopentadienyl)zirconium divinyl and borazine, introducing Zr, B, C, N together. The formation and concentration of elements Zr, C, B, N in the precursor and ceramic were detected through FTIR NMR, XRD, SEM and TEM. From the analysis, the Cp₂Zr(CH?CH₂)₂ and borazine linked together via the addition reaction between C?C and B-H. And after pyrolysis at 1200 °C, the precursor turned to ZrC/ZrB₂/BN composite ceramics, with a yield of 52 wt%. EDX resulted showed that the elements were well dispersed in the ceramics. According to SEM and TEM, the ceramic had a relatively dense structure with nano crystalline areas of ZrC embedded in the amorphous Zr-C-B-N matrix. TGA in air demonstrated that the ceramic had a favorable property on oxidation resistance.     

Anisotropy oxidation of textured ZrB2–MoSi2 ceramics

Oxidation behavior of hot forged textured ZrB₂–20 vol% MoSi₂ ceramics with platelet ZrB₂ grains was investigated at 1500 °C for exposure time from 0.5 to 12 h. Compared to untextured ceramics, the textured ceramics showed obvious anisotropic oxidation behavior and the surface normal to the hot forging pressure demonstrated better oxidation resistance. Such improvement in the oxidation resistance is primarily considered as a higher intrinsic ZrB₂ atomic density on the orientated {0 0 l} planes in the textured ceramics. It is expectable that the anisotropic textured ZrB₂–MoSi₂ ceramics can offer better oxidation resistance when a certain surface with higher oxidation resistance is exposed to air at elevated temperature.     

Tetragonal Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3: Lead-free ferroelectric transparent ceramics with electrical and optical multifunctional performances

The lead-free Er³⁺-doped (K_0.48Na_0.48Li_0.04)(Nb_0.96Bi_0.04)O₃ (KNLNB-Er-x) ceramics were fabricated by conventional pressureless sintering. They possess a tetragonal perovskite phase with dense microstructure. High transmittances of the ceramics are obtained both in the visible and infrared regions. The optical band gap energies of them are 3.07–3.11 eV, close to other KNN-based materials. The relaxor-like characteristics, good dielectric, ferroelectric and piezoelectric properties of the ceramics have been obtained. The up-conversion photoluminescence spectra have been studied and obvious color-tunable emissions have been observed by modulating temperature. The fluorescence intensity ratio (FIR) value based on green emissions at 533 and 555 nm in the temperature ranging from 300 to 750 K have been investigated, giving the maximum sensitivity of ~ 0.0038 K^?1. Furthermore, the FIR technique opens up a method to detect the Curie temperature of the ceramics. Owing to both optical and electrical properties, the KNLNB-Er-x transparent ceramics could be a promising candidate in the application of color-tunable solid-state lightings, optical temperature sensors, and electrical-optical coupling devices.     

Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB₂-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB₂ grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB₂ grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB₂-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB₂-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB₂-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB₂. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.     

Densification and mechanical properties of hot-pressed ZrN ceramics doped with Zr or Ti

The densification of hot-pressed ZrN ceramics doped with Zr or Ti have been investigated at 1500–1700 °C. It is shown that either Zr or Ti additive can facilitate the densification process. ZrN with 20 mol% Zr or Ti (named ZNZ and ZNT) sintered at 1700 °C can achieve above 98% relative densities whereas densification temperature up to 2000 °C is necessary for pure ZrN. The densification improvements are attributed to solid solution of Zr or Ti into ZrN to form non-stoichiometric ZrN_1?x or (Zr, Ti)N_1?x. The microstructures and mechanical properties of ZNZ and ZNT samples have been examined. Large grain size and flat fracture surface existed in ZNT sample sintered at 1700 °C, which lead to poor toughness as low as 2.3 MPa m^1/2. On the contrary, the fracture toughness of ZNZ sample sintered at 1700 °C was up to 5.9 MPa m^1/2, attributed to fine and uniform grain size distribution.     

Domain structures in hot-press sintered SrBi2Ta1.6Nb0.4O9 and SrBi2Ta2O9 ferroelectric ceramics

SrBi₂Ta_1.6Nb_0.4O₉ (SBTN) and SrBi₂Ta₂O₉ (SBT) ceramics with typical bismuth layered perovskite structure were synthesized by hot-press sintering at 1000 °C for 2 h. The maximum relative density of as-sintered SBTN and SBT materials is 98.97%. The domain structure of SBTN and SBT was systemically characterized by means of TEM and HRTEM. The 90° domain walls were identified by the 90° rotation relationship of the electron diffraction pattern along the [0 0 1] zone axis. Irregular shaped and highly curved 180° domain wall were also observed in SBTN ceramics. The traditional α-fringes can be found in SBT, which are the evidence of large strains in hot-press sintering ceramics. Rod-like SrTa₂O₆ precipitates are also analyzed as well as its interface with the matrix.     

Synthesis,structure and catalytic properties of a novel zirconium guanidinato complex [Zr{ArNC(NMe2)N(SiMe3)}(μ2-Cl)Cl2]2[Ar = 2,6-iPr2-C6H3]

A novel non-symmetric zirconium guanidinato complex, [{Zr{ArNC(NMe₂)N(SiMe₃)}(μ₂-Cl)Cl₂}₂] (Ar = 2,6-ⁱPr₂-C₆H₃), was synthesized and structurally characterized. Catalytic studies showed that the zirconium complex was active for ethylene polymerization with the activity of 4.98 × 10⁵ g PE/mol Zr h. The influences of cocatalysts, Al/Zr molar ratios and ethylene pressures on the activities were investigated.