Synthesis and microwave dielectric properties of BaO-Sm2O3-5TiO2 ceramics with NdAlO3 additions

BaO-Sm₂O₃-5TiO₂ (BST5) ceramics with NdAlO₃ additions of up to 15 wt% were produced with a solid state reaction method, and their structural and microwave dielectric properties were determined. Experimental results showed that NdAlO₃ neither merged nor altered the orthorhombic tungsten bronze structure of the main phase of the produced ceramics (except for a shrinkage in the crystal lattice), but it was segregated in distinct grains in the microstructure of the produced ceramics. However, the amount of NdAlO₃ strongly influenced the densification and the microstructure (i.e. grain shape and size) of the produced ceramics. Analysis of the experimental results suggests that the microstructural features can be correlated to the dielectric properties of these ceramics. Accordingly, the dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) of the produced BLT5 ceramics can be tuned with the amount of NdAlO₃ additions and the sintering process parameters. The best dielectric properties were achieved for BaO-Sm₂O₃-5TiO₂ ceramics with 7.5% NdAlO₃ (ε_r=73.22, Q×f =10,300 GHz, and τ_f=−1.05 ppm/°C).     

Sintering behavior,phase evolution and microwave dielectric properties of thermally stable Li2O-3MgO-mTiO2 ceramics (1≤m≤6)

Li₂O-3MgO-mTiO₂ (1≤m≤6) ceramics were prepared by the solid state reaction method. X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy techniques were used to investigate the phase composition and crystal structure. With increasing m values, the phase structures of ceramics changed as: (Li₂Mg₃TiO₆, m=1)→(Li₂Mg₃Ti₄O₁₂ and Mg₂TiO₄, m=2,3)→(Li₂Mg₃Ti₄O₁₂, m=4)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃ and Li₂MgTi₃O₈, m=5)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃, Li₂MgTi₃O₈ and MgTi₂O₅, m=6). The optimized sintering temperature was lowered from 1275 °C to 1050 °C. When m=5, Li₂O-3MgO-5TiO₂ ceramics showed good microwave dielectric properties at a wide sintering temperature range of 1000–1200 °C, and the best microwave dielectric properties of Q×f=71,726 GHz, ε_r=21.9 and τ_f=−20.9 ppm/°C were obtained at a relatively low sintering temperature of 1050 °C.     

Preparation,structure and microwave dielectric properties of novel La2MgGeO6 ceramics with hexagonal structure and adjustment of its τf value

A novel La₂MgGeO₆ ceramic was synthesized through a solid-state reaction process within a sintering temperature range of 1450–1550 °C. By a combination of X-ray diffraction and Rietveld refinement analyses, the ceramics were found to have a pure hexagonal phase structure belonging to space group R3/146. The scanning electron microscopy images revealed that the ceramic grains were closely connected. The effects of internal (lattice energy, valence bond, and fraction packing) and external factors (density) on the microwave properties of ceramics were also studied. The ceramic exhibited excellent microwave dielectric performances, with a relative permittivity (?_r) of 21.2, a quality factor (Q × f) of 52 360 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?44.2 ppm/°C, when sintered at 1500 °C for 4 h. The τ_f value of the La₂MgGeO₆ ceramic doped with CaTiO₃ could be adjusted to zero. Particularly, 0.2La₂MgGeO₆-0.8CaTiO₃ ceramics have good microwave dielectric properties with τ_f = +2.1 ppm/°C, Q × f = 15 610 GHz, and ?_r = 40.3.     

Microwave dielectric properties of CaWO4–Li2TiO3 ceramics added with LBSCA glass for LTCC applications

In this work, 0.86CaWO₄–0.14Li₂TiO₃ ceramics were prepared via a traditional solid-state process. The effects of Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) addition on the phase formation, sintering character, microstructure and microwave dielectric properties of the ceramics were investigated. A small amount of LBSCA addition could effectively lower the sintering temperature of the ceramics. X-ray diffraction analysis revealed that CaWO₄ and Li₂TiO₃ phases coexisted without producing any other crystal phases in the sintered ceramics. The dielectric constant and Qf values were related to the amount of LBSCA addition and sintering temperatures. All specimens could obtain near-zero temperature coefficient (τ_f) values through the compensation of the positive τ_f of Li₂TiO₃ and the negative τ_f of CaWO₄. The 0.86CaWO₄–0.14Li₂TiO₃ ceramic with 0.5 wt% LBSCA addition and sintered at 900 °C for 3 h exhibited excellent microwave dielectric properties of ε_r=12.43, Qf=76,000 GHz and τ_f=−2.9 ppm/°C.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

BaMnV2O7：A novel microwave dielectric ceramic for LTCC applications

A novel low-temperature fired BaMnV₂O₇ ceramic was fabricated with solid-state reaction. Rietveld refinements based on X-ray diffraction data and TEM analysis indicated that BaMnV₂O₇ exhibited a monoclinic structure with a P21/n space group. The dense microstructure of the BaMnV₂O₇ ceramic obtained at 850 °C was examined by scanning electron microscopy. The increasing content of V⁴⁺ affected the quality factor, which was confirmed by X-ray photoelectron spectroscopy. The intrinsic dielectric properties were obtained from far infrared reflectivity spectra. Additionally, the BaMnV₂O₇ ceramic showed good chemical stability with a Ag electrode and desirable microwave dielectric performance at 850 °C: ϵ_r = 11.7, Q × f =20040 GHz and τ_f = −48.2 ppm/°C, which can potentially be applied in LTCC technology.     

Sintering behavior and dielectric properties of SiO2–BPO4 glass-fluxed ceramics

(1–x)SiO₂–xBPO₄ (x?=?23–70?wt%) glass-fluxed ceramics have been prepared by a traditional ceramic process. The phase assemblage, sintering, crystallization behavior, microwave dielectric properties and chemical compatibility with Ag/Cu have been studied. The SiO₂-rich compositions (x?=?23–50?wt%) could be well densified at ～975?°C/2?h, while the BPO₄-rich compositions demonstrated poor sinterability and porous microstructure. The SiO₂-rich compositions sintered at 975?°C/2?h contained BPO₄, low temperature cristobalite and glassy phases. Crystallization of BPO₄ occurred at lower temperature than that of SiO₂. Good combined microwave dielectric properties with ?_r?=?～5, Q?×?f?=?25,000?GHz and τ_f value of –7.3?ppm/°C could be obtained when 10?wt% TiO₂ was added after sintering at 975?°C/2?h. The x?=?23?wt% composition chemically reacted with Ag, but exhibited good chemical compatibility with Cu after sintering at 975?°C/2?h.     

A low-εr and high-Q microwave dielectric ceramic Li2SrSiO4 with abnormally low sintering temperature

A novel low-temperature co-fired ceramic Li₂SrSiO₄ with low-ε_r and high-Q was synthesized through solid-state reaction. XRD results showed that the Li₂SrSiO₄ ceramic formed a single hexagonal structure with a space group of P3₁21. The microwave dielectric properties of Li₂SrSiO₄ ceramic sintered at 880 °C were ε_r = 7.4 ± 0.1, Q × f = 100,700 ± 2000 GHz, and τ_f = −85.4 ± 2.4 ppm/℃. The PVL chemical bond theory indicated that the low sintering temperature of Li₂SrSiO₄ ceramic might be ascribed to the weak covalency (18 %) of SiO bond. A near-zero τ_f (‒1.9 ± 1.8 ppm/℃) combined with ε_r = 8.2 ± 0.1, and Q × f = 63,200 ± 1900 GHz was obtained by adding SrTiO₃ to form 0.94Li₂SrSiO₄ - 0.06SrTiO₃ composite ceramic. Given their chemical compatibility with Ag powders, the Li₂SrSiO₄ ceramics can be a candidate dielectric material for LTCC applications.     

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.     

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg_0·5Ti_0.5)³⁺ for Al³⁺ lowered the sintering temperature and improved the Q × f value of MgAl₂O₄ ceramic. The MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with ε_r = 9.1, Q × f = 98,000 GHz, τ_f = −61.36 ppm/°C. The sintering temperature of MgAl_1·94(Mg_0·5Ti_0.5)_0.06O₄ microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl₂O₄ ceramics.     

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.     

Microwave dielectric properties of Ca1.15Sm0.85Al0.85Ti0.15O4 ceramics prepared by reaction sintering

In the present work, a high quality (Q) Ca_1.15Sm_0.85Al_0.85Ti_0.15O₄ (CSAT) ceramic was prepared via reaction sintering (RS) method. The phase structure, surface morphology, packing fraction (PF), and valence bond of the ceramic were systematically investigated. By studying the X-ray diffraction (XRD) pattern of the ceramic, it was determined to exhibit a single-phase tetragonal structure with the dimensions of a = b = 3.6943(13)Å and c = 12.0320(23)Å and volume of V = 164.22(10) ų. The influence of the intrinsic quality loss factor on the Q × f value was investigated by calculating the PF. Simultaneously, the bond valence of the samarium (Sm) sites was evaluated to elucidate the relationship with the temperature coefficient of resonant frequency (τ_f). The CSAT ceramic was sintered at 1550 °C for 6 h and exhibited exceptional characteristics in terms of the relative dielectric constant (ε_r) = 17.5, quality factor (Q × f) = 66700 GHz, and τ_f = ?6.93 ppm/^°C. These results highlighted the excellent suitability of the RS method for preparing CSAT ceramics with outstanding microwave dielectric properties.     

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.