Microwave dielectric properties of neodymium tin oxide

The microwave dielectric properties of Nd₂Sn₂O₇ ceramics were investigated with a view to their application in mobile communication. Nd₂Sn₂O₇ ceramics were prepared by the conventional solid-state method with various sintering durations. A maximum density of 7.11 g/cm³, a dielectric constant (_εr${\epsilon }_r$) of 17.02, a quality factor (Qf) of 33,100 GHz, and a temperature coefficient of resonant frequency (_τf${\tau }_f$) of −55 ppm/°C were obtained when Nd₂Sn₂O₇ ceramics were sintered at 1550 °C for 9 h.     

Microwave dielectric properties of temperature-stable zircon-type (Bi,Ce)VO4 solid solution ceramics

In the (Bi_1 − xCe_x)VO₄ (0 ≤ x ≤ 1) system, we found that the (Bi_1 − xCe_x)VO₄ (0 ≤ x ≤ 0.1) belongs to the monoclinic scheelite phase and the (Bi_1 − xCe_x)VO₄ (0.7 ≤ x ≤ 1) belongs to the tetragonal zircon phase, while the (Bi_1 − xCe_x)VO₄ (0.1 < x < 0.7) belongs to the mixed phases of both monoclinic scheelite and tetragonal zircon structure. Interestingly, two components with near-zero temperature coefficient of resonant frequency (TCF) appeared in this system. In our previous work, a near-zero TCF of ~+15 ppm/°C was obtained in a (Bi_0.75Ce_0.25)VO₄ ceramic with a permittivity (ε_r) of ~47.9 and a Qf (Q = quality factor = 1/dielectric loss; f = resonant frequency) value of ~18 000 GHz (at 7.6 GHz). Furthermore, in the present work, another temperature-stable microwave dielectric ceramic was obtained in (Bi_0.05Ce_0.95)VO₄ composition sintered at 950°C and exhibits good microwave dielectric properties with a ε_r of ~11.9, a Qf of ~22 360 GHz (at 10.6 GHz), and a near-zero TCF of ~+6.6 ppm/°C. The results indicate that this system might be an interesting candidate for microwave device applications.     

The dielectric constant and quality factor calculation of the microwave dielectric ceramic solid solutions

The dielectric constant of the microwave dielectric ceramic solid solution is usually predicted by the Clausius-Mosotti equation but the quality factor (Q) cannot be precisely calculated. In this paper, it finds that the dielectric constant of the solid solutions also could be well calculated by the Maxwell-Wagner formula, and that the Q of solid solutions can be precisely calculated, by assuming a solid solution as a two or more materials’ mixture.     

Microwave processing and properties of ceramics with different dielectric loss

Microwave sintering behaviors of three kinds of ceramics with different dielectric loss [Al₂O₃, Ce–Y–ZrO₂ and lead-based relaxor ferroelectrics (PMZNT)] in 2·45 GHz microwave furnace were described. Measurement of sample densities showed an enhancement of the sintering processing for all materials studied. For PMZNT and Ce–Y–ZrO₂ with high dipolar loss or ionic conductive loss, the associated microstructure examined using scanning electron microscopy showed that microwave-sintered compacts produced much finer grain sizes at near theoretical density compared to conventional sintering. Resulting material properties, such as flexure strength and breakdown strength, were also increased due to developed microstructure in microwave processing. However, a comparable grain size and properties were observed for high pure Al₂O₃ with low dielectric loss in microwave and conventional methods. ©     

Microwave dielectric properties of BaTi4O9 thin film

BaTi₄O₉ thin films have been prepared by RF magnetron sputtering on the Pt/Ti/SiO₂/Si substrates and the dielectric properties of the BaTi₄O₉ film have been investigated at microwave frequency range. The homogeneous BaTi₄O₉ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The circular-patch capacitor (CPC) was used to measure the microwave dielectric properties of the film. The dielectric constant (ɛ_r) and the dielectric loss (tan δ) were successfully measured up to 6 GHz. The ɛ_r of the BaTi₄O₉ thin film slightly increased with the increase of the film thickness. However, the tan δ decreased with increasing the thickness of the film. The ɛ_r of BaTi₄O₉ thin film was similar to that of the BaTi₄O₉ ceramics, which is about 36–39. The tan δ of the film with 460 nm thickness was very low approximately, 0.0001 at 1–3 GHz. Since BaTi₄O₉ film has a high ɛ_r and a low tan δ, the BaTi₄O₉ film can be used as the microwave devices.     

Microwave dielectric properties of Co2P2O7 ceramics

Co₂P₂O₇ ceramics were prepared through the traditional solid-state sintering technique. The phase composition, grain size distribution, and densification were researched via X-ray diffraction and scanning electron microscope. The influence of pores on permittivity was described by various models. The dielectric loss was found highly dependent on porosity. Moreover, the low ε_r (<10) values of Co₂P₂O₇ ceramics were explained by the covalent feature of P–O bonds. Raman spectroscopy was used for exploring the relationship between polar phonon modes and dielectric properties in terms of intrinsic factors. The optimum dielectric properties (ε_r = 6.76, Qf = 36,400 GHz and τ_f = ?23.9 ppm/°C) were obtained at 1160 °C for 4 h.     

Microwave dielectric properties of glass-ceramic composites for low temperature co-firable ceramics

Ba–B–Si glass was added to Ba–Nd–Sm–Bi–Ti–O (BRT₁₁₄) microwave dielectric material for LTCC applications. Conventional one-step processing method for preparing glass-BRT₁₁₄ composite materials yields low dielectric constant, since the glass was easy to react with BRT₁₁₄ and forms a low dielectric constant phase, Ba₃B₆Si₂O₁₆. A large proportion of pores appeared. The nature of glass, whether it is sol-gel derived or fused, shows marked influence on the microstructure and microwave dielectric properties of the composites. A two-step process containing precoating the BRT₁₁₄ powders with a thin layer of glass, followed by conventional samples preparation process, tremendously improved the densification behaviour of the material. The formation of pores and interactions between glass and BRT₁₁₄ was greatly suppressed such that materials with high dielectric constant (ε_r=40) were achieved by sintering 9 wt.% glass-containing composite at 950 °C for 2.5 h.     

Microwave dielectric properties and sintering behaviors of scheelite compound CaMoO4

The microwave dielectric properties, sintering behaviors of scheelite compound CaMoO₄ were investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. To improve the sintering property of CaMoO₄, samples with different Ca/Mo ratio were prepared. The bulk densities of CaMo_(x)O₄ (for x = 1.02, 1.05, 1.08) samples were higher than those of pure CaMoO₄ over all temperature range. The well-sintered CaMo_(x)O₄ (for x = 1.02) sample with ∼95.7% of the theoretical density shows Q × f value of 71,000 GHz and dielectric constant (ɛ_r) = 10.3. The effects of liquid phase formation on sintering process and dielectric properties were discussed. Sample with hot-press sintering was also prepared in order to investigate dielectric properties of fully dense CaMoO₄, which exhibited quality factor (Q × f), ∼55,000 GHz and dielectric constant (ɛ_r) = 11.7.     

Microwave dielectric properties of MgTiO3–SrTiO3 layered ceramics

MgTiO₃–SrTiO₃ layered ceramics with different stacking were fabricated and the microwave dielectric properties were evaluated with TE₀₁₁ mode. With increasing SrTiO₃ thickness fraction, the resonant frequency (f₀) decreased, while the effective dielectric constant (ɛ_r,eff) and temperature coefficient of resonant frequency (τ_f) increased for the bi-layer ceramics. The stacking arrangement also had significant effect on the microwave dielectric properties. For the same SrTiO₃ thickness fraction of 0.333, the tri-layer MgTiO₃/SrTiO₃/MgTiO₃ ceramics had lower f₀, higher ɛ_r,eff and τ_f. The result was not consistent with the previous report on the layered ceramics with TE₀₁₁ mode [Cho, J. Y., Yoon, K. H. and Kim, E. S., Effect of stress on microwave dielectric properties of layered Mg_0.93Ca_0.07TiO₃–(Ca_0.3Li_0.14Sm_0.42)TiO₃ ceramics. Mater. Chem. Phys. 2003, 79, 286; Cho, J. Y., Yoon, K. H. and Kim, E. S., Correlation between arrangement of dielectric layers and microwave dielectric properties of Mg_0.93Ca_0.07TiO₃–(Ca_0.3Li_0.14Sm_0.42)TiO₃ ceramics. J. Am. Ceram. Soc. 2003, 86, 1330], where the effective dielectric constant was only determined by the thickness fraction and was independent of the stacking arrangement. Finite element analysis gave an explanation for the different microwave dielectric behaviors of the bi- and tri-layer ceramics in the present experiment.     

Microwave dielectric properties of Al-doped ZnO powders synthesized by coprecipitation method

Al-doped ZnO (AZO) powders with different Al concentrations were synthesized by coprecipitation method. Crystal structural and dielectric properties of the powders as a function of aluminum doping concentration as well as annealing temperature were investigated. The XRD results reveal that Al atoms are doped into ZnO lattice successfully and all the samples are polycrystalline with a hexagonal wurtzite structure. The real part (ε′) and imaginary part (ε″) of the complex permittivity of the powders were carried out by a vector network analyzer in the microwave frequency range of 8.2–12.4 GHz. The results show that the ε′ and ε″ of the doped ZnO powders increase with the doping content and both of them are higher than that of the pure one. For the AZO powder with 7 mol% Al, both ε′ and ε″ increase firstly and then decrease with increasing annealing temperature.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.     

Influences of dielectric and conductive fillers on dielectric and mechanical properties of solid silicone rubber composites

Dielectric elastomers are materials being used for electromechanical transduction applications. Their electromechanical response depends on permittivity, Young’s modulus and electric breakdown strength. A factor that limits its application is high operating voltages that can be reduced through improvement in permittivity. One of the methods is by incorporating high permittivity fillers into polymer matrix to obtain dielectric–dielectric composites (DDC).These composites show high permittivity at the cost of reduced flexibility. An alternative solution is development of composites by incorporating organic or inorganic conductive fillers into polymer matrix. These composites show high permittivity with high dielectric loss and low breakdown strength. To overcome both the above limitations both dielectric and conductive fillers are incorporated into dielectric polymer matrix to obtain conductor–dielectric composites (CDC). In this study, high temperature vulcanized solid silicone rubber as matrix has been used to prepare DDC composites with barium titanate (BT) filler and CDC composites with both BT as dielectric and ketjenblack as conductive fillers, using Taguchi design of experiments. The effect of factors such as amount of fillers and curing agent, mixing time in roll mill and curing temperature on the dielectric and mechanical properties are reported. Lichtenecker model predicts the permittivity of the DDC composite more accurately. For the CDC composites permittivity increased by 390%, effective resistivity decreased by 80%, Young’s modulus increased by 368% and Shore A hardness increased by 90% as compared to those of reference matrix. Important interaction effects are observed among both the fillers that are uniformly dispersed without any aggregation.

     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO₂ bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti³⁺ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO₂ specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛ_r) and quality factor (Q × f) of SPS-TiO₂ re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛ_r ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO₂ specimens prepared using SPS were discussed in terms of grain size variation and Ti⁴⁺ reduction.     

Thermal properties of ZrB2-TiB2 solid solutions

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB₂. The resulting (Zr,Ti)B₂ ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB₂ resulted in grain size decreasing from 22 μm for nominally pure ZrB₂ to 7 μm for ZrB₂–50 vol.% TiB₂. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB₂ to 58 W/m⋅K for ZrB₂–50 vol.% TiB₂. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB₂ at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB₂. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB₂ decreasing to 57 W/m⋅K when 50 vol.% TiB₂ was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB₂. Additions of ≥25 vol.% TiB₂ reduced the phonon contribution to nearly zero for all temperatures.     

Stability of rhombohedral structure and improved dielectric and ferroelectric properties of Ba,Na, Ti doped BiFeO3 solid solutions

A study of the (1-x)BiFeO₃-(x)Ba_1/2Na_1/2TiO_2.75 (BFO-BNT) solid solutions obtained using the solid state reaction method, for different molar relative concentration of Ba_1/2Na_1/2TiO_2.75 in the 0.0 ≤ x ≤ 0.12 composition range, is presented. The crystal structure and the dielectric and ferroelectric properties are studied in detail. Results of the Rietveld refinement of the X-ray diffraction data demonstrate that the system is single phase with R3c symmetry up to x = 0.09 while for x = 0.12, a small quantity of a secondary phase with P4mm symmetry appears. Scanning electron microscopy demonstrates that BNT presence promotes grain growth resulting in larger grains. Raman spectroscopy shows that, with increasing x, some of the A and E Raman modes slightly reduce their intensity while shifting in frequency, evincing the structural changes caused by the Ba, Na, and Ti incorporation on the BFO lattice. The X-ray photoelectron spectroscopy study confirms the successful substitution and gradual structural distortion in the samples. The improvement in dielectric properties with increasing BNT concentration can be attributed to stable dipole moment formation. Compared with pure BFO ceramics, doped BFO samples exhibit remarkably enhanced ferroelectric properties.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Microwave dielectric properties of flexible butyl rubber–strontium cerium titanate composites

Butyl rubber–strontium cerium titanate (BS) composites have been prepared by hot pressing. The tensile tests show that the BS composites are flexible. The dielectric properties of the composites have been investigated at 1 MHz and 5 GHz as a function of ceramic contents. The composite with volume fraction 0.43 of ceramic filler has a dielectric constant (ε_r) of 11.9 and dielectric loss (tan δ) 1.8 × 10^?3 at 5 GHz. The measured values of ε_r are compared with the effective values calculated using different theoretical models. The thermal conductivity of the composites is found to increase with ceramic contents and reaches a value of 4.5 Wm^?1 K^?1 for maximum filler loading 0.43 volume fraction. The coefficient of thermal expansion of the composites decreases gradually with filler loading and reaches a minimum value of 30.2 ppm °C^?1 at a volume fraction 0.43. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Microwave dielectric properties of mineral sillimanite obtained by conventional and cold sintering process

The sillimanite (Al₂SiO₅) mineral has been sintered by conventional ceramic route and by cold sintering methods. The mineral has very poor sinterability and transformed to mullite on sintering above 1525 °C. The dielectric properties of sillimanite mineral (Al₂SiO₅) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 °C has low ε_r of 4.71 and tanδ of 0.002 at 1 MHz and at microwave frequency ε_r = 4.43, Q_u × f = 41,800 GHz with τ_f = −17 ppm/°C. The sintering aid used for cold sintering Al₂SiO₅ is sodium chloride (NaCl). The Al₂SiO₅NaCl composite was cold sintered at 120 °C. XRD analysis of the composite revealed that there is no additional phase apart from Al₂SiO₅ and NaCl. The densification of the Al₂SiO₅NaCl composite was confirmed by using microstructure analysis. The Al₂SiO₅NaCl composite has ε_r of 5.37 and tanδ of 0.005 at 1 MHz whereas at microwave frequency it has ε_r = 4.52, Q_u × f = 22,350 GHz with τ_f = −24 ppm/°C. The cold sintered NaCl has ε_r = 5.2, Q_u × f = 12,000 GHz with τ_f = −36 ppm/°C.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.