Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Deuterium NMR study of the MP-2269: albumin interaction — a step forward to the dynamics of non-covalent binding

MP-2269, the Gd(III) complex of 4-pcntylbicyclo[2.2.2] octane-l-carboxyl-di-L-aspartyl-lysinc-derived-DTPA, is a small Gd-agent that binds non-covalently to scrum albumin in vivo to assume the enhanced relaxivities associated with macromolecular agents, (due in part to increased rotational correlation time, τ_R). To further explore the fundamental parameters that govern the dynamics of water proton relaxation enhancement by this prototypical albumin-binding agent, the rotational correlation time (τ_R) for the deuterated La(III) analog of MP-2269 has been independently measured in the presence and absence of 4% albumin using²H-NMR approaches. The diamagnetic La(III) analog of MP-2269 was deuterated at the α-position of the carbonyl groups.²H-NMR studies were conducted at 7.05T (46 MHz) and 310°K on a Bruker NMR spectrometer. Spectral deconvolution permitted calculation of transverse relaxation rates, 1/T₂, from the NMR linewidths and subsequently, τ_R. The results yielded a τ_R of the albumin bound complex of ∼ 8 ns. This value is intermediate between those earlier estimated by¹⁷O-NMR (∼ 1 ns) and¹H-NMRD (∼ 20–50 ns) and significantly shorter than that of albumin. The²H-NMR study results also indicate that the exchange between free and albumin-bound forms of the La(III) analog is slow (exchange lifetimes > 1 ms). This slow exchange does not affect the water residence lifetimes (τ_M 140-280 ns).     

Forsterite ceramics for millimeterwave dielectrics

The wireless communications have been tremendously developed in the recent ubiquitous age. The utilizable frequency region has been expanded to millimeterwaves because of shortage of conventional frequency regions. For the ultra high frequencies of millimeterwaves, dielectrics with ultra high quality factor Q and low dielectric constant ε _r are desired. Low ε _r is necessary to short the delay time of signals and to make devices small in size. Silicates for candidate of millimeterwave materials have low dielectric constant, because of silica-oxygen tetrahedra composed of half covalent bonds. Forsterite (Mg₂SiO₄) as such kind of silicates has high Q ⋅ f of 270000 GHz and low ε _r of 6.8. The temperature coefficient of resonant frequency τ _f was improved by adding rutile TiO₂ with high τ _f of plus 450 ppm/^∘C. In this paper, these results for forsterite are summarized.     

Sintering and compositional effects on the microwave dielectric characteristics of Mg(Ta1−x Nb x )2O6 ceramics with 0.25 ≦ x ≦ 0.35

1,500 °C−sintered MgTa₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 30.5, Q × f = 56,900 GHz, and τ _f = 28.3 ppm/°C, whereas 1,400 °C-sintered MgNb₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 21.7, Q × f = 89,900 GHz, and τ _f = −68.5 ppm/°C. In order to find the dielectric resonators with τ _f value close to 0 ppm/°C, the effects of sintering condition and composition on the microwave dielectric characteristics of Mg(Ta_1−x Nb _x )₂O₆ ceramics (0.25 ≦ x ≦ 0.35) prepared under sintering temperature of 1,300–1,450 °C are investigated. The results show that as the sintering temperature increases from 1,300 to 1,450 °C, the ɛ _r, Q × f and τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics all increase and saturate at 1,450 °C. On the other hand, as the Nb₂O₅ content decreases, the τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics will shift to near 0 ppm/°C. The optimized sintering conditions and composition to obtain the Mg(Ta_1−x Nb _x )₂O₆ dielectrics with τ _f close to 0 ppm/°C are sintering temperature of 1,450 °C, sintering duration of 4 h, and composition of x = 0.25, which exhibits the microwave dielectric characteristics of ɛ _r = 27.9, Q × f = 33,100 GHz, and τ _f = −0.7 ppm/°C.     

Microwave dielectric properties of Ca[(Li1/3Nb2/3)1−x Tix]O3−δ ceramics with glass

The effect of the addition of glass on the densification, low temperature sintering, and microwave dielectric properties of the Ca[(Li_1/3Nb_2/3)_1−x Ti_x]O_3−δ(CLNT) was investigated. Addition of glass (B₂O₃-ZnO-SiO₂-PbO system) improved the densification and reduced the sintering temperature from 1150^∘C to 900^∘C of Ca[(Li_1/3Nb_2/3)_1−x -Ti_x]O_3−δ microwave dielectric ceramics. As increasing glass contents from 10 wt% to 15 wt%, the dielectric constants (ε_r) and bulk density were increased. The quality factor (Q⋅f₀), however, was decreased slightly. The temperature coefficients of the resonant frequency (τ_f) shifted positive value as increasing glass contents over Ti content is 0.2 mol. The dielectric properties of Ca[(Li_1/3Nb_2/3)_0.75Ti_0.25]O_3−δ with 10 wt% glass sintered at 900^∘C for 3 h were ε_r = 40 Q·f₀ = 11500 GHz, τ_f = 8, ppm/°C. The relationship between the microstructure and dielectric properties of ceramics was studied by X-ray diffraction (XRD), and scanning electron microscope (SEM).     

Optimising the design of paramagnetic MRI contrast agents: influence of backbone substitution on the water exchange rate of Gd-DTPA derivatives

Among other factors influencing the residence time of the coordinated water (τ _M ) of paramagnetic contrast agents, the steric hindrance around the gadolinium ion seems to play a beneficial role. Such a crowding can be achieved by substituting the Gd-DTPA backbone on the C4 position. Several Gd-DTPA complexes carrying diverse groups at this position have thus been synthesised and characterised: Gd(S)-C₄-Me-DTPA, Gd(S)-C₄-n-Bu-DTPA, Gd(S)-C₄-iBu-DTPA, Gd(S)-C₄-iPr-DTPA, and Gd-C₄-diMe-DTPA. τ _M has been measured through the evolution of the water oxygen-17 transverse relaxation rate as a function of the temperature. The data show a reduction of τ _M of Gd(S)-C₄-Me-DTPA, Gd(S)-C₄-n-Bu-DTPA, Gd(S)-C₄-iBu-DTPA, Gd(S)-C₄-iPr-DTPA, and Gd-C₄-diMe-DTPA (τ _M ³¹⁰=91,82, 108,98, and 57 ns respectively, as compared to Gd-DTPA (τ _M ³¹⁰=143 ns)). At 310 K, the nuclear magnetic dispersion relaxation profiles of water protons are very similar for the five complexes which present longitudinal relaxivities slightly higher than those of Gd-DTPA. Regarding zinc transmetallation, C4-monosubstituted derivatives are more stable than Gd-DTPA. These results confirm that a judicious substitution of the DTPA skeleton allows for an acceleration of the coordinated water exchange rate. This observation can be useful for the design of vectorised contrast agents for molecular imaging.     

Dielectric anomalies of Pb0.7La0.2TiO3-based perovskite

Dielectric transitions from normal ferroelectric to relaxor-like and then to quantum paraelectric-like behaviour were observed by substituting 10 to 60% of Ca²⁺ for Pb²⁺ into A-site of Pb_0.7La_0.2TiO₃ with the required stoichiometry of Pb_0.7(1−x)Ca_0.7x La_0.2TiO₃ (PCLT). The quantum ferroelectric relation that commonly applies to SrTiO₃-based perovskites, T _maxα (X − X _c )^1/2, fails to describe the observed dielectric anomalies in PCLT, whereby the transition temperature vanishes with a finite slope. Quantum ferroelectric behaviour that exhibits a sharp dielectric peak but violates the Curie-Weiss law was not observed throughout the composition range studied. Unlike the typical quantum paraelectric SrTiO₃, the quantum paraelectric-like behaviour observed for PCLT with x = 0.55 and 0.60 exhibit negative transition temperatures, as shown by the fittings to both the Curie-Weiss and Barrett’s relations. Thermal hysteresis was surprisingly observed in substituted PCLT with x = 0.30 and 0.40 that exhibit frequency dispersive relaxation. To establish a correlation between the dielectric anomalies and structural parameters, analyses on global and local perovskite lattices were carried out using X-ray diffraction technique and micro-Raman spectroscopy.     

Sintering and dielectric properties of Al2O3 ceramics doped by TiO2 and CuO

The effects of CuO and TiO₂ additives on the microstructure and microwave dielectric properties of Al₂O₃ ceramics were investigated. Al₂O₃ ceramics with CuO and TiO₂ additions can be well sintered to achieve 93∼98% theoretical densities below 1,360 °C due to Ti₄Cu₂O liquid phase sintering effect. The Qf values decreased with increasing CuO and TiO₂ content, due to the formation of the second phase Ti₄Cu₂O. However, the varying behaviors of the dielectric constant (ɛ _r ) and temperature coefficients (τ _f ) were associated with phase constitutions, as a result of the change of CuO and TiO₂content. The τ _f can be shifted close to 0 ppm/°C by controlling the content of CuO and TiO₂. The specimens with 0.5 wt.% CuO and 7 wt.% TiO₂ sintered at 1,360 °C for 4 h showed ɛ _r of 11.8, Qf value of 30,000 GHz, and τ _f of −7 ppm/°C.     

Dielectric relaxation and variable-range-hopping conduction in BaSn1−x Cr x O3 system

The possibility of formation of a solid solution in the system BaSn_1−x Cr _x O₃ has been explored upto x ≤ 0.20. It has been confirmed that single phase solid solution forms upto x ≤ 0.10. Dielectric and conduction behaviour of single phase samples have been studied in the temperature range 400–610 K and frequency range 10 Hz–2 MHz. Two dielectric relaxation processes in two different frequency ranges have been observed. The temperature dependence of both dc and ac resistivity obey relation ρ = ρ _o exp(B/T^1/4), indicative of variable range hopping conduction mechanism. The activation energy for dc conduction is higher than that for relaxation time (τ) of low frequency dielectric relaxation process. It has been observed that activation energy for dielectric relaxation matches with activation energy for ac conductivity (at 100 kHz) for both the dielectric relaxation processes. Seebeck coefficient ‘α’ of the samples have been measured in the temperature range 350–650 K. Negative value of ‘α’ in the entire range of temperature measurement shows that conduction species are negatively charged. On the basis of value of activation energy for dc conduction and sign of Seebeck coefficient, conduction in the low temperature region (below 500 K) is attributed to hopping of weakly bonded electrons among Sn²⁺ ⇔ Sn⁴⁺ or Sn³⁺ ⇔ Sn⁴⁺ and that in the high temperature region (above 500 K) to hopping of doubly ionized oxygen vacancies .     

Structural evolution and microwave dielectric properties of (1 − x)BaZn1/3Nb2/3O3 + xBaMg1/2W1/2O3 ceramics

Structural evolution and microwave dielectric properties of (1 − x)BaZn_1/3Nb_2/3O₃ + xBaMg_1/2W_1/2O₃ (0 ≤ x ≤ 1) system have been investigated in this work. All samples exhibit single perovskite phase except for the samples with x = 0 and x ≥ 0.8 in which barium niobate and BaWO₄ second phase existed, respectively. 1:1 cations ordering existed in the samples with x ≥ 0.1, and the ordering degree increases with the increase of x. Liquid phase sintering was observed in the sample with x ≥ 0.8. Dielectric constant decreases almost linearly from 40.8 to 17.4 with increasing x. Q × f value monotonically increases from 26,162 GHz to 64,705 GHz with increasing x. The τ_f value changes from +30 ppm/°C to −27.8ppm/°C. Near zero τ_f value of −1.4 ppm/°C could be obtained at x = 0.4 composition.     

Phase transformation and microwave dielectric properties of BiPO4 ceramics

Monazite-type compounds, BiPO₄ polymorphs were prepared by the solid-state reaction method. The phase transformation and microwave dielectric properties of sintered ceramics were investigated using the X-ray powder diffraction (XRD) and a network analyzer, respectively. The low-temperature phase of BiPO₄ has monoclinic structure, and was transformed into the high-temperature phase with a slight distortion of monoclinic when it is heated above 600^∘C. The effect of the transformation on the microwave dielectric properties was examined. It was found that the dielectric properties of each phase were significantly different. In particular, the high-temperature phase sintered at 950^∘C has good microwave dielectric properties; the relative dielectric constant (ε _r ) = 22, the quality factor (Q× f) = 32,500 GHz and the temperature coefficient of resonant frequency (τ _f ) = − 79 ppm/ ^∘C.     

Oxygen exchange and transport properties of yttria-stabilized zirconia coated with LaCrO3

The oxygen permeation flux through YSZ (Yttria-Stabilized Zirconia) in reducing Po ₂ is mostly controlled by the surface-exchange kinetics in spite of very high temperature [1]. In order to study the kinetics, the YSZ surface was coated with LaCrO₃ on feed side, permeate side, or two sides, and the oxygen fluxes were measured under controlled Po ₂ gradient generated by different CO/CO₂ mixtures (permeate side: ∼3 × 10⁻¹² atm, feed side: 2 × 10⁻¹⁰∼2 × 10⁻⁸ atm) at 1600^∘C. The oxygen flux was determined by measuring the change in CO₂ content of the permeate-side gas. When both feed and permeate surfaces were coated, the oxygen flux increased by ∼6 times. For either permeate- or feed-side coating, the increases were ∼4 times and ∼1.5 times, respectively. A model was proposed in order to estimate the surface-exchange coefficient ( ) of feed and permeate side with or without coating.     

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of the BaTi4O9 ceramics

The effect of B₂O₃ and CuO on the sintering temperature and microwave dielectric properties of BaTi₄O₉ ceramics was investigated. The BaTi₄O₉ ceramics were able to be sintered at 975^∘C when B₂O₃ was added. This decrease in the sintering temperature of the BaTi₄O₉ ceramics upon the addition of B₂O₃ is attributed to the formation of BaB₂O₄ second phase whose melting temperature is around 900^∘C. The B₂O₃ added BaTi₄O₉ ceramics alone were not sintered below 975^∘C, but were sintered at 875^∘C when CuO was added. The formation of BaCu(B₂O₅) second phase could be responsible for the decrease in the sintering temperature of the CuO and B₂O₃ added BaTi₄O₉ ceramics. The BaTi₄O₉ ceramics containing 2.0 mol% B₂O₃ and 5.0 mol% CuO sintered at 900^∘C for 2 h have good microwave dielectric properties of ε_r = 36.3, Q× f = 30,500 GHz and τ_f = 28.1 ppm/^∘C     

Thermal expansion and electrochemical properties of La0.7AE0.3CuO3−δ (AE=Ca, Sr, Ba) cathode materials for IT-SOFCs

Alkaline earth metals (Ca, Sr, Ba) substituted lanthanum copper oxides were investigated to evaluate their potentials as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion and electrochemical performance of La_1−x AE _x CuO_3−δ (x = 0.3, AE=Ca, Sr, Ba) were studied by X-ray diffraction, thermal dilatometer and impedance spectra, respectively. By lowering the size of the A-site cation in ABO₃ perovskite, a lower thermal expansion and polarization resistance were obtained forCa-doped LaCuO_3−δ cathode which showed an area specific resistance of 0.19 Ω cm⁻² under open circuit potential conditions at 800°C, and a polarization overpotential of 52 mV at a current density of 0.1 A/cm² at 700°C, being a potential candidate of cathode material for IT-SOFCs.     

The effect of alumina addition on the electrical conductivity of Gd-doped ceria

Acceptor doped-ceria is a possible electrolyte material for the IT-SOFC (intermediate temperature solid oxide fuel cell) due to its high oxygen-ion conductivity. However, its use has been limited by its mechanical weakness and the appearance of electronic conductivity in reducing condition. In this study, alumina was selected as an additive in the doped-ceria to see if it increases the oxygen-ion conductivity and mechanical strength. Effects of alumina addition in doped ceria were studied as a function of alumina content and acceptor (Gd) content. The electrical conductivity of (Ce_1−x Gd _x O_2−δ)_1−y + (Al₂O₃) _y (x = 0–0.35, y = 0–0.10) was measured by using impedance spectroscopy. The grain conductivity of Ce_0.8Gd_0.2O_2-δ (GDC20) with 5 mol% alumina increased ∼3 times from that of GDC20 at 300^∘C. The grain conductivity was even ∼2 times higher than that of Ce_0.9Gd_0.1O_2−δ (GDC10) at 300^∘C. The electrical conductivity of GDC20 without alumina addition, measured at 500^∘C in air, rapidly decreased after exposure to reducing condition (Po₂∼10⁻²² atm) at 800^∘C. However, the decrease was much slower in GDC20 with alumina addition, indicating the improved mechanical strength. Among the examined compositions, (Ce_0.75Gd_0.25 O_2-δ)_0.95 + (Al₂O₃)_0.05 (GDC25A5) showed the highest conductivity at most temperatures.     

Ni-Zn ferrite films synthesized from aqueous solution usable for sheet-type conducted noise suppressors in GHz range

Ni_0.2Zn_0.3Fe_2.5O₄ films (1–5 μm thick) were deposited by spin spray ferrite plating from an aqueous solutions onto polyimide sheets at 90^∘C. Their peel test and high-frequency permeability as well as noise suppression effects were investigated. The oxygen plasma treatment on polyimide sheet surface improved the film adhesion. There was not visible crack on the bended film surface for the Ni-Zn ferrite film thinner than 2 μm and was not peeled off even after the bending test of a million times. The films exhibited excellent high-frequency permeability profile and a natural resonance frequency (where the imaginary permeability reaches a maximum) f _r was 370 ± 30 MHz. The transmission loss increased with the film thickness, reaching the maximum Δ P _loss = 70% at 8 GHz for the 5 μm-thick film. The reflection loss in the measured frequency range was S ₁₁ < 10 % which is small enough for films to be used as the conducted noise suppressors. The value of Δ P _loss obtained for the 5-μm thick film was about 15% higher than that (Δ P _loss = 55 %) attained by the commercialized 50-μm thick noise suppressing sheet.     

1H-NMRD and17O-NMR assessment of water exchange and rotational dynamics of two potential MRI agents: MP-1177 (an extracellular agent) and MP-2269 (a blood pool agent)

The parameters that govern water proton magnetic relaxation (e.g. water exchange rates, and rotational and electronic correlation times) of representatives of two classes of Gd(III) complexes have been estimated, using two different approaches and the results compared with those derived for known analogs. The complexes studied are: (i) the non-ionic GdDTPA-bis(methoxyethyl-amide) [Gd(DTPA-BMEA)], a typical small-molecule extracellular MR agent, and (ii) the ionic Gd(III) complex of 4-pentylbicyclo[2.2.2]octane-1-carboxyl-di-l-aspartyl-lysine-derived-DTPA [GdL]⁴⁻, a prototype MR blood pool agent, which binds to serum albumin in vivo through non-covalent hydrophobic interactions. An¹⁷O-NMR study of [Gd(DTPA-BMEA)] gives a water exchange rate constant ofk _ex ²⁹⁸ =(0.39±0.02)×10⁶ s⁻¹, identical to that for the bismethylamide analog [Gd(DTPA-BMA)]. Both approaches yield longer rotational correlation times for [Gd(DTPA-BMEA)], consistent with its higher molecular weight. An¹⁷O-NMR study of [GdL]⁴⁻ gives a water exchange rate constant ofk _ex ²⁹⁸ =(4.2±0.1)×10⁶ s⁻¹, identical to that for [Gd(DTPA)]²⁻. The water exchange rate on [GdL]⁴⁻ did not decrease considerably when bound to albumin, the lowest limit isk _ex,GdL-BSA=k _ex,GdL/2. Both approaches yield identical rotational correlation times for [GdL]⁴⁻, however, it was difficult to derive a consistent rotational constant for the albumin-bound [GdL]⁴⁻ using the different approaches (values ranged between 1.0 and 23.0 ns).     

Zeolite coating on foamed stainless steel by in-situ crystallization method

Faujasite-type zeolite films were prepared on foamed stainless steel by the in-situ crystallization method. Precursor solutions were prepared by dissolving water glass and NaAlO₂in a NaOH solution and aged at room temperature for two days. The concentrations of the starting materials were varied from 0.29 to 2.3 M (in SiO₂concentration) keeping the molar ratios of Na₂O:SiO₂:Al₂O₃fixed at 3.6:3.0:1.0. The foamed stainless steel substrate had about 90% of porosity and an average pore size of 600 μ m. It was dip-coated in the precursor solution four times, then hydrothermally treated at 80^∘, 110^∘ and 150^∘C for 6–48 h. The XRD patterns and SEM photographs revealed that faujasite-type zeolite was formed predominantly at a SiO₂concentration of 1.1 M, temperature of 110^∘C and duration of 24 h, with a product particle size of 2–5 μ m. At higher concentrations of the precursor solution, hydroxylsodalite becomes the major product rather than faujasite-type zeolite. The adherence strength of the zeolite grains deposited on the foamed stainless steel is higher in the in-situ crystallization method than when a conventional solution method is used. Thus, the in-situ crystallization method is concluded to be effective for preparing zeolite films even on metal substrates.