Search

Composite dielectrics comprising the crystal phases of Co-doped magnesium titanates MgTiO3 and Mg2TiO4, and CaTiO3 were examined with regard to their phase composition and its effect on the microwave dielectric properties. In the case of MgTiO3-based composites an increase in the Co concentration results in increasing amount of an additional phase MgTi2O5, which significantly deteriorates the product Q × f. In contrast, the materials based on the spinel Mg2TiO4 never contain MgTi2O5, and consequently demonstrate excellent Q × f magnitude which is as high as 100,000 GHz at low Co doping. At higher Co concentration, however, an additional phase MgTiO3 appears in these materials that is accompanied by a slight reduction of the Q × f magnitude. A slight addition (5 wt.%) of the glass-forming zinc borate into the Mg2TiO4-based composite allows a noticeable reduction of its sintering temperature by 150–200 °C without a significant degradation of its dielectric properties.

A study of K x Ba 1− x Ga 2− x Ge 2+ x O 8 ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P 2 1 / a ⇔ C 2/ m phase transition. The temperature of this phase transition decreases with an increase in x , in a similar way to the sintering temperature, which decreases from 1100 °C ( x = 0) to 970 °C ( x = 1). The temperature of the P 2 1 / a ⇔ C 2/ m phase transition is below the sintering temperatures of K x Ba 1− x Ga 2− x Ge 2+ x O 8 (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x ( x = 0.4 and 0) remain in the P 2 1 / a modification over a wide temperature range above the sintering temperature. Compared to the C 2/ m modification of K x Ba 1− x Ga 2− x Ge 2+ x O 8 (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P 2 1 / a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q × f values of ∼100,000 GHz (at ∼12 GHz). The temperature coefficient of the resonant frequency is ∼−25 ppm/K, regardless of the composition.

Dense, single-phase Sr x Ba 1− x Al 2 Si 2 O 8 (0 ≤ x ≤ 1) and Ca y Ba 1− y Al 2 Si 2 O 8 (0 ≤ y ≤ 0.05) ceramics were synthesized and then structurally and dielectrically characterized. The substitution of Ba with Ca or Sr promotes the transformation of hexacelsian to monocelsian. The substitution with Ca led to a more-than-two-times-lower Q × f value at y = 0.05, whereas the Q × f value of the Sr 0.05 Ba 0.95 Al 2 Si 2 O 8 ( x = 0.05) was only 5% lower than the Q × f value of the BaAl 2 Si 2 O 8 , which exhibited Q × f values of 70,600 and 77,700 GHz when sintered at 1500 °C for 12 and 40 h, respectively. The improvement in the Q × f value with the prolongation of the heat-treatment time, which can be attributed to an increase in the Al:Si ordering, was also noticed for other Sr x Ba 1− x Al 2 Si 2 O 8 solid solutions. In contrast to the dielectric losses, the permittivity ( ɛ = 7.3 and 7.2 at 1 MHz and 10.5 GHz, respectively) did not change significantly with the composition and the sintering conditions.

Composite ceramics based on the spinel Mg2TiO4 were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO3. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q×f value of around 150–160 THz was obtained for the undoped Mg2TiO4, whereas a reduced Q×f value was observed for an increase in the Co concentration in the system (1−x)Mg2TiO4−xCo2TiO4. Upon doping with 7 mol% CaTiO3, the Q×f value passed through a maximum with increasing Co concentration. Adding ZnO–B2O3 to the composite system based on Co-doped Mg2TiO4 resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q×f value.

Ceramics with a composition close to BaZn 2 Ti 4 O 11 were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn 2 Ti 4 O 11 was formed in the homogeneity range corresponding to the formula BaZn 2 − x Ti 4 O 11 − x (0 x 2 − x Ti 4 O 11 − x (0 x τ f = −30 ppm/K and high Q × f values, which increased from ∼68,000 GHz at x = 0 to ∼83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn 2 − x Ti 4 O 11 − x (0 x 2 Ti 4 O 11 -based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO 3 , hollandite-type solid solutions (Ba x Zn x Ti 8 − x O 16 ) and BaTi 4 O 9 caused a decrease in Q × f values.

NaxCa1� xAl2� xSi21xO8 plagioclase solid solutions (0rxr1) were synthesized under sub-solidus conditions using a solid-state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl2Si2O8; x 5 0) to the albite (NaAlSi3O8; x 5 1). Microwave (MW) dielectric measurements revealed that

Albite and anorthite, the end-members of the plagioclase feldspar structural family (Na x Ca 1− x Al 2− x Si 2+ x O 8 ), were synthesized under sub-solidus conditions using the solid-state reaction technique. The structural investigations revealed that both the plagioclase-phase formation and sintering temperature decreased from the anorthite (CaAl 2 Si 2 O 8 ; x = 0), which started to form at 1000 °C and sintered at 1300 °C, to the albite (NaAlSi 3 O 8 ; x = 1), which started to form at 800 °C and sintered at 1000 °C. Dielectric measurements in the microwave (MW) frequency region revealed that a high temperature (1500 °C) and an extended heat-treatment time were needed to obtain low-loss anorthite ceramics (Qxf = 10 000–12 000 GHz). In contrast, the albite attained Qxf = 11 200 GHz at a lower temperature (1025 °C). A temperature coefficient of resonant frequency ( τ f ) close to zero (−5 ppm/°C) was another advantage of the albite over the anorthite, which exhibited τ f = −130 ppm/°C. The dielectric measurements also revealed that a slow cooling rate considerably improved the Qxf values of the anorthite and albite.