Low Loss Dielectrics in the Ca(B'1/2Nb1/2)O3 [B'=Lanthanides, Y] System

The Ca(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] complex perovskites have been prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscopy methods. The ceramics have dielectric constant (ɛ_r) in the range 23–32, normalized Q -factor ( Q _u× f ) 11 000–38 000 GHz and temperature coefficient of resonant frequency (τ_f) −43–5.2 ppm/°C. The microwave dielectric properties of Ca(B'_1/2Nb_1/2)O₃ ceramics are found to depend on the ionic radii of B'-site elements and tolerance factor ( t ). The substitution of Ba²⁺ and Sr²⁺ for Ca²⁺ resulted a phase transition in Ca(B'_1/2Nb_1/2)O₃ ceramics. The (Ca_0.05Ba_0.95) (Y_1/2Nb_1/2)O₃ has τ_f close to zero (1.2 ppm/°C) with ɛ_r=35 and Q _u× f =48 500 GHz and is proposed as a useful material for base station applications. Dielectric properties of the Ca(B'_1/2Nb_1/2)O₃ ceramics were tailored by the addition of TiO₂ and CaTiO₃.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

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.     

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High-Performance Li/Ta-Modified (Na,K)NbO3 Ceramics

Microstructure characteristics, phase transition, and electrical properties of (Na_0.535K_0.485)_0.926Li_0.074(Nb_0.942Ta_0.058)O₃ (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d ₃₃ is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO₃ ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k _p=46.2%, thickness electromechanical coupling factor k _t=36%, mechanical quality factor Q _m=18, remnant polarization P _r=21.1 μC/cm², and coercive field E _c=1.85 kV/mm.     

Co2+-Substitution Effect in Ce-TZP/La M-type Hexaferrite Composites

A Ce-TZP/platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO powders. Platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)₁₂O₁₉ composites was expanded widely by substituting Co²⁺ ions for Fe²⁺ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)₁₂O₁₉ composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 Ceramics

(Ca_{1+ x} Sm_{1− x} )(Al_{1− x} Ti _x )O₄ (0≤ x ≤0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca_{1+ x} Sm_{1− x} ) (Al_{1− x} Ti _x )O₄ and minor amount of (Ca,Sm)(Al,Ti)O₃ secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO₄ ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ɛ_r=19–23, Q × f =49 100–118 700 GHz, and τ_f=−15–15 ppm/°C.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

A/B Site Modified CaTiO3 Dielectric Ceramics for Microwave Application

(Li_1/2Nd_1/2)²⁺ substitution into the A site and (Mg_1/3Ta_2/3)⁴⁺ substitution into the B site of CaTiO₃ ceramic were investigated, respectively. The modified CaTiO₃ dielectric ceramics prepared by conventional solid-state method exhibit single perovskite structure and improved dielectric properties. Optimal microwave dielectric properties of ɛ_r=112.6, Q × f =4480 GHz, τ_f=8.2 ppm/°C in [Ca_0.4(Li_1/2Nd_1/2)_0.6] TiO₃ and ɛ_r=60.2, Q × f =36900 GHz, τ_f=−10.1 ppm/°C in Ca[Ti_0.4(Mg_1/3Ta_2/3)_0.6] O₃ are obtained, which indicates their potential for microwave application. The effects of change of crystal structure on dielectric properties are also discussed.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.     

Synthesis of Highly Oriented Lead Zirconate–Lead Titanate Film Using Metallo-organics

Crack-free Pb(Zr,Ti)O₃ (PZT) thin films with preferred orientation were prepared successfully on MgO (100), SrTiO₃ (100), and Pt/Ti/SiO₂/Si substrates from metal alkoxide solutions. Calcination of precursor films in a H₂O─-O₂ gas mixture was found to be effective not only for low-temperature crystallization of perovskite PZT, but also for obtaining the preferred orientation of PZT films. Single-phase PZT films with high preferred orientation were synthesized on MgO (100) and Pt/Ti/SiO₂/Si substrates at 550° and 600°C for 2 h, respectively. The PZT film on the Pt/Ti/SiO₂/Si substrate showed a permittivity of 520, tan δ of 0.03, a remanent polarization of 24 μC/cm², and a coercive field of 54 kV/cm.     

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.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Microstructure and Microwave Dielectric Properties of (1−x)Ca(Mg1/3Ta2/3)O3/xCaTiO3 Ceramics

Dense (1− x )Ca(Mg_1/3Ta_2/3)O₃/ x CaTiO₃ ceramics (0.1≤ x ≤0.9) were prepared by a solid-state reaction process. The crystal structures and microstructures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Single-phase solid solutions were obtained in the entire composition range. Long-range 1:2 ordering of B-site cations and oxygen octahedra tilting lead to the monoclinic symmetry with space group P 2₁/ c for x =0.1. For x above 0.1, the long-range ordering was destroyed and the crystal structure became the orthorhombic with space group Pbnm . The microwave dielectric properties showed a strong dependence on the composition and microstructure. The dielectric constant and temperature coefficient of resonant frequency increased nonlinearly as the CaTiO₃ content increased while the Qf values decreased approximately linearly. Good combination of microwave dielectric properties was obtained at x =0.45, where ɛ_r=45.1, Qf =34 800 GHz, and τ_f=17.4 ppm/°C.     

Low-Dielectric Loss Characteristics of Nd(Co1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ (NCT) ceramics using starting powders of Nd₂O₃, CoO, and TiO₂ prepared by the conventional solid-state route have been researched. The dielectric constant values (ɛ_r) saturated at 24.8–27. Quality factor ( Q × f ) values of 37 900–140 000 (at 9 GHz) and the measured τ_f values ranging from −45 to −48 ppm/°C can be obtained when the sintering temperatures are in the range of 1410°–1500°C. The ɛ_r value of 27, the Q × f value of 140 000 (at 9 GHz) and the τ_f value of −46 ppm/°C were obtained for NCT ceramics sintered at 1440°C for 4 h. For applications of high selective microwave ceramic resonator, filter, and antenna, NCT is proposed as a suitable material candidate.     

Low-Temperature Firing and Microwave Dielectric Properties of Ca[(Li1/3Nb2/3)0.84Ti0.16]O3−δ Ceramics for LTCC Applications

The effects of LiF and ZnO–B₂O₃–SiO₂ (ZBS) glass combined additives on phase composition, microstructures, and microwave dielectric properties of Ca[(Li_1/3Nb_2/3)_0.84Ti_0.16]O_3−δ (CLNT) ceramics were investigated. The LiF and ZBS glass combined additives lowered the sintering temperature of CLNT ceramics effectively from 1150° to 880°C. The main diffraction peaks of all the specimens split due to the coexistence of the non-stoichiometric phase (A) and stoichiometric phase (B), which all possess CaTiO₃-type perovskite structures. The transformation from A into B became accelerated with the increase of LiF or ZBS content. ZBS glass restrained the volatilization of lithium salt, which greatly affected the microstructures and microwave dielectric properties. CLNT ceramics with 2 wt% LiF and 3 wt% ZBS sintered at 900°C for 2 h show excellent dielectric properties: ɛ_r=34.3, Q × f =17 400 GHz, and τ_f=−4.6 ppm/°C. It is compatible with Ag electrodes, which makes it a promising ceramic for low-temperature cofired ceramics technology application.     

Chemical Vapor Deposition of Polycrystalline Al2O3

Polycrystalline Al₂O₃ was chemically vapor-deposited onto sintered Al₂O₃ substrates by reaction of AlCl₃ with (1) H₂O, (2) CO:H₂, and (3) O₂ at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al₂O₃, the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al₂O₃ was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface.     

Photoluminescent Properties of (Ca,Zn)TiO3:Pr,B Particles Synthesized by the Peroxide-Based Route Method

(Ca_{1− x} ,Zn _x )TiO₃:Pr, B red phosphor particles were prepared using the peroxide-based route and their photoluminescent (PL) properties were investigated by changing the sintering temperature, the concentration of the activator, the ratio of Ca to Zn, and the amount of H₃BO₃ flux. For the CaTiO₃:Pr phosphor, a pure perovksite-type CaTiO₃ phase was formed when the sintering temperature was 700°–800°C. It was found that the substitution of Zn atoms instead of Ca considerably enhanced the 614-nm red emissions. The PL intensity of (Ca_{1− x} ,Zn _x )TiO₃:Pr phosphor was also additionally improved by adding an H₃BO₃ flux. Finally, the optimized phosphor Ca_0.85Zn_0.15TiO₃:0.001Pr,0.1B showed the highest PL intensity.     

Dielectric Characteristics of Nd(Zn1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Zn_1/2Ti_1/2)O₃ (NZT) ceramics prepared by the conventional solid-state route have been studied. The prepared NZT exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. The dielectric constant values (ɛ_r) saturated at 29.1–31.6. The quality factor ( Q × f ) values of 56 700–170 000 (at 8.5 GHz) can be obtained when the sintering temperatures are in the range of 1300°–1420°C. The temperature coefficient of resonant frequency τ_f was not sensitive to the sintering temperature. The ɛ _r value of 31.6, the Q × f value of 170 000 (at 8.5 GHz), and the τ_f value of −42 ppm/°C were obtained for NZT ceramics sintering at 1330°C for 4 h. For applications of high selective microwave ceramic resonators, filters, and antennas, NZT is proposed as a suitable material candidate.