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.

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.

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.

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.

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.

Ultralow-loss (1-x)CaWO_(4)-xNa_(2)WO_(4) (x=0.1,0.2) microwave dielectric ceramic for LTCC applications

In this work,the(1-x)CaWO_(4)-xNa_(2)WO_(4)(x=0.1,0.2,denoted as 0.9CW-0.1NW and 0.8CW-0.2NW,respectively)ultralow-loss microwave dielectric ceramics were prepared via solid-state reaction method.Using low melting-point Na_(2)WO_(4) as sintering aid to prepare CaW_(O4)eNa_(2)WO_(4) composite ceramics,the sintering temperature of CaWO_(4) was successfully reduced while maintaining excellent microwave performance.The optimal microwave dielectric properties have been achieved at 900C for 0.9CW-0.1NW ceramic:εr=9.0,Q×f=105660 GHz,tandδ=1.1×10^(-4)and tf=35.4 ppm/℃ at a frequency of 12.0 GHz.For the 0.8CW-0.2NW ceramic,the optimal microwave dielectric properties have been obtained at 740C,withεr=8.5,Q×f=97014 GHz,tandδ=1.2×10^(-4)and tf=37.4 ppm/℃ at a frequency of 11.8 GHz.In summary,both composite ceramics exhibit low sintering temperatures,excellent dielectric properties and chemical compatibility with the Ag electrode.The findings of this study provide an effective approach to prepare novel composite ceramics as promising candidates for LTCC applications.

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.

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.

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.

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.

Crystal structure and enhanced microwave dielectric properties of the Ce_(2)[Zr_(1−x)(Al_(1/2)Ta_(1/2))_(x)]_(3)(MoO_(4))_(9) ceramics at microwave frequency

Dense microwave dielectric ceramics of Ce_(2)[Zr_(1−x)(Al_(1/2)Ta_(1/2))_(x)]_(3)(MoO_(4))_(9)(CZMAT) (x = 0.02–0.10) were prepared by the conventional solid-state route. The effects of (Al1/2Ta1/2)^(4+) on their microstructures, sintering behaviors, and microwave dielectric properties were systematically investigated. On the basis of the X-ray diffraction (XRD) results, all the samples were matched well with Pr_(2)Zr_(3)(MoO_(4))_(9) structures, which belonged to the space group R3¯c. The lattice parameters were obtained using the Rietveld refinement method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips—Van Vechten—Levine (P—V—L) theory. Excellent dielectric properties of Ce_(2)[Zr_(0.94)(Al_(1/2)Ta_(1/2))_(0.06)]_(3)(MoO_(4))_(9) with a relative permittivity (ε_(r)) of 10.46, quality factor (Q × f) of 83,796 GHz, and temperature coefficient of resonant frequency (τ_(f)) of −11.50 ppm/℃ were achieved at 850 ℃.

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.

Synthesis,crystal structure and microwave dielectric properties of self-temperature stable Ba_(1-x)Sr_(x)CuSi_(2)O_(6) ceramics for millimeter-wave communication

Ba_(1-x)Sr_(x)CuSi_(2)O_(6) compounds with a tetrahedral structure(I41/acd)were prepared through the solid-state reaction method.The phase building process,structural evolution and microwave dielectric properties of Ba_(1-x)Sr_(x)CuSi_(2)O_(6) were investigated.Single BaCuSi_(2)O_(6) phase can be obtained when calcined at 1050℃ for 3 h or 950℃ for 10 h.The substitution of Ba^(2+) by Sr^(2+) can effectively promote the sintering process and the maximum solubility of Ba_(1-x)Sr_(x)CuSi_(2)O_(6) was located between 0.25 and 0.30.Rietveld refinement,Raman-spectra and P-V-L complex chemical bond theory were used to explain the correlations between the crystal structures and microwave dielectric properties.The dielectric constant was dominated by the susceptibility(Σχ^(μ))and ionic polarizability.The quality factor(Q×f)was determined by the bond strength,packing fraction and lattice energy,especially the Si-O bond.The susceptibility of Cu-O bond and Si-O bond played an important role in controlling the temperature coefficient of the resonant frequency(τf).A near zero τf value was obtained at x=0-0.10 and the optimum microwave dielectric properties for Ba_(1-x)Sr_(x)CuSi_(2)O_(6) were achieved at x=0.20 when sintered at 1000℃ for 3 h:ε_(r)=8.25,Q×f=47616 GHz and τf=9.6 ppm/℃.

Understanding microwave dielectric properties of (1-x)CaTiO3-xLaAlO3 ceramics in terms of A/B-site ionic-parameters

We prepared (1-x)CaTiO3-xLaAlO3 (0 ≤x≤ 1) microwave dielectric ceramics using a conventional two-step solid-state reaction method,and investigated microwave dielectric properties of the ceramics in terms of A/B-site ionic-parameters.Ionic-parameters such as ionic polarizability,A-site bond valence,and ionic rattling were linked to the microwave dielectric properties.As the LaAlO3 content x in the (1-x)CaTiO3-xLaAlO3 ceramics increased from 0.3 to 0.7,the dielectric constant gradually decreased,which was attributed to the decrease of polarizability deviation and suppression of the cation rattling.The temperature coefficient of the resonant frequency decreased as the content of LaAlO3 increased because of the increase of A-site cation bond valence.The quality factor value (Q × f) increased as LaAlO3 content increased because of the enhancement of the order degree of B-site cation.A significant deterioration of the temperature coefficient of the resonant frequency and Q ×fvalue was observed at the composition x =0.5.These decreases were attributed to a phase transition from orthorhombic crystal (for x ≤ 0.5) to rhombohedral crystal (for x ＞ 0.5).

Crystal structures and microwave dielectric properties of Sr_(2)MgWO_(6) ceramics at different sintering temperatures

Sr_(2)MgWO_(6)(SMW)is a typical perovskite oxide compound,but there has been little research on the effects of processing on its dielectric properties.In this work,SMW ceramics were prepared by solid-state synthesis with sintering at 1450℃,1475℃,1500℃and 1525℃,respectively.XRD results confirmed that the samples possessed double perovskite structure(Fm-3m).The Raman and FTIR spectra were used to study the lattice vibrational modes.The FPSQ model was used to obtain the fitting curves of the FTIR spectra and derive the intrinsic properties of the material that were found to be in agreement with the measured data.The structure-property relationships were successfully established based on the Raman mode results.The optimal sintering temperature of SMW ceramics was identified as 1475℃due to the excellent performances characteristics(ε_(r)=16.97,Q×f=23,872 GHz,τ_(f)=-35.38 ppm/℃)obtained at this temperature.This study explored the relationships among the crystal structures,lattice vibrational characteristics and dielectric properties of SMW ceramics,so as to further understand their dielectric response mechanism and lay a solid theoretical foundation for the development of microwave ceramics.

Microwave dielectric properties of Ba_(6-3x)La_(8+2x)(Ti_(1-z)Zr_z)_(18)O_(54)(x=2/3) ceramics

Zr substitution for Ti was investigated to modify the dielectric properties of Ba6-3xLa8+2xTi18O54(x=2/3) ceramics.A single-phase solid solution with tungstenbronze-like structure was formed in the range of 0<z<0.2 in Ba6-3xLa8+2x(Ti1-zZrz)18O54 ceramics.As Zr content exceeded this range,a secondary phase of Ba2ZrO4 was detected.This is correlated with the decrease of tolerance factor.As Zr content increased,there was an expansion of the b-axis and c-axis,and increase in the cell volume.Incorporation ...

Preparation, crystal structure, and dielectric characterization of Li_(2)W_(2)O_(7) ceramic at RF and microwave frequency range

Single phase Li_(2)W_(2)O_(7) with anorthic structure was prepared by the conventional solid-state reaction method at 550℃ and the anorthic structure was stable up to 660℃.The dielectric properties at radio frequency(RF)and microwave frequency range were characterized.The sample sintered at 640℃ exhibited the optimum microwave dielectric properties with a relative permittivity of 12.2,a quality factor value of 17,700 GHz(at 9.8 GHz),and a temperature coefficient of the resonant frequency of232 ppm/℃ as well as a high relative density~94.1%.Chemical compatibility measurement indicated Li_(2)W_(2)O_(7) did not react with aluminum electrodes when sintered at 640℃ for 4 h.

Sintering characteristics and microwave dielectric properties of 0.5Ca_(0.6)La_(0.267)TiO_3-0.5Ca(Mg_(1/3)Nb_(2/3))O_3 ceramics prepared by reaction-sintering process

0.5 Ca（0.6La0.267TiO3-0.5 Ca（Mg1/3Nb2/3）O3（5 CLT-5 CMN） ceramics were prepared by a reaction-sintering process and their sintering characteristics, microwave dielectric properties were investigated in detail.Without any calcination stage involved,a mixture of CaCO_3, La_2 O_3, TiO_2, MgO and Nb_2 O_5 was pressed and sintered directly. Pure phase 5 CLT-5 CMN ceramics with high density and dense microstructure can be obtained after sintered at 1400 ℃ for 4 h. Compared with those prepared by the conventional ceramic route, 5 CLT-5 CMN ceramics produced by the reaction-sintering process exhibit slightly higher dielectric constant and Q×f value. Fine microwave dielectric properties of ε_r= 56.4, Q×f= 48,550 GHz and T_f = ＋8.7 ppm/℃ for 5 CLT-5 CMN ceramics sintered at 1400 ℃ for 4 h are obtained, suggesting reactionsintering process is a simple and efficient method to produce pure phase 5 CLT-5 CMN ceramics as a potential candidate for the fabrication of microwave devices.

Molten Salt Synthesis of Ba(Mg_(1/3)Nb_(2/3))O_3 Powder

The single-phase Ba(Mg_(1/3)Nb_(2/3))O_3(BMN) powder was successfully prepared by the KCl molten salt synthesis(MSS) method.The temperature for single-phase BMN powders by MSS was about 400℃ lower than that by the solid-phase method.The average particle size(APS) was about 0.91μm at 900℃ and increased with increasing synthesis temperature.Based on the APS,the activation energy for particle growth in the MSS,whose value was 64.1kJmol^(-1),was attained.The sinterability of the powder prepared by MSS method was better than that prepared by solid-phase method.