Crystal structure and dielectric properties of germanate melilites Ba2MGe2O7 (M = Mg and Zn) with low permittivity

Two melilite-structured ceramics, Ba₂MgGe₂O₇ and Ba₂ZnGe₂O₇, were fabricated using the simple solid-state sintering route and their microwave dielectric properties were reported for the first time. Rietveld refinements revealed that both ceramics crystallized in a tetragonal system with a space group P-42₁m. In contrast to Ba₂MgGe₂O₇ that was single phase in the whole sintering range, Ba₂ZnGe₂O₇ exhibited a second phase BaZnGeO₄ which was confirmed by SEM and Raman analysis. The sintering temperature could optimize the relative density and dielectric properties. Dense Ba₂MgGe₂O₇ and Ba₂ZnGe₂O₇ ceramics could be obtained at 1280?°C and 1180?°C with low relative permittivity(ε_r) of 7.76 and 9.0, high quality factor (Q?×?f) of 20,700 and 13,950?GHz, and negative temperature coe?cient of resonant frequency (τ_f) of ?55 and ?75?ppm/°C, respectively. Their differences in microwave dielectric properties were analyzed based on ionic polarizability and packing fraction. The thermal stability of Ba₂MgGe₂O₇ was tuned through the solid solution formation with TiO₂ and the τ_f value was adjusted successfully.     

Low relative dielectric permittivities of polyimides and copolyimides derived from non-coplanar diamines and 4,4′-(hexafluoroisopropyl)diphthalic anhydride

Abstract

Two series of polyimides and copolyimdes were solution cast into transparent, flexible and tough films. The glass transition temperatures T _g obtained from dynamic mechanical analysis were moderate compared with commercial polyimides such as Dupont Vespel®. The T _gs for the majority of polymers derived from monomer 1 (2′,5′-bis(p-aminophenoxy)-[1,1′;4′,1≥]terphenyl) (1A-F) were higher than those of polymers derived from monomer 2 (2,5-bis(p-aminophenoxy)biphenyl) (2A-F). The thermal expansion coefficients of the two series of polymers were moderate and comparable. The maximum elongations and moduli of polymers 2A-F were larger than those of polymers 1A-F possibly because polymers 2A-F were more flexible than polymers 1A-F. The relative dielectric permittivities of both series of polymers were measured and exhibited a low value from 2 49 to 3 01 at 100 kHz. In contrast, the relative permittivities calculated using the Clausius-Mosotti relationship were higher. Incorporation of non-coplanar diamines into the polymer chain causes an increase in free volume, which may account for the decrease in relative permittivities.     

Crystal structure of corundum type Mg4(Nb2–xTax)O9 microwave dielectric ceramics with low dielectric loss

New microwave dielectric ceramics, i.e. Mg₄(Nb_2−xTa_x)O₉ (MNT) solid solutions, were synthesized and their microwave dielectric properties and crystal structure were investigated in this study. From the discrete variational Xα (DV-Xα) method, it was found that the Ta–O bonds in the TaO₆ octahedron become more covalent than Nb–O bonds in the NbO₆ octahedron; this result leads to the decrease of the ionicity in the Ta⁵⁺ ion. The dielectric constants of MNT were slightly decreased from 12.4 to 11.5; this result might be due to the covalent interaction of Ta–O bonding. The quality factors of the samples were found to exhibit high value (Q·f≒350 000 GHz for x=2) which is comparable to those of Al₂O₃.     

A novel one-dimensional single helix derived from 2,2′-bipyridine based Zn(II) species directed self-assembly with 1,2-benzenedicarboxylate

The self-assembly assisted by 2,2^′-bipyridine (2,2^′-bpy) protected zinc(II) species with 1,2-benzenedicarboxylate (1,2-BDC) has yielded a novel one-dimensional single helical coordination polymer of [Zn(2,2′-bpy)(1,2-BDC)(H₂O)]_n in a racemate of right- and left-handed helices, which was crystallographically studied and thermally analyzed.     

Phase formation and dielectric properties of Ln2(Ln′0.5Nb0.5)2O7 (Ln = rare earth element)

Weberites and pyrochlores (A₂B₂O₇), both fluorite-related superstructures, are attractive dielectric ceramics due to their ability to accommodate diverse cations, thus allowing their properties to be tailored. This study focuses on the fundamental understanding of the structure–dielectric property relationships in fluorite-related oxides. Specifically, Ln₃NbO₇ and Ln₂(Ln′_0.5Nb_0.5)₂O₇ (where the ionic radius of Ln′ is smaller than that of Ln) compounds are investigated. It has been previously shown that weberite-type Ln₃NbO₇ exhibits a composition dependent dielectric relaxation above room temperature. It is here shown that a dielectric relaxation also occurs in La₂(Ln′_0.5Nb_0.5)₂O₇ (Ln′ = Yb³⁺, Er³⁺, and Dy³⁺) compounds near or below ?158 °C. The temperature, at which the maximum permittivity occurs, is different for different compositions (?132 °C for La₂(Yb_0.5Nb_0.5)₂O₇, ?197 °C for La₂(Er_0.5Nb_0.5)₂O₇, and ?187 °C for La₂(Dy_0.5Nb_0.5)₂O₇ at 1 MHz) and is correlated with the distortion of the NbO₆ octahedra. The room temperature dielectric permittivity of all three compounds was measured to be between 40 and 50 at 1 MHz.     

Two novel low permittivity microwave dielectric ceramics Li2TiMO5 (M = Ge,Si) with abnormally positive τf

Two tetragonal natisite structured Li₂TiMO₅ (M = Ge, Si) ceramics fabricated using the conventional solid-state reaction method were investigated in terms of the thermal stability, sintering behavior and dielectric properties at radio (RF) and microwave frequency region. At the optimum sintering temperature of 1140 °C, Li₂TiGeO₅ (LTG) has ε_r ˜ 9.43, Q × f ˜ 65,300 GHz (at 14.7 GHz), and τ_f ˜ +24.1 ppm/°C, while Li₂TiSiO₅ (LTS) sintered at 1180 °C exhibits ε_r ˜ 9.89, Q × f ˜ 38,100 GHz (at 14.2 GHz), and τ_f ˜ +50.1 ppm/°C. The positive τ_f values of the present LTG and LTS are abnormal and extremely important for low-ε_r microwave dielectric ceramics, which could behave as a promising τ_f compensator. Moreover, the dielectric spectra of both ceramics revealed a phase transition at low-temperature, exhibiting a dielectric peak, which could account for the negative τ_ε and positive τ_f in operating temperature ranges.     

Organosoluble and colorless fluorinated poly(ether imide)s derived from a highly contorted biphenyl-2,2′-diol bis(ether anhydride) and aromatic bis(ether amine)s with trifluoromethyl substituents

A series of new fluorinated aromatic poly(ether imide)s (PEIs) were prepared from highly contorted 2,2′-bis(3,4-dicarboxyphenoxy)biphenyl dianhydride and various trifluoromethyl-containing bis(ether amine)s via chemical or thermal imidization. All the PEIs showed good solubility in a variety of organic solvents and could afford flexible, colorless, and transparent thin films via solution-casting. They showed cut-off wavelengths of Uv-vis absorptions below 380 nm and very low yellowness index (b* < 7.0). They also showed high thermal stability with the 10% weight loss temperature from 503 to 585 °C of in nitrogen or air atmosphere. The glass transition temperatures (T _gs) are recorded in the range of 194–254 °C by DSC. For a comparative study, another series of non-fluorinated PEIs derived from the same biphenyl-2,2′-diol bis(ether anhydride) with various bis(ether amine)s without the trifluoromethyl substituents were also synthesized and characterized.     

Studies on the Separation and Recovery of Palladium from Simulated High Level Liquid Waste (SHLW) Solution with Novel Extractant N,N,N′,N′-tetra (2-ethylhexyl) Dithiodiglycolamide DTDGA

The extraction properties of N,N,N',N'-tetra-(2-ethylhexyl) dithiodiglycolamide (DTDGA) have been evaluated for separation and recovery of palladium from simulated high-level liquid waste (SHLW) solution. Extraction of Pd was found to increase with increase in nitric acid concentration up to 4.0 M, above which the extraction remains almost constant. Acid uptake studies show 1:1 stoichiometry between DTDGA and HNO₃ at nitric acid concentration above 3.0 M. The acid uptake constant (K_H) was found to be 0.60 which could be due to the presence of two carbonyl groups of amidic moiety. DTDGA has shown very high extractability and selectivity for Pd over other metal ions present in SHLW. The separation factor (SF) for Pd over other metal ions was found to be more than 10⁴. Almost complete back extraction of palladium from organic phase was achieved with 0.01 M thiourea in 0.1 M nitric acid. Reusability studies of the extractant indicate that D_Pd remains almost constant even after five successive cycles of extraction and stripping. Two extraction stages will be required for complete extraction of 100 mg/L Palladium in 3.0 M nitric acid solution using 0.0025 M DTDGA/n-dodecane solvent system.     

Self-assembly of two discrete polynuclear iron(II) metallomacrocycles from a ligand containing two 2,2′:6′,2″-terpyridine binding domains

The homoditopic ligand L contains two tpy metal-binding domains and reacts with iron(II) chloride to form two multinuclear metallomacrocycles having [3 + 3] and [4 + 4] metal–ligand stoichiometries.     

Porous γ-(Y1-xHox)2Si2O7 thermal insulator with excellent high-temperature strength retention and very low thermal conductivity

High-temperature thermal insulation materials challenge extensive candidates with good mechanical, thermal and chemical reliability at high temperatures. Recently, porous γ-Y₂Si₂O₇ was indicated a promising thermal insulator in harsh environment; however, its strength at 1300?°C reduced to 34% of that at room temperature. In this work, we significantly improved its high-temperature strength by doping Ho. Highly porous γ-(Y_1-xHo_x)₂Si₂O₇ solid solution was fabricated by in-situ foam-gelcasting method. Especially, porous γ-(Y_2/3Ho_1/3)₂Si₂O₇ demonstrated the optimal high-temperature strength, for instance 65% retention at 1300?°C, as well as high compressive strength (13.9?MPa) and low thermal conductivity (0.186?W/(m?K)) at room temperature, at the porosity of 79.3%. Interestingly, porous solid solution sample displayed obviously lower thermal conductivity than the two end pure-phase porous materials. Porous γ-(Y_1-xHo_x)₂Si₂O₇ solid solution is clearly highlighted as a promising high-temperature thermal insulator with outstanding high-temperature strength retention and optimal low thermal conductivity.     

Spectroscopic evaluation of a novel multi-element sensitive fluorescent probe derived from 2-(2′-phenylbenzamide)benzimidazole: Selective discrimination of Al3 + and Cd2 + from their congeners

A novel fluorescent chemosensor 2 exhibiting excited-state intramolecular proton transfer (ESIPT) process has been synthesized by condensation approach, among which 2 could be used as a multielement sensitive fluorescent probe for both Al^3 + and Cd^2 +. The emission wavelength of 2 underwent blue shift of 78 nm upon binding with these ions. The cation-induced inhibition of ESIPT phenomenon was responsible for these fluorescence changes. Moreover, the absorption spectra of 2 showed changes with F⁻ and AcO⁻ ions also. TD-DFT studies well displayed the blue shift in the emission wavelength of 2 upon binding with Al^3 + ions.     

N,N,N′,N′-Tetra(2-Ethylhexyl) Thiodiglycolamide T(2EH)TDGA: A Promising Ligand for Separation and Recovery of Palladium from High Level Liquid Waste (HLLW) Solutions

The extraction properties of N,N,N′,N′-tetra(2-ethylhexyl)thiodiglycolamide T(2EH)TDGA have been evaluated for the separation and recovery of palladium from simulated high-level liquid waste (SHLW). T(2EH)TDGA has shown very high selectivity for Pd (II) over other metal ions present in SHLW. The separation factor (SF) for Pd (II) over other metal ions was found to be more than 10⁵. Reusability studies of the extractant indicate that D_Pd remained almost constant even after five successive cycles of extraction and stripping. Palladium was quantitatively recovered from thiourea strip solution by treating it with ammonia and filtering the precipitate of palladium sulphide. The acid uptake constant (K_H) was found to be 0.62 which could be due to the presence of two carbonyl groups of amidic moiety. To account for very high extractability of palladium with T(2EH)TDGA over other ‘S’ donor extractants, namely Bis-(2-ethylhexyl)sulphoxide (BESO), FTIR, as well as Raman studies were carried out. FTIR and Raman studies suggested the ligation through carbonyl as well as the thio-ether group. Conditional extraction constants (log K′_ex) were determined and the thermodynamic parameters were calculated from the dependence of the conditional extraction constant (log K′_ex) on temperature. The calculated values of ΔG_ex, ΔH_ex, and ΔS_ex were ?41.78 kJmol^?1, ?55.12 kJmol^?1 and ?44.04 JK^?1 mol^?1 respectively. The extraction process is indicated to be enthalpy driven with the entropy factor counteracting it.     

Fabrication and in vitro degradation study of novel optically active polymers derived from amino acid containing diacids and 4,4′-thiobis(2-tert-butyl-5-methylphenol)

Chiral bioactive poly(ester-imide)s (PEI)s were synthesized from N,N′-(pyromellitoyl)-bis-(L-phenylalanine) and N,N′-(pyromellitoyl)-bis-(L-leucine) diacids derived from amino acids with 4,4′-thiobis(2-tert-butyl-5-methylphenol) via direct polycondensation reaction in a system of tosyl chloride, pyridine and N,N-dimethylformamide as a condensing agent. The structures of these polymers were confirmed by FT-IR, ¹H-NMR, specific rotation, elemental and thermogravimetric analysis (TGA) techniques. TGA showed that the 10% weight loss temperature in a nitrogen atmosphere was more than 390 °C, which indicates that the resulting PEIs have a good thermal stability. The biodegradability of the monomers and prepared polymers was investigated in culture media and soil burial test for assessing the susceptibility of these compounds to microbial degradation. The results showed that the synthesized monomers and theirs derived polymers are biologically active and they are nontoxic to microbial growth.     

The preparation of graphene hybrid films decorated with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] particles prepared by non-solvent induced precipitation

A hybrid film of graphene decorated with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) nanoparticles is fabricated by a simple non-solvent induced precipitation method. The structure of the graphene film and graphene/MEH-PPV hybrid film are investigated by scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The large specific surface area of graphene sheets makes it an ideal support for depositing MEH-PPV nanoparticles as well as preventing aggregation of the nanoparticles. The graphene/MEH-PPV hybrid film is superhydrophobic, which is attributed to the increase in surface roughness as a result of random decoration of MEH-PPV nanoparticles on the graphene sheets. The MEH-PPV nanoparticles endow the graphene hybrid film with strong photoluminescence. The hybrid film also shows excellent conductivity and high thermal stability. The graphene/MEH-PPV hybrid film thus prepared may find potential applications in various devices where integrated properties of superhydrophobicity, conductivity and luminescence are required.     

Structure,spectral analysis and microwave dielectric properties of novel x(NaBi)0.5MoO4-(1-x)Bi2/3MoO4 (x = 0.2 ∼ 0.8) ceramics with low sintering temperatures

Herein, the x(NaBi)_0.5MoO₄-(1-x)Bi_2/3MoO₄ (xNBM-(1-x)BMO, x = 0.2 ∼ 0.8) microwave dielectric ceramics with low sintering temperatures were prepared via the traditional solid-state method to adjust the τ_f value and dielectric constant. The crystal structure was determined using X-Ray diffraction and Raman spectroscopy, the microstructure was investigated using scanning electron micrograph and energy disperse spectroscopy, and the dielectric properties were studied using a network analyser and infrared spectroscopy. For the xNBM-(1-x)BMO composite ceramics, the (NaBi)_0.5MoO₄ tetragonal phase coexisted with the Bi_2/3MoO₄ monoclinic phase. With the rise of x value, the permittivity increased from 23.7–29.8, and the τ_f value shifited from -53.3 ppm/°C to -13.7 ppm/°C. The 0.8NBM-0.2BMO ceramic sintered at 680 °C possessed excellent microwave dielectric properties with a ε_r = 29.8 (6.7 GHz), a Qf = 11,800 GHz, and a τ_f = -13.7 ppm/°C. These results made the xNBM-(1-x)BMO composite ceramics great candidates in low temperature co-fired ceramics technology.     

Porous Co3O4 nanowires derived from long Co(CO3)(0.5)(OH)·0.11H2O nanowires with improved supercapacitive properties

Porous Co(3)O(4) nanowires with large aspect ratio have been obtained by annealing long Co(CO(3))(0.5)(OH)·0.11H(2)O precursor nanowires synthesized by a facile hydrothermal method. The results show that the amount of the additive (urea) has an important impact on the morphology of the as-synthesized cobalt-carbonate-hydroxide intermediate, where the uniformity and the overall structure can be controlled by changing the urea concentration. After the heat treatment, the as-obtained phase-pure Co(3)O(4) nanowires with a well retained structure are applied as the electrode material for supercapacitors, and the sample exhibits excellent performance with a high specific capacitance of 240 F g(-1) after 2000 charge/discharge cycles, corresponding to a retention of 98% of the initial capacitance.     

Products and microwave dielectric properties of ceramics with nominal composition (Ba0.9Ca0.1)(YxB′1/2)O(3x+4.5)/2 (B′=Nb5+, Ta5+)

The products and microwave dielectric properties of ceramics with nominal composition (Ba_0.9Ca_0.1)(Y_xB′_1/2)O_(3x+4.5)/2 (B′=Nb⁵⁺, Ta⁵⁺) are investigated. When x=0.5, i.e. (Ba_0.9Ca_0.1)(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺), the product contains a considerable amount of Y₂O₃ as well as the main perovskite phase. When x=0.3 the product is single phase, equivalent to Ba(Ca_1/9Y_3/9Nb_5/9)O₃ or Ba(Ca_1/9Y_3/9Ta_5/9)O₃. The lattice parameters of these new compounds are smaller than those of Ba(Y_1/2Nb_1/2)O₃ and Ba(Y_1/2Ta_1/2)O₃. The relative permittivities (ε_r) of these new compounds are larger than those of Ba(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺). The increase in ε_r of the Nb-system is about 4 times larger than that of the Ta-system. The Q f values of the present ceramics are larger than the Ca-containing perovskite in the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. The sharp increase of ε_r in this study cannot be explained by the Ca²⁺ rattling ion model at the A-site, which applies to the case of the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. A new method to explain the increase in ε_r is discussed.