Preparation and Properties of Si2N2O Ceramics for Microwave Sintering Furnaces

Si2N2O ceramics were prepared using amorphous Si3N4 as the raw material and Li2CO3 as the sintering additive through vacuum multi-stage sintering.The influence of the Li2CO3 addition(0%,1%,2%,3%,and 5%,by mass)on the phase composition,the microstructure,the porosity,the mechanical properties,the dielectric constant and the tangent of the dielectric loss angle of the porous Si2N2O ceramics was investigated.The results reveal that a suitable addition of Li2CO3 can promote the generation of Si2N2O but excessive or inadequate Li2CO3 causes decomposition of Si2N2O ceramics.The prepared porous Si2N2O ceramics have good mechanical properties,good thermal shock resistance,and low dielectric properties,which have excellent potential for application in microwave sintering furnaces.

RESEARCH ON MICROWAVE SINTERING ALUMINA CERAMICS

This paper introduces the characteristic of microwave sintering,introduces and analyses the process of microwave sintering of thealumina ceramics, moreover, compared with the experiments ofconventional sintering method. Based on the research, some resultsare obtained, and experimental basis for microwave sintering ofceramics is put forward.

Low-Temperature Sintering and Microwave DielectricProperties of Ba_2Ti_3Nb_4O_(18) Ceramics

The effects of CuO and H3BO3 additions on the low-temperature sintering,microstructure,and microwave dielectric properties of Ba2Ti3Nb4O18 ceramics were investigated.The addition of less amount of CuO （ 〈1 wt%） considerably facilitated the densification of Ba2Ti3Nb4O18 ceramics.Appropriate addition of H3BO3 （ 〈3.5 wt%） remarkably improved the microwave dielectric properties of ceramics.The addition of H3BO3 and CuO successfully reduced the sintering temperature of Ba2Ti3Nb4O18 ceramics from 1300 to 1050 ℃.Ba2Ti3Nb4O18 ceramics sintered at 1 050 ℃ for 4 h with the addition of 1.0 wt% CuO and 3.5 wt% H3BO3 exhibited good microwave dielectric properties：er=33.74,Q？f=13 812 GHz,and tf=-5.35 ppm/°C at about 5.0 GHz.

Microstructure and microwave dielectric properties of ZnO-B_2O_3 added (Ca_(0.254)Li_(0.19)Sm_(0.14))TiO_3 ceramics

The effects of ZnO-B2O3 （ZB2） on the sintering behavior and microwave dielectric properties of （Ca0.254Li0.19Sm0.14）TiO3 ceramics were investigated. The densities of the specimens reached the maximum value by adding 3 wt.% ZB2 and then decreased. The sintering temperature of the specimens was lowered from 1300 to 1100℃ without degradation of the microwave dielectric properties. The （Ca0.254Li0.19Sm0.14）TiO3＋ 3 wt.% ZB2 sintered at 1100℃ for 3 h showed good microwave dielectric properties, εr= 108.2, Qf= 6545 GHz, and rf= 6.5 ppm/℃, respectively, indicating that ZB2 was an effective sintering aid to improve the densification and microwave dielectric properties of （Ca0.254Li0.19Sm0.14）TiO3 ceramics.

FINITE ELEMENT COMPUTER MODEL OF MICROWAVE HEATED CERAMICS

At present,the concerned papers appeared in special magazines about discussing or introducing finite element method for calculating temperature distribution in ceramic body during microwave sintering, are not a good many, but it seems that finite element method is more convenient than finite difference method in dealing with special or complex geometry of ceramic body. In this paper, we describe a 3D finite element model simulating the heating pattern of ceramic microwave sintering in TE(10N) single-mode rectangular cavity in which the microwave energy deposition pattern in the samples can be expressed as an analysis function of space, and present a series of transient temperature distributions and heating rates of ceramic cylinders and cuboids tender variable thermal conductivities, dieletric loss factors,power consumer-levels, etc. These digital solutions may provide a better understanding of eliminating thermal runaway and improving temperature homogeneity.

Microstructure and strength modification of relaxor ferroelectric ceramics through microwave sintering for multilayer capacitors

Relaxor ferroelectric ceramics with the composition of xPb( Mg1/3 Nb2/3O3-yPb (Zn1/3Nb2/3O3-zPbTiO3 was fast sintered in a 2.45 GHz microwave system. Microwave-sintered samples illustrate more rapid densification and much smaller grain size microstructure than eonventional sintered samples. Also the microwave processing significantly increases the dielectric strength and flexural strength of the relaxor so that its strength becomes comparable with modified BaTiO3, and could obtain comparable dielectric properties in comparison with conventional sintering process. Microwave processing has many advantages for sintering of relaxor ferroelectrie ceramics used as multilayer capacitors.

Sintering characteristics, phase transitions, and microwave dielectric properties of low-firing [(Na_(0.5)Bi_(0.5))_(x)Bi_(1-x)](W_(x)V_(1-x))O_(4) solid solutionceramics

A series of high-k[(Na_(0.5)Bi_(0.5))_(x)Bi_(1-x)](W_(x)V_(1-x))O_(4)(abbreviated as NBWV(x value))solid solution ceramics with a scheelite-like structure are synthesized by a modified solid-state reaction method at the temperature range of 680-760 C.A monoclinic(0≤x<0.09)to tetragonal scheelite(0.09≤x≤1.0)structural phase transition is confirmed by X-ray difraction(XRD),Raman,and infrared(IR)analyses.The effect of structural deformation and order-disorder caused by Na^(+)/Bi^(3+)/W^(6+) complex substitution on microwave dielectric properties is investigated in deail.The compositional series possess a wide range of variable relative permittivity(er=24.8-80)and temperature coefficient of resonant frequency(TCF value,-271.9-188.9 ppm/℃).The maximum permittivity of 80 and a high Qxf value of~10,000 GHz are obtained near the phase boundary at x=0.09.Furthermore,the temperature-stable dielectric ceramics sintered at 680 C with excellent microwave dielectric properties of ε_(r)=80.7,Qxf=9400 GHz(at 4.1 GHz),and TCF value=-3.8 ppm/℃ are designed by mixing the components of x=0.07 and 0.08.In summary,similar sinterability and structural compatibility of scheelite-like solid solution systems make it potential for low-temperature co-fired ceramic(LTCC)applications.

Effect of LiF addition on sintering behavior and dielectric breakdown mechanism of MgO-based microwave dielectric ceramics

Glass-free MgO-based microwave dielectric ceramics(1-x)wt%(0.98 MgO-0.02 Al2O3)-x wt%LiF(x=0.5,1,2,3,4)were synthesized where LiF and Al2O3 were utilized as sintering additive and reinforcement phase in MgO matrix respectively.It was found that ion substitution is apt to occur between LiF and MgO,leading to the formation of oxygen vacancies and MgF2.Nevertheless,different from ordinary liquidphase sintering,morphologies of ceramics were distinctly altered with grains changing from polyhedron to sphere-like shape and densities underwent obvious decrease when excessive amount of LiF was introduced and we call it excessive liquid-phase sintering.Grain boundary weakening caused by this circumstance would exert an adverse effect on physical properties.Moreover,LiF addition dramatically reduced dielectric breakdown strength and altered the dielectric breakdown behavior of MgO-based ceramics,which is dominated by electrical breakdown mechanism.Combination of good properties were achieved in 1 wt%LiF modified MgO-based ceramics sintered at 950℃ which exhibited superior microwave properties(εr=9.56,tanδ=9.2×10^(-5),Qf=124,600 GHz),high flexural strength(184.5 MPa),high thermal conductivity(21.3 W/(m,K)),high coefficient of thermal expansion(-12 ppm/℃)and moderate electrical properties(Eb=35.9 kV/mm,r=4.9×10^(12) U cm).

Sintering behaviour and microwave dielectric properties of MgO-2B_(2)O_(3)--xwt%BaCu(B_(2)O_(5))-ywt%H_(3)BO_(3) ceramics

This study investigates the bulk density,sintering behaviour,and microwave dielectric properties of the MgO-2B_(2)O_(3) series ceramics synthesised by solid-state reaction.According to the X-ray diffraction and microstructural analyses,the as-prepared MgO-2B_(2)O_(3) ceramics possess a single-phase structure with a rod-like morphology.The effects of different quantities of H_(3)BO_(3) and BaCu(B_(2)O5)(BCB)on the bulk density,sintering behaviour,and microwave dielectric properties of the MgO-2B_(2)O_(3) ceramics were investigated.Accordingly,the optimal sintering temperature was obtained by adding 30 wt%H_(3)BO_(3) and 8 wt%BCB.We also reduced the sintering temperature to 825°C.Furthermore,the addition of 40 wt%H_(3)BO_(3) and 4 wt%BCB increased the quality factor,permittivity,and temperature coefficient of resonance frequency of MgO-2B_(2)O_(3) to 44,306 GHz(at 15 GHz),5.1,and-32 ppm/℃,respectively.These properties make MgO-2B_(2)O_(3) a viable low-temperature co-fired ceramic with broad applications in microwave dielectrics.

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.

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.

Effect of Sb-site nonstoichiometry on the structure and microwave dielectric properties of Li_(3)Mg_(2)Sb_(1-x)O_(6) ceramics

The non-stoichiometric Li_(3)Mg_(2)Sb_(1-x)O_(6)(0.05≤x≤0.125)compounds have been prepared via the mixed oxide method.The influences of Sb nonstoichiometry on the sintering behavior,microstructure,phase composition along with microwave dielectric performances for Li_(3)Mg_(2)Sb_(1-x)O_(6) ceramics were studied.Combined with X-ray diffraction(XRD)and Raman spectra,it was confirmed that phase composition could not be affected by the Sb nonstoichiometry and almost pure phase Li_(3)Mg_(2)SbO_(6) was formed in all compositions.Appropriate Sb-deficiency in Li_(3)Mg_(2)SbO_(6) not only lowered its sintering temperature but also remarkably improved its Q×f value.In particular,non-stoichiometric Li_(3)Mg_(2)Sb_(0.9)O_(6) ceramics sintered at 1250℃/5 h owned seldom low dielectric constant ε_(r)=10.8,near-zero resonant frequency temperature coefficient τ_(f)=-8.0 ppm/℃,and high quality factor Q×f=86,300 GHz(at 10.4 GHz).This study provides an alternative approach to ameliorate its dielectric performances for Li_(3)Mg_(2)SbO_(6)-based compounds through defect-engineering.

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.

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 ℃.

Synthesis and microwave dielectric behavior of (Bi_(1-x)Pb_(x))NbO_(4) ceramics

Ceramic samples of(Bi_(1-x)Pb_(x))NbO_(4)(x=0,0.025,0.05,0.10,0.15,0.20)with 0.75 wt.%V2O5 addition sintered at 920℃,940℃ and 960℃ are investigated.Pb is selected as a substitute for Bi^(3+)in BiNbO_(4) ceramics as it exists in two stable valence states of+2 and+4 and the average valency matches to that of Bi^(3+).The average Shannon radius(for octahedral coordination)of Pb^(2+)(1.19Å)and Pb^(4+)(0.775Å)cations is 0.9825Å,which is similar to that of Bi^(3+)ion(1.03Å).The dense(>94%)polycrystalline(Bi_(1-x)Pb_(x))NbO_(4) samples fabricated mostly reveal orthorhombic(Pnna)phase(α-BiNbO_(4),Sp.Gp.52)by powder XRD.Presence of satellite Pb2Nb2O7 phase,the amount of which is increasing with increase in Pb content,is also noticed.The microwave dielectric constant(ε'r)values of the niobates are found to increase from 42 to 71,whereas the quality factor(Qu.f)values are found to decrease from 5400 to 550 GHz with increasing substitution of Pb.The compositions so synthesized are important as hardly there are any microwave dielectric ceramics available with 45<ε'r<75.

Effect of TiO_2 content on the microwave dielectric properties of BaO-ZnO-TiO_2 ceramics

In this paper,the BaO-0.6ZnO-xTiO2 ceramics with x=2.5-2.8 have been prepared by the conventional solid-state ceramic route for the purpose of investigating the effect of TiO2 content on the microwave dielectric properties.The XRD results showed that the main crystal phase in the sintered ceramics was Ba4ZnTi11O27 and that the additional phases:Ba2ZnTi5O13 and BaZn2.03Ti3.93O10.89 were presented,depending on the TiO2 contents.The SEM photographs of the samples sintered at 1200℃ for 2 h showed a high compact microstructure.Because the phase composition of ceramics samples was changed with TiO2 content,the dielectric constant(εr),the quality factor values(Q×f) and the temperature coefficient of resonant frequency(τf) were first increased,continuously came up to a peak value,and then let up.It was lucky to find that the sample with composition BaO-0.6ZnO-2.7TiO2 had both the maximum dielectric constant εr=36.1 and the maximum Q×f value of 29320 GHz,and more importantly,it had an acceptable temperature coefficient of resonant frequency τf=10.45 ppm/℃.

Ba_(4)Ti_(13)O_(30)陶瓷的制备及其介电性能表征

利用烧结过程中固相反应产生的额外烧结推动力,在1200℃、常压烧结的条件下,通过混相烧结的方法成功制备了晶粒均匀、致密的纯相Ba_(4)Ti_(13)O_(30)陶瓷,研究了Ba_(4)Ti_(13)O_(30)相的合成规律、烧结性能及其显微结构的演变规律,并表征了Ba4Ti13O30材料的介电性能。结果表明,用固相反应法合成的BaTiO_(3)粉体与TiO_(2)在1150℃下即可合成Ba_(4)Ti_(13)O_(30)相,且在1150~1250℃温度范围内均生成单相。当烧结温度为1200℃时,Ba_(4)Ti_(13)O_(30)陶瓷的相对密度达到最大值95%,此时试样的微波性能最佳,其介电常数和品质因数Q×f分别为40和66273,其品质因数Q×f值远高于文献报道的13000;但当烧结温度提高到1230℃时陶瓷试样体密度略有下降并出现晶粒异常长大的现象,微波性能也随之急剧恶化,这是由其显微结构的变化所致。

Sr_(0.5)Zr_(2)(PO_(4))_(3)-(Ce,Sm)PO_(4)复相陶瓷核废物固化体的制备及化学稳定性

为同时固化高放废物中的模拟放射性核素Sr、Ce和Sm,采用一步微波烧结工艺成功制备了Sr_(0.5)Zr_(2)(PO_(4))_(3)-(Ce,Sm)PO_(4)复相磷酸盐陶瓷固化体,采用XRD、Raman、SEM-EDS和密度表征研究了其物相组成、微观结构以及致密性,并利用PCT法评估了化学稳定性。结果表明:Sr_(0.5)Zr_(2)(PO_(4))_(3)相和(Ce,Sm)PO_(4)独居石相兼容性好,两相间不发生相互反应;所制备的复相陶瓷固化体晶粒尺寸小,相对密度高于96%,改变Sm/Ce比对固化体的微观结构和致密性无明显影响;PCT测试结果表明Sr、Ce和Sm的元素归一化元素浸出率都较低,与单相磷酸盐陶瓷固化体相比,复相磷酸盐陶瓷固化体具有较为优异的化学稳定性。

氮化硅陶瓷微波烧结几何参数对电磁场分布影响规律的研究

在微波烧结过程中,烧结试样内部电场分布情况对试样烧结过程起决定性的作用。采用HFSS仿真软件,对一种加载氮化硅试样的5馈口微波烧结腔进行模拟仿真。同时,研究了试样的半径、高度及放置位置对微波电场的影响规律。模拟结果表明:当试样半径在42—44 mm左右时,内部电磁场分布均匀;随着试样高度的增加,电场的均匀性先下降后上升到最大值后又下降,试样高度在92—94 mm处附近时内部电磁场分布较为均匀、场强梯度小;随着放置高度的增加,试样内均匀性上升,但超过一定高度后电磁场均匀性下降;经比较研究,得出电磁场分布均匀、符合烧结要求的最佳烧结试样尺寸为高度93 mm、半径44mm、放置高度215 mm。