Effect of ZnO doping on the microstructure and microwave dielectric properties of 0.2CaTiO3-0.8(Li0.5Sm0.5)TiO3 ceramics

0.2CaTiO₃-0.8(Li_0.5Sm_0.5)TiO₃-xZnO(x = 0, 0.3, 0.6, 0.9, 1.2 wt%, 0.2CT-0.8LST-xZnO) with orthogonal perovskite structure were fabricated by the solid state method. The effects of ZnO additives on the microwave dielectric properties of 0.2CT-0.8LST ceramics were systematically investigated. With increasing the dopant (x) concentration, the dielectric constant (ε_r) and the temperature co-efficient of resonance frequency (τ_f) decreased, however, the Q × f values increased. The relationship between vibration mode and microwave dielectric properties was studied using Raman spectroscopy. The Q × f value of ceramics was related to the half-height width of Raman scattering. Narrower Raman scattering peaks corresponded to longer microwave energy propagation decay times and higher Q × f value. Based on X-ray photoelectron spectroscopy (XPS), the addition of Zn²⁺ ions limited the reduction of Ti⁴⁺ cations. The excellent dielectric properties were obtained when x = 1.2 wt% with ε_r = 100.25, Q × f = 6525 GHz, and τ_f = ?12.12 ppm/°C.     

The effects of dispersants on sinterability and microwave dielectric properties of Zr0.8Sn0.2TiO4 ceramics

This study investigated the effects of dispersants (deionised water and ethanol) on the sinterability, phase compositions and microwave dielectric properties of Zr_0.8Sn_0.2TiO₄ ceramics prepared by a solid-state reaction. Results showed the presence of impurity phases in low-density ceramics with ethanol as dispersant sintered from 1500?°C to 1550?°C. However, pure phase was detected in samples prepared with deionised water as dispersant when sintering temperature ranged from 1512?°C to 1550?°C. The microwave dielectric properties of the samples with deionised water significantly improved compared with those with ethanol. Thus, deionised water was suitable for preparing Zr_0.8Sn_0.2TiO₄ ceramics with a high density of approximately 98%, ε_r of 39.83, Q ×?f of 33,700?GHz and τ_f of +?3.5?ppm/°C.     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 ppm/^°C.     

Phase composition,crystal structure,and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y_3?xAl₅Nb_xO_12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO₄ secondary phase at the grain boundary of Y₃Al₅O₁₂. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb⁵⁺ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of ε_r ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τ_f ~ ? 34.7 ppm/°C can be obtained in x = 0.1 (Y_2.9Al₅Nb_0.1O_12.1) sintered at 1700 °C for 6 h.     

Sintering behaviour,phase composition,microstructure, and dielectric properties of BaSm2O4 microwave ceramics

BaSm₂O₄ was prepared by the conventional solid-phase reaction method. Single-phase dense BaSm₂O₄ ceramic (space group: Pnam) was obtained at 1500 °C. Crystal refinement results show that BaSm₂O₄ ceramics have a CaFe₂O₄ structure. The change of Q × f is explained by calculating the stack fraction and radial shrinkage of BaSm₂O₄ ceramics. When the sintering temperature was 1500 °C, the packing fraction and radial shrinkage of the BaSm₂O₄ ceramic reached the maximum values of 52.194% and 36%. Due to secondary recrystallization, the relative density of the ceramics increases and then decreases, reaching a maximum of 1500 °C (96.65%). In addition, the τ_? value is affected by SmO₆ octahedral distortion. The ceramics have the best combined dielectric properties after sintering at 1500 °C for 4 h: ε_r = 10.99, Q × f = 54598 GHz, τ_? = ?25.4 ppm/°C. BaSm₂O₄ ceramics have good prospects for applications in the field of mobile communication base stations.     

Effects of Sn substitution on the crystal structures,microstructures, and microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

A series of Ce₂(Zr_1?xSn_x)₃(MoO₄)₉ (0.02 ≤ x ≤ 0.1) (CZ_1?xS_xM) ceramics were synthesized to investigate the effect of Sn⁴⁺ doping on the crystal structure, chemical bond parameters, and dielectric properties of Ce₂Zr₃(MoO₄)₉ ceramics. X-ray diffraction patterns illustrated the formation of the single-phase trigonal system solid solution in all samples. Rietveld refinement result showed that the lattice volume decreased linearly, which can be explained by the fact that the effective radius of Sn ion is smaller than that of Zr ion. As the Sn content increased, scanning electron microscope images showed that the CZ_1?xS_xM ceramics transformed from bar-like grains to disk-like grains and the grain size declined gradually. The structure–property correlation was estimated by using P–V-L theory; the descending ε_r was mainly consistent with the reduced polarizability and total bond ionicity. The Q × f was associated with the lattice energy of the Ce–O1 bond. The change of τ_f value was mainly attributed to the bond energy (E_Mo1–O1 and E_Mo1–O4) and the coefficients of thermal expansion (α_Mo1–O1 and α_Mo1–O4). Infrared analysis indicated that the dielectric properties of the CZ_1?xS_xM ceramics were primarily ascribed to the absorption of phonon oscillation. Notably, when x = 0.08, outstanding microwave dielectric properties could be achieved, namely, ε_r = 10.22, Q × f = 72,390 GHz, τ_f = ?7.54 ppm/°C.     

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.     

Microstructure and dielectric properties of high entropy Ba(Zr0.2Ti0.2Sn0.2Hf0.2Me0.2)O3 perovskite oxides

A series of high entropy Ba(Zr_0.2Ti_0.2Sn_0.2Hf_0.2Me_0.2)O₃ (Me=Y³⁺,Nb⁵⁺,Ta⁵⁺,V⁵⁺,Mo⁶⁺,W⁶⁺) perovskite oxides were synthesized by using a solid state reaction method. Three multiple-cation solid solutions formed pure phase compounds, and only two compounds were sintered into ceramics. Microstructure analysis showed the influence of configurational entropy on phase stability and grain growth. Dielectric measurements showed that the high entropy ceramics possessed decent temperature stability of permittivity from 25 °C to 200 °C, low dielectric loss (<0.002) from 20 Hz to 2 MHz, high resistance and moderate breakdown strength (290 kV/cm, 370 kV/cm). Evidence strongly confirmed that controlling configurational entropy could be a feasible perspective to set up highly tunable perovskite structures and explore novel species of dielectric materials.     

Effects of LiF additive on crystal structures,lattice vibrational characteristics and dielectric properties of CaWO4 microwave dielectric ceramics for LTCC applications

In this paper, CaWO₄-x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics are prepared by conventional solid-state reaction at 850 °C. All the X-ray diffraction peaks are attributed to CaWO₄, combined with the results of X-ray photoelectron spectroscopy and scanning electron microscope. They indicate that LiF forms liquid phase at high temperature, exists in amorphous phase, and the CaWO₄-x wt.% LiF samples belong to the composite ceramics. With the increase of the LiF content, the liquid phase fills the pores of the ceramics and increases the compactness. Therefore, the quality factor increases from 59,055 GHz (x = 0.5) to 77,855 GHz (x = 3.0), and the dielectric constant decreases from 9.26 (x = 0.5) to 8.41 (x = 3.0). The lattice vibration characteristics of the CaWO₄-x wt.% LiF samples are studied by Raman and infrared spectroscopy. The external modes associated with the vibration of the Ca–O bonds have the greatest influence on the dielectric response, as shown by the fitting results based on the four-parameter semi-quantum (FPSQ) model. In addition, the simulated intrinsic properties and measured values are in general agreement. When the x value increases, the dielectric constant decreases with the decrease of the bond length. The quality factor has a negative correlation with the FWHM value of the B_g mode and a positive correlation with the packing fraction. Finally, the CaWO₄-3 wt.% LiF sample has good compatibility with the silver powders at 850 °C, and high application value in the LTCC field.     

Correlation between the structure and modified microwave dielectric characteristics of Zr4+ substituted Ni0.4Zn0.6Ti(1-x)ZrxNb2O8 ceramics

Microwave ceramics are an important classes of materials that are used in microwave communication systems, especially in the area of 5G wireless communication and the internet of things. In this work, to improve the Q×f values and enhance the temperature stability of Ni_0.4Zn_0.6TiNb₂O₈ ceramics, the influence of the substitution of Zr⁴⁺ ions at the Ti site in Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics was investigated. The Q×f value increases from 32114 GHz to 45733 GHz and the τ_f value changes from 38.1 ppm/°C to 3 ppm/°C with a slight Zr⁴⁺ ion substitution (x = 0.1). Meanwhile, the sample with the Zr⁴⁺ ion substitution (x = 0.3) that was sintered at 1120 °C shows a very high Q×f value of 92078 GHz. Furthermore, the XRD results reveal that the phase and structure of the Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics change with the different Zr⁴⁺ ion contents. The substitution of the Zr⁴⁺ ion can promote the sintering process for the Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics and restrain the Ni_0.5Ti_0.5NbO₄ phase formation. The results obtained from Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics can offer useful information for the study and application of high-frequency microwaves.     

Structural evolution and microwave dielectric properties in Sr2(Ti1-xSnx)O4 ceramics

The structure and microwave dielectric properties of Sr₂(Ti_1-xSn_x)O₄ ceramics were determined in the entire composition range of x?=?0–1.0. X-ray diffraction patterns and Raman spectra indicated a composition-induced onset of octahedral tilting at x?=?0.75, and the crystal structure transformed from tetragonal (I4/mmm) to orthorhombic (Pccn). An obvious change of grain morphology was observed in the phase transformation region as well. The variations of the microwave dielectric properties with composition were systematically investigated and the effect of octahedral tilting on the evolution of τ_f value was emphasized. Moreover, the relationship between τ_ε and tolerance factor of the present ceramics was revealed and compared with the empirical rule in perovskite structure. The role of tolerance factor in designing the materials with required performance was highlighted.     

Microstructure and dielectric properties of perovskite-structured high-entropy ceramics of Pb(Zr0.25Ti0.25Sn0.25Hf0.25)O3

A dielectric high-entropy ceramic with a composition of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ was designed through B-site doping, and then prepared by solid phase reaction method combined with conventional sintering in air for 3 h at 1200 ^°C, 1250 ^°C and 1300 ^°C, respectively. All the high-entropy ceramics of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ possess a perovskite structure with uniform elemental distribution and their average grain size falls within the range of 3.19–5.5 μm. For the sample sintered at 1250 ^°C, the dielectric loss is less than 0.07 in the testing frequency of 1 kHz∼1 MHz in 30–350 ^°C, and the dielectric constant reaches a peak of 14356 at about 270 ^°C at 1 kHz. At room temperature, the remnant polarization P_r reaches 28.8 μC/cm². The results demonstrate that the high-entropy ceramic of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ has great potentials in the dielectric and ferroelectric field.     

The effect of residual stresses and dislocations on microwave dielectric loss in rutile-related (Ti0.6Zr0.4)0.8(Zn1/3Nb2/3)0.2O2

Strain and stress play an important role in functional materials, but they are often neglected in electroceramics. Here we report the stress effect on microwave dielectric loss in rutile-related (Ti_0.6Zr_0.4)_0.8(Zn_1/3Nb_2/3)_0.2O₂ solid solutions. The macroscopic, microscopic and nano-scale residual stresses have been systematically investigated, including their relationships. The average Raman spectra have demonstrated considerable macro-residual stresses, especially for the heavily cracked sample. The high-resolution Raman images reveal the inhomogeneity of residual stress distribution at the grain-level, which is reflected by the shifts of Raman-active modes. The high-resolution transmission electron microscope (HRTEM) images and geometric phase analysis (GPA) analysis show the aggregation of dislocations and resulting distortion of crystal. It can be found that the dielectric loss depends strongly on residual stresses, that samples with greater residual stress exhibit much higher dielectric loss because the residual stress increases the vibration anharmonicity of the lattice.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

Parallel preparation and properties investigation on Li2O-Nb2O5-TiO2 microwave dielectric ceramics

A parallel preparation method was developed using dry powders as starting materials to synthesize multi-compositional microwave dielectric ceramics. The Li₂O-Nb₂O₅-TiO₂ ternary system was investigated as a model material. The validity of the parallel ceramic preparation process was confirmed by synthesizing a group of LiNb_0.6Ti_0.5O₃ ceramics in parallel, which showed the same crystalline structure and close dielectric properties. The ceramic libraries with M-phase-rich samples and Li₂TiO₃-rich samples were prepared using the parallel process, and the microwave dielectric properties and crystal phases were investigated systematically. An excellent microwave ceramic with a composition of 0.55Li₂O-0.05Nb₂O₅-0.40TiO₂ was obtained, which has a dielectric constant of 18.4 and a high quality value (Q × f) of 79000 GHz. This parallel process can be applied extensively to explore a variety of bulk ceramic libraries for discovering new functional materials with high performances.     

Preparation,phase assemblage and microwave dielectric properties of AlPO4-BPO4-SiO2 ternaries

Microwave dielectric ceramics with low dielectric permittivities (?r<6), high quality values and temperature sable resonator frequencies are strongly desired with the development of millimeter wave communication. In this paper, a compositional design for low-k materials was proposed from the point view of crystal chemistry. AlPO₄-BPO₄-SiO₂ glass-ceramics were prepared by traditional solid state and sol-gel processes, respectively. The sintering behaviors, phase assemblages, microstructures and microwave dielectric properties of AlPO₄-BPO₄-SiO₂ ternaries have been studied. The solid solubility, glass forming ability and crystallization of the AlPO₄–BPO₄–SiO₂ ternaries can be understood by considering the structural flexibility via the degree of ionicity i of the bonds in the ternaries. All compositions demonstrate low dielectric permittivity less than five and negative temperature coefficient of resonant frequency. Maximum Q × f value could be obtained for the 0.45AlPO₄–0.45BPO₄–0.10SiO₂ composition prepared by sol-gel process after sintering at 1175 °C/2 h: ?_r～4.16, Q × f～59,519.     

Effect of particle size and ZnO addition on microwave dielectric properties of nanocrystalline (Zr0.8Sn0.2) TiO4 ceramics

Abstract

Polycrystalline samples of (Zr_0.8Sn_0.2)TiO₄ have been synthesised by solid state reaction method. The effects of ball milling and the addition of ZnO (1–2.5 wt-%) on the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ ceramics are investigated. As the ball milling time increases, the density of the (Zr_0.8Sn_0.2)TiO₄ ceramics increases. The influence of ZnO as a sintering aid on the sintering temperature of (Zr_0.8Sn_0.2)TiO₄ is also studied. As the weight percentage of ZnO increases, the density increases up to 1.5 wt-% of ZnO and thereafter it starts decreasing. The dielectric constant εγ _r and the product of quality factor Q and resonant frequency f_o of TE_{01 °} mode (Qxf_o ) followed the same trend with increasing density, irrespective of whether the density increased with ball milling time or by addition of ZnO. The value of Qxf _o is found to increase with increasing grain size. The microwave dielectric constant and the Qxf _o values were respectively ranged from 32 to 39.3 and from 32 000 to 49 600 at 10.5 GHz for (Zr_0.8Sn_0.2)TiO₄ ceramics. Furthermore, the influence of the processing parameters on the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ is discussed.     

Correlations between structure and microwave dielectric properties of Co doped MgMoO4 ceramics

Mg_1-xCo_xMoO₄ (x = 0.01–0.15) ceramics were prepared by traditional solid-state methods. The phase composition, crystalline structure, micromorphology, and microwave dielectric properties of Mg_1-xCo_xMoO₄ ceramics were comprehensively studied. Mg_1-xCo_xMoO₄ ceramics present monoclinic wolframite structures from x = 0.01 to x = 0.15 with Co occupying the Mg-site. With the addition of Co²⁺, ε_r of Mg_1-xCo_xMoO₄ ceramics increase. Q × f is maximal at 5 mol% Co²⁺ content. The Mg_0.95Co_0.05MoO₄ ceramic exhibits an optimal microwave dielectric property: ε_r = 7, Q × f = 59247 GHz, τ_f = −68 ppm/°C. The Q × f values increase by 20% compared with the pure MgMoO₄ ceramics (~49149 GHz). Doping Co²⁺ effectively promotes the densification of ceramics and increases ε_r and Q × f. However, when the Co content exceeds 5 mol%, the decreased packing fraction and disorder distribution of ions contribute to the increase in dielectric losses. The correlations between Co²⁺ substitution and wolframite structure have been discussed by Raman spectroscopy, FT-IR spectroscopy and Rietveld refinement.     

Effects of LiF addition on the densification and microwave dielectric properties of LiInO2 ceramics

Low-temperature co-fired ceramics (LTCC) LiInO₂ + xwt% LiF (x = 0, 1, 2, 3, 4, 5) was synthesized by a traditional solid-state reaction method. XRD, TEM, and SEM show that all specimens form a pure-phase tetragonal structure with a space group of I4₁/amd. The addition of LiF can effectively optimize the microstructure and improve the relative density of LiInO₂ ceramics. As x value increased, the ε_r increased from 9.6 to 13.6, Q×f increased from 39,600 GHz to 52,500 GHz, and all the specimens exhibit a positive τ_f value at the range of 9.6–19.1 ppm/°C. The low-ε_r and positive τ_f in LiInO₂ ceramics was investigated by bond valence and P–V-L chemical bond theory, indicating that it was closely related to the rattling effect of In and Li. The concentration of oxygen vacancies was studied by dielectric spectroscopy, indicating that doping LiF can compensate for the volatilization of Li during high temperature sintering. Notably, excellent microwave dielectric properties (ε_r ~13.6, Q×f = 52,500 GHz, and τ_f ~ 18.1 ppm/°C) were achieved in the LiInO₂ + 3 wt% LiF sintered at 890 °C.