Improved microwave dielectric properties of MgAl_(2)O_(4)spinel ceramics through(Li_(1/3)Ti_(2/3))^(3+)doping

A series of nominal compositions MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)(x=0,0.04,0.08,0.12,0.16,and 0.20)ceramics were successfully prepared via the conventional solid-state reaction route.The phase compositions,microstructures,and microwave dielectric properties were investigated.The results of x-ray diffraction(XRD)and scanning electron microscopy(SEM)showed that a single phase of MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)ceramics with a spinel structure was obtained at x≤0.12,whereas the second phase of MgTi_(2)O_(5)appeared when x>0.12.The cell parameters were obtained by XRD refinement.As the x values increased,the unit cell volume kept expanding.This phenomenon could be attributed to the partial substitution of(Li_(1/3)Ti_(2/3))^(3+)for Al^(3+).Results showed that(Li_(1/3)Ti_(2/3))^(3+)doping into MgAl_(2)O_(4)spinel ceramics effectively reduced the sintering temperature and improved the quality factor(Q_f)values.Good microwave dielectric properties were achieved for a sample at x=0.20 sintering at 1500℃in air for 4 h:dielectric constantε_(r)=8.78,temperature coefficient of resonant frequencyτ_(f)=-85 ppm/℃,and Q_(f)=62300 GHz.The Q_(f)value of the x=0.20 sample was about 2 times higher than that of pure MgAl_(2)O_(4)ceramics(31600 GHz).Thus,MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)ceramics with excellent microwave dielectric properties can be applied to 5G communications.

Dielectric Properties and Defect Structure of Bi-doped SrTiO_3 Ceramics

The dielectric properties of ceramics with composition of (Sr 1-x Bi x )TiO 3+x/2 (where x =0.05～0.70 ) were measured at frequency of 1 MHz. The experimental results indicate that the dielectric properties of (Sr 1-x Bi x )TiO 3+x/2 system are greatly varied with an increase of the stoichiometric amounts of Bi 2O 3. The relative permittivity of the solid solutions is high, and the dissipation factor is low. The positron annihilation technique(PAT) was adopted to study the defect structure. An explanation of the dielectric properties of Bi-doped SrTiO 3 ceramics has been suggested in terms of electron-compensation and vacancy or defect-compensation mechanisms and space-charge polarization mechanism.

Dielectric Properties of Unfilled Tetragonal Tungsten Bronze Ba4PrFe(0.5)Nb(9.5)O(30) Ceramics

In order to found new dielectrics ceramics in tungsten bronze structure, unfilled tungsten bronze（TB） ceramics with nominal formula Ba4PrFe（0.5）Nb（9.5）O（30） were prepared by the solid state reaction method. The microstructure and dielectric properties were studied using powder X-ray diffraction, field emission scanning electron microscope, and variable temperature dielectric test system. The results show that the ceramics have a single phase and belong to the space group of P4bm with the cell of a = b = 12.4839（3） ？, c = 3.9409（5） ？, V = 614.192（2） ？3. The frequency dependent dielectrics properties show that the ceramics have a Debye-like relaxation at room temperature, while the temperature dependent dielectrics properties indicate that the ceramics are a relaxor and the relaxation is due to the nanopolars and oxygen vacancies. The ceramics have a potential application in electronic ceramics as temperature-stable multilayer ceramic capacitors.

Effect of La_2O_3-Dopant on Microstructure and Microwave Dielectric Properties of CaO-MgO-Nb_2O_5-TiO_2 System Ceramics

La_2O_3-doped CaO-MgO-Nb_2O_5-TiO_2 system ceramics were prepared by solid-state ceramic technique. The microstructure and microwave dielectric properties of CaO-MgO-Nb_2O_5-TiO_2-La_2O_3 ceramics can be adjusted by varying the amount of La^(3+) ions. The results show that the replacement of Ca^(2+) by La^(3+) at A-site of the ceramics can increase the quality factor Q·f value as well as the temperature coefficient of resonant frequency τ_f and decrease the dielectric constant ε_r. With increase of La^(3+) contents, the dielectric constant decreases from 57 to 35 and Q·f value increases from 33400 GHz to 35000 GHz (at 7.6 GHz). Meanwhile, the temperature coefficient of resonant frequency is improved towards near zero. The dielectric properties of these compounds are related to octahedra tilting due to deficient vacancies at A-site.

Effect of La^(3+), Sr^(2+) on Microwave Dielectric Properties of CaO-MgO-Nb_2O_5^-TiO_2 System Ceramics

The effect of La^3 ＋ , Sr^2＋ on the microstructure and microwave properties of CaO-MgO-Nb2O5-TiO2 system ceramics was investigated. The result shows that a single complex perovskite structure formed within investigated composition range in La^3＋ , Sr2-doped CaO-MgO-Nb2O5-TiO2 system ceramics. With increasing of La^3＋ , Sr^2＋ content, the structure of La^3＋ , Sr2-doped CaO-MgO-Nb2O5-TiO2 system ceramic respectively maintain orthorhombic type.

Study on the dielectric properties of Mg-doped NaBiTi6O14 ceramics

With the interest in using lead-free materials to replace lead-containing materials increasing,the use of Na_（0.5）Bi_（0.5）TiO_3（NBT） has come into our sight.We studied the composition of NBT and found that NaBiTi_6O_（14） ceramics can be compositionally tuned by Mg-doping on the Ti-site to optimize the dielectric properties.In this study,Mg-doped NaBiTi_6O_（14）（NaBi（Ti_（0.98）Mg_（0.02））_60_（14-d）） ceramics were prepared by a conventional mixed oxide route at different sintering temperatures,and their dielectric properties have been studied at a wide temperature range.X-ray diffraction（XRD） patterns of the NBT-based ceramics indicate that all samples have a pure phase without any secondary impurity phase.The experimental data show that after Mg-doping,the relative permittivity and dielectric loss become lower at 1040,1060,and1080 ℃ except 1020 ℃ and at different frequencies from 10 kHz,100 kHz to 1 MHz.Take 1060 ℃ for example,when the sintering temperature is 1060 ℃ at 1 MHz,the minimum relative permittivity of NaBiTi_6O_（14） is 32.9 and the minimum dielectric loss is 0.01417,the relative permittivity of NaBi（Ti_（0.98）Mg_（0.02））_60_（14-δ） under the same condition is 25.8 and the dielectric loss is 0.000104.We explored the mechanism of Mg-doping and surprisingly found that the dielectric property of NaBi（Ti_（0.98）Mg_（0.02））_60_（14-δ） becomes better owing to Mg-doping.Thus,NaBi（Ti_（0.98）Mg_（0.02））_60_（14-δ） can be used in microwave ceramics and applied to new energy materials.

Effect of Rare Earth Oxides on the Microstructure and Microwave Dielectric Properties of CaO-MgO-Nb_2O_5-TiO_2 System Ceramics

La2O3 and SrO-doped CaO-MgO-Nb2O3-TiO2 system ceramics were prepared by solid-state ceramic technique.The microstructure and microwave dielectric properties of CaO-MgO-Nb2O5-TiO2-La2O3 cermics can be adjusted by varying the amount of La^3＋ or Sr^2＋ ions respectively.The replacement of Ca^2＋ by La^3＋ at A-site of the ceramics increases the quality factor Q value（ at 7.6GHz）as well as the temperature coefficient of resonant frequency τf and decreases the dielectric constant εr and the substitution of Sr^2＋ at A-site in this ceramics system exhibits opposite characteristics.The microwave properties of La^3＋,Sr^2＋-doped CaO-MgO-Nb2O5-TiO2 system ceramics depend on the degree of octahedral distortion inside materials.

Structural and Dielectric Properties of Y_2O_3-Doped Ba_(0.92)Sr_(0.08)Ti_(0.95)Sn_(0.05)O_3 Ceramics

The structural and dielectric properties of Ba0.92Sr0.08Ti0.95Sn0.0503 （BSTS） ＋x（molar ratio, %） Y^3＋ceramics are investigated. Combining the lattice parameters and the distortion of crystal lattice, an alternation of substitution preference of Y^3＋ ion for the host cations in perovskite lattice is found. Owing to Y^3- ion entering the A site, the maximum dielectric constant is 5 627 for 1.25% Y^3＋-doped samples; when Y^3- ion is more than 1.25%, it tends to occupy the B site in perovskite lattice, causing a drop in the dielectric constant. Owing to the appearance of oxygen vacancy, the optimized dielectric loss is 0.004 for 1.25% Y^3＋-doped samples. The thermal stability of BSTS ceramics is significantly improved and the Curie temperature shifts to lower value with the amount of Y2O3 increased, making it a superior candidate for capacitor applications.

Dielectric and Magnetic Properties of (1-x)CaTiO3-xNi0.5Zn0.5Fe2O4 Composite Ceramics

（1-x）CaTiO3-xNi0.5Zn0.5Fe2O4（0 x 1.0） composite ceramics were synthesized by a conventional solid state reaction method.The phase formation,microstructure,and dielectric and magnetic properties were investigated by X-ray diffraction,scanning electron microscopy,precision impedance analysis,and vibrating sample magnetometry,respectively.The results indicate that the composite ceramics are composed of both perovskite phase Ca TiO3 and spinel phase Ni0.5Zn0.5Fe2O4.The maximal relative density for 0.5CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics reaches 97.8%,as it has been sintered at the temperature of 1260 ℃ for 3 h.Dielectric constant and loss tangent of（1-x）CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics show dispersion in the low frequency range.Their phase transition temperature of the dielectric constant shifts to lower temperatures with the increase of Ni0.5Zn0.5Fe2O4 content.This phenomenon is attributed to that the phase transition temperature of CaTiO3 is higher than that of Ni0.5Zn0.5Fe2O4.The saturation magnetization of （1-x）CaTiO3-xNi0.5Zn0.5Fe2O4 composite ceramics increases with the Ni0.5Zn0.5Fe2O4 ferrite content.

UltrahighQ Sr_(1+x)Y_(2)O_(4+x)(x=0-0.04)microwave dielectric ceramics for temperature-stable millimeter-wave dielectric resonator antennas

Microwave dielectric ceramics should be improved to advance mobile communication technologies further.In this study,we prepared Sr_(1+x)Y_(2)O_(4+x)(x=0-0.04)ceramics with nonstoichiometric Sr^(2+)ratios based on our previously reported SrY_(2)O_(4) microwave dielectric ceramic,which has a low dielectric constant and an ultrahigh quality factor(Q value).The ceramic exhibited a 33.6% higher Q-by-frequency(Q×f)value(Q≈12,500)at x=0.02 than SrY_(2)O_(4).All Sr_(1+x)Y_(2)O_(4+x)(x=0-0.04)ceramics exhibited pure phase structures,although variations in crystal-plane spacings were observed.The ceramics are mainly composed of Sr-O,Y1-O,and Y_(2)-O octahedra,with the temperature coefficient of the resonant frequency(τ_(f))of the ceramic increasing with Y_(2)-O octahedral distortion.The ceramic comprises uniform grains with a homogeneous elemental distribution,clear grain boundaries,and no obvious cavities at x=0.02.The Sr_(1+x)Y_(2)O_(4+x)(x=0-0.04)ceramics exhibited good microwave dielectric properties,with optimal performance observed at x=0.02(dielectric constant(εr)=15.41,Q×f=112,375 GHz,and τ_(f)=-17.44 ppm/℃).The τ_(f) value was reduced to meet the temperaturestability requirements of 5G/6G communication systems by adding CaTiO_(3),with Sr_(1.02)Y_(2)O_(4.02)+2wt% CaTiO_(3) exhibiting ε_(r)=16.14,Q×f=51,004 GHz,andτf=0 ppm/℃.A dielectric resonator antenna prepared using Sr_(1.02)Y_(2)O_(4.02)+2wt%CaTiO_(3) exhibited a central frequency of 26.6 GHz,with a corresponding gain and efficiency of 3.66 dBi and 83.14%,respectively.Consequently,Sr_(1.02)Y_(2)O_(4).02-based dielectric resonator antennas are suitable for use in 5G millimeter-wave band(24.5-27.5 GHz)applications.

Ultrahigh energy storage with superfast charge-discharge capability achieved in linear dielectric ceramic

Ceramic capacitors designed for energy storage demand both high energy density and efficiency.Achiev-ing a high breakdown strength based on linear dielectrics is of utmost importance.In this study,we present the remarkable performance of densely sintered(1-x)(Ca_(0.5)Sr_(0.5)TiO_(3))-xBa4Sm28/3Ti18O54 ceramics as energy storage materials,with a measured energy density(Wrec)of 4.9 J/cm^(3)and an ultra-high ef-ficiency(η)of 95%which is almost optimal in linear dielectric that has been reported.To unravel the underlying mechanisms,we conducted a systematic investigation on the influence of adding paraelec-tric Ba_(4)Sm_(28/3)Ti_(18)O_(54)(BST)on both microstructure and macroscopic electrical properties of Ca_(0.5)Sr_(0.5)TiO_(3)(CST).Notably,the addition of BST effectively reduces the grain size of CST.The conduction mechanism is primarily governed by grain boundaries,where high-density grain boundaries act as barriers to charge carrier transport due to their elevated resistivity.Moreover,the activation energy associated with grain boundaries increases with rising resistivity,implying a lower concentration of free vacancies within these regions.The increased barrier height for oxygen vacancy migration at grain boundaries compensates for the grain boundary defects,thereby resulting in enhanced breakdown strength.This characteristic offers a substantial advantage in terms of thermal and frequency stability(25-175℃,1-100 Hz).This work introduces a candidate material with outstanding comprehensive energy storage properties.

EFFECTS OF STATIC ELECTRIC FIELD ON THE FRACTURE BEHAVIOR OF PIEZOELECTRIC CERAMICS

The paper gives an overview on experimental observations of the failure behavior of electrically insulating and conducting cracks in piezoelectric ce- ramics.The experiments include the indentation fracture test,the bending test on smooth samples,and the fracture test on pre-notched(or pre-cracked)compact ten- sion samples.For electrically insulating cracks,the experimental results show a com- plicated fracture behavior under electrical and mechanical loading.Fracture data are much scattered when a static electric field is applied.A statistically based frac- ture criterion is required.For electrically conducting cracks,the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to mea- sure the electrical fracture toughness.Furthermore,the electrical fracture toughness is much higher than the mechanical fracture toughness.The highly electrical fracture toughness arises from the greater energy dissipation around the conductive crack tip under purely electric loading,which is impossible under mechanical loading in the brittle ceramics.

Realization of Low Temperature Densification of Nickelate Ceramics for MLCC Application

We report an improved method for the preparation of highly dense nickelate ceramics at relatively low temperature. It is found that the introduction of appropriate additives during the ball-milling process facilitates the formation of nickelate phase through solid state reaction. Moreover, although high-purity nickelate powders can only be obtained by calcining the mixture of starting materials at temperature higher than 1100 ℃. The adoption of powders calcined at 1000 ℃, rather than those calcined at higher temperature, is conductive to the low-temperature densification of nickelate ceramics, which is attributed to the small and dispersive particles, and the solid state reaction of the residual starting materials during sintering. Compared with the conventional process, the improved method can reduce the sintering temperature of nickelate ceramics by about 100 ℃ and decrease the grain size of the obtained ceramics, and therefore makes nickelate meet the fabrication requirements of multi-layer ceramic capacitors（MLCC）.

Crystal structure,chemical bond characteristics,infrared reflection spectrum,and microwave dielectric properties of Nd_(2)(Zr_(1−x)Ti_(x))_(3)(MoO_(4))_(9) ceramics

Microwave dielectric ceramics(MWDCs)with low dielectric constant and low dielectric loss are desired in contemporary society,where the communication frequency is developing to high frequency(sub-6G).Herein,Nd_(2)(Zr_(1−x)Ti_(x))_(3)(MoO_(4))_(9)(NZ_(1−x)T_(x)M,x=0.02-0.10)ceramics were prepared through a solid-phase process.According to X-ray diffraction(XRD)patterns,the ceramics could form a pure crystal structure with the R3c(167)space group.The internal parameters affecting the properties of the ceramics were calculated and analyzed by employing Clausius-Mossotti relationship,Shannon’s rule,and Phillips-van Vechten-Levine(P-V-L)theory.Furthermore,theoretical dielectric loss of the ceramics was measured and analyzed by a Fourier transform infrared(IR)radiation spectrometer.Notably,when x=0.08 and sintered at 700℃,optimal microwave dielectric properties of the ceramics were obtained,including a dielectric constant(ε_(r))=10.94,Q·f=82,525 GHz(at 9.62 GHz),and near-zero resonant frequency temperature coefficient(τ_(f))=−12.99 ppm/℃.This study not only obtained an MWDC with excellent properties but also deeply analyzed the effects of Ti^(4+)on the microwave dielectric properties and chemical bond characteristics of Nd_(2)Zr_(3)(MoO_(4))_(9)(NZM),which laid a solid foundation for the development of rare-earth molybdate MWDC system.

The latest process and challenges of microwave dielectric ceramics based on pseudo phase diagrams

The explosive process of 5G communication evokes the urgent demand of miniaturized and integrated dielectric ceramics filter It is a pressing need to advance the development of dielectric ceramics utilization of emerging technology to design new materials and understand the polarization mechanism.This review provides the summary of the study of microwave dielectric ceramics(MWDCs)sintered higher than 1000℃ from 2010 up to now,with the purpose of taking a broad and historical view of these ceramics and illustrating research directions.To date,researchers endeavor to explain the structure-property relationship of ceramics with multitude of approaches and design a new formula or strategy to obtain excellent microwave dielectric properties.There are variety of factors that impact the permittivity,dielectric loss,and temperature stability of dielectric materials,covering intrinsic and extrinsic factors.Many of these factors are often intertwined,which can complicate new dielectric material discovery and the mechanism investigation.Because of the various ceramics systems,pseudo phase diagram was used to classify the dielectric materials based on the composition.In this review,the ceramics were firstly divided into ternary systems,and then brief description of the experimental probes and complementary theoretical methods that have been used to discern the intrinsic polarization mechanisms and the origin of intrinsic loss was mentioned.Finally,some perspectives on the future outlook for high-temperature MWDCs were offered based on the synthesis method,characterization techniques,and significant theory developments.

Sintering behavior and microwave dielectric properties of (1-x)CaTiO_3-xLaAlO_3 ceramics

The sintering behavior,microstructure and microwave dielectric properties of （1–x）CaTiO3–xLaAlO3 （x=0.1,0.3,0.5,0.7,0.9,respectively） ceramics were investigated systematically by thermogravimetry-differential thermal analysis （TG-DSC）,X-ray diffraction （XRD）,scanning electron microscopy （SEM） and a network analyzer.The result showed that forming temperature of the perovskite type crystal increased with increasing of x value.0.9CaTiO3-0.1LaAlO3 ceramics were sintered well from 1 400 to 1 550 oC,its bulk density increased with sintering temperature,and microwave dielectric properties of the ceramics at 1 400 oC was shown as follows： relative dielectric constant εr= 45.1,Q×f= 46 087 GHz and τf=–14.1×10–6/oC,respectively.But 0.7CaTiO3-0.3LaAlO3 ceramics were sintered well only when sintering temperature rose to 1 500 oC.（1–x）CaTiO3–xLaAlO3 （x=0.5,0.7 and 0.9,respectively） were not sintered well up to 1 550 oC and the sintered samples exhibited porous characteristic and with low bulk density.

Recent progress of ecofriendly perovskite-type dielectric ceramics for energy storage applications

Increasing concern has been focused on the search for ecofriendly dielectric ceramics to meet the extensive demands of pulsed capacitors.Due to the advantages of high-energy storage density,efficiency,and excellent temperature stability,optimization of energy storage performance in dielectric ceramics has been a goal in the past decades.This review summarizes the recently reported progress in energy storage properties of typical perovskite-type lead-free ceramics.The advantages and shortcomings in the various kinds of ceramics are discussed.Finally,future prospects are presented to provide some guidelines for the exploration of new materials.

Machine learning approaches for permittivity prediction and rational design of microwave dielectric ceramics

Low permittivity microwave dielectric ceramics(MWDCs)are attracting great interest because of their promising applications in the new era of 5G and IoT.Although theoretical rules and computational methods are of practical use for permittivity prediction,unsatisfactory predictability and universality impede rational design of new high-performance materials.In this work,based on a dataset of 254 single-phase microwave dielectric ceramics(MWDCs),machine learning(ML)methods established a high accuracy model for permittivity prediction and gave insights of quantitative chemistry/structureproperty relationships.We employed five commonly-used algorithms,and introduced 32 intrinsic chemical,structural and thermodynamic features which have correlations with permittivity for modeling.Machine learning results help identify the permittivity decisive factors,including polarizability per unit volume,average bond length,and average cell volume per atom.The feature-property relationships were discussed.The optimal model constructed by support vector regression with radial basis function kernel was validated its superior predictability and generalization by verification dataset.Low permittivity material systems were screened from a dataset of~3300 materials without reported microwave permittivity by high-throughput prediction using optimal model.Several predicted low permittivity ceramics were synthesized,and the experimental results agree well with ML prediction,which confirmed the reliability of the prediction model.

Structure,defects,and microwave dielectric properties of Al-doped and Al/Nd co-doped Ba_(4)Nd_(9.33)Ti_(18)O_(54)ceramics

Low-loss tungsten–bronze microwave dielectric ceramics are dielectric materials with potential application value for miniaturized dielectric filters and antennas in the fifth-generation(5G)communication technology.In this work,a novel Al/Nd co-doping method of Ba_(4)Nd_(9.33)Ti_(18)O_(54)(BNT)ceramics with a chemical formula of Ba_(4)Nd_(9.33+z/3)Ti_(18−z)Al_(z)O_(54)(BNT–AN,0≤z≤2)was proposed to improve the dielectric properties through structural and defect modulation.Together with Al-doped ceramics(Ba_(4)Nd_(9.33)Ti_(18−z)Al_(4z/3)O_(54),BNT–A,0≤z≤2)for comparison,the ceramics were prepared by a solid state method.It is found that Al/Nd co-doping method has a significant effect on improving the dielectric properties compared with Al doping.As the doping amount z increased,the relative dielectric constant(εr)and the temperature coefficient of resonant frequency(τf)of the ceramics decreased,and the Q×f values of the ceramics obviously increased when z≤1.5.Excellent microwave dielectric properties ofεr=72.2,Q×f=16,480 GHz,andτf=+14.3 ppm/℃were achieved in BNT–AN ceramics with z=1.25.Raman spectroscopy and thermally stimulated depolarization current(TSDC)technique were firstly combined to analyze the structures and defects in microwave dielectric ceramics.It is shown that the improvement on Q×f values was originated from the decrease in the strength of the A-site cation vibration and the concentration of oxygen vacancies(VO××),demonstrating the effect and mechanism underlying for structural and defect modulation on the performance improvement of microwave dielectric ceramics.

Room-temperature-densified H_(3)BO_(3) microwave dielectric ceramics with ultra-low permittivity and ultra-high Qf value

With the rapid development of mobile communication technology towards 5G and 6G,the microwave dielectric materials with ultra-low permittivity and ultra-high Qf value are urgently demanded.Here,the excellent microwave dielectric properties are reported in H3BO3 ceramics with the molecular crystal structure,whose permittivity(2.84)and density(1.46 g/cm^(3))are record-low among the low-loss ceramics.The ultra-high Qf value of 146,000 GHz(or the ultra-low dielectric loss of 1.03×10^(-4) at 15 GHz)is also distinguished.Besides,the H_(3)BO_(3) ceramics can be densified at room temperature by a simple cold sintering process in a short time of 10 min,and this brings many advantages for the integration with microwave circuits.The large molecule volume originating from the molecular crystal structure and the low dielectric polarizabilities of H^(+) and B^(3+) are responsible for the ultra-low permittivity of H_(3)BO_(3) ceramics,and more microwave dielectric materials with ultra-low permittivity and ultra-high Qf value are expected to be explored in the molecular crystals.