Acid-base catalysis of N-[(morpholine)methylene]daunorubicin

The stability of N-[(morpholine)methylene]-daunorubicin hydrochloride (MMD) was investigated in the pH range 0.44-13.54, at 313, 308, 303 and 298 K. The degradation of MMD as a result of hydrolysis is a pseudo-first-order reaction described by the following equation: ln c = ln c(0) - k(obs)? t. In the solutions of hydrochloric acid, sodium hydroxide, borate, acetate and phosphate buffers, k(obs) = k(pH) because general acid-base catalysis was not observed. Specific acid-base catalysis of MMD comprises the following reactions: hydrolysis of the protonated molecules of MMD catalyzed by hydrogen ions (k(1)) and spontaneous hydrolysis of MMD molecules other than the protonated ones (k(2)) under the influence of water. The total rate of the reaction is equal to the sum of partial reactions: k(pH) = k(1) ? a(H)+ ? f(1) + k(2) ? f(2) where: k(1) is the second-order rate constant (mol(-1) l s(-1)) of the specific hydrogen ion-catalyzed degradation of the protonated molecules of MMD; k(2) is the pseudo-first-order rate constant (s(-1)) of the water-catalyzed degradation of MMD molecules other than the protonated ones, f(1) - f(2) are fractions of the compound. MMD is the most stable at approx. pH 2.5.     

Solid-state stability studies of faropenem based on chromatography,spectroscopy and theoretical analysis

Aim: The purpose of this study was to investigate the stability of faropenem in solid state.

Results: The kinetic and thermodynamic parameters of degradation of faropenem were studied using an RP-HPLC method while the changes of spectral properties were investigated using derivative UV and FT-IR. Quantum-chemical calculations, based on the density functional theory, were carried out to support the estimation of the intra-ring stresses of faropenem and for theoretical interpretation of the spectra. The degradation of faropenem was a first-order reaction depending on the substrate concentration at an increased relative humidity and in dry air. The dependence ln k = f(1/T) became the ln k?=?(2.03?±?3.22)?×?10⁴–(9761?±?3052)(1/T) in dry air and ln k?=?(1.25?±?0.22)?×?10⁵–(9004?±?3479)(1/T?) at 90.0% RH. The thermodynamic parameters E_a, ΔH^≠a, and ΔS^≠a of the degradation of faropenem were calculated. The dependence ln k?=?f(RH%) assumed the form ln k?=?(7.58?±?1.88)?×?10^?2 (RH%) – (5.90?±?3.90)?×?10^?8.

Conclusions: Stability studies of faropenem showed that the fusion of β-lactam and thiazolidine rings reduces the intra-ring stress, leading to a lower susceptibility to degradation in dry air and at increased RH.     

Sulphur,nitrogen-doped TiO2/graphene oxide composites as a high performance photocatalyst

Sulphur (S), nitrogen (N)-doped TiO₂/graphene oxide (GO) composites were environmentally friendly synthesised using thiourea (CS(NH₂)₂) as a binary-element doping reagent by a simple colloidal blending method. The S, N-doped TiO₂/GO composites show higher photocatalytic degradation rates on methyl orange (MO) compared to TiO₂. The average value of k (the apparent rate constant) for the S, N-doped TiO₂/5%GO (k?=?0.035?min^?1) was found to be eight times higher than that of TiO₂ grown in solution (k?=?0.0038?min^?1) and four times higher than that of P25 (k?=?0.008?min^?1). The investigation showed that the contribution of GO to the high photocatalytic activities of the composites come from its high specific surface area, oxygen-containing functional groups and large aromatic domains that were inclined to be bound by conjugated MO molecules via π–π stacking. The doping of S and N can increase the numbers of photo-generated electrons and holes, which produce more charge carriers to form reactive species, and thus promote the degradation of MO. This work could offer a route to improve the photocatalytic activities of TiO₂ and facilitate their application in reality.     

The effect of 2-hydroxypropyl-ß-cyclodextrin on the solubility,stability and dissolution rate of famotidine

Abstract

Famotidine was found to form an inclusion complex with 2-hydroxypropyl-ß-cyclodextrin (HPCD). Phase-solubility diagram was classified as type A_L with a stability constant for complex formation (K_st) of 100.50 M¹ at pH 7.4. Famotidine undergoes specific acid catalysis in strongly acidic solutions. Addition of HPCD to these solutions decreased the rate of drug degradation. The rate constant for degradation of complexed famotidine (k_c) and K_st were estimated from the relationship between the observed rate constant for overall drug degradation (K_obs) and HPCD concentration. An increase in ionization of famotidine resulted in a decrease in the magnitude of K_st. The dissolution rate of the prepared complex was significantly greater than that of the pure drug.     

Micro-structural,dielectric, ferroelectric and piezoelectric properties of mechanically processed (Pb<Subscript>1−x</Subscript>La<Subscript>x</Subscript>)(Zr<Subscript>0.60</Subscript>Ti<Subscript>0.40</Subscript>)O<Subscript>3</Subscript> ceramics

In this work, (Pb_1?xLa_x)(Zr_0.60Ti_0.40)O₃ (PLZT x/60/40, x?=?at.%) ceramics were prepared by using high energy mechanical ball milling followed by cold isostatic pressing (CIP), investigated for their micro-structural, dielectirc, ferroelectric and piezoelectric properties. Mechanical activation results in the highly reactive nature of the nano size milled PLZT powders, which enables the partial perovskite phase formation, confirmed by room temperature XRD patterns. CIP leads to a higher density with a closely packed dense microstructure of sintered PLZT ceramics shown in SEM images. The grain size of PLZT x/60/40 ceramics was found to be decreasing with increasing La³⁺ content. The highest relative density of ~?97% was found to be for PLZT 8/60/40 ceramics with grain size of ~?1.35 µm. The PLZT 8/60/40 system also shows the highest dielectric constant of ~?1976, remnant polarization of 29.1 µC/cm², piezoelectric coefficients (d₃₃?\(~ \cong ~\)?570 pC/N, g₃₃?\(~ \cong ~\)?28.03?×?10^?3 Vm/N) and electromechanical coupling factors (k_p?=?k₃₃?=?64.1% and k₃₁?=?54%). The elastic compliances for the PLZT x/60/40 ceramics were also obtained.     

A Cross-Performance Relationship Between Carr's Index and Dissolution Rate Constant: The Study of Acetaminophen Batches

The aim of this paper is to promote a simple and scalable approach to accelerate the formulation development of wet granules using acetaminophen batches as a model system. Only two thorough experiments with five processing steps of: crystallization → dry blending → wet granulation → drying → dissolution, were required to establish a specific linear relationship between the overall effect of the particle size distribution and the dissolution performance for a given formulation of any batch of acetaminophen. With this specific linear relationship at hand, dissolution rates of the granules prepared from batches of acetaminophen with various particle size distribution could be predicted without the need of doing any wet granulation, drying and dissolution for the same formulation. It was found that the Carr's Index, C, an overall manifestation of particle size distribution, of only a few grams of the dry blended acetaminophen was good enough to be linearly related to the dissolution rate constant, k, of the formulated granules by ln k?=?α ln C?+?ln A (or exponentially by a power law of k?=?AC^α) where A was the exponential factor and α was the power index. A and α were dependent on the mass transfer of acetaminophen powders and the rheological properties of the formulated dry blended powders, respectively. The three linear relationships for 75, 62, and 30 wt % formulations were ln k?=?2.9 ln C –12.3, ln k?=?2.8 ln C –12.5, and ln k?=?4.2 ln C –18.0, respectively. The power laws for 75, 62, and 30 wt % formulations were k?=?4.7 × 10^?6 C^2.9, k?=?3.9 × 10^?6 C^2.8, and k?=?1.5 × 10^?8 C^4.2, respectively. The formulation used in our study contained acetaminophen, microcrystalline cellulose, and polyvinylpyrrolidone. The validation of the linearity between k and C was verified (1) by acetaminophen batches from different processes and sources, (2) by the various formulation compositions of acetaminophen of 75, 62, and 30 wt%, and (3) by the growth mechanisms of wet granulation and the resultant granular structures determined by dry sieve analysis, optical microscopy (OM), mercury intrusion porosimetry (MIP), the Brunauer-Emmett-Teller (BET) method, scanning electron microscopy (SEM), and Fourier transformed infrared (FT-IR) microscopic mapping. In general, granules grown from the small-size ranged acetaminophen powders of a given formulation had a higher C. Since the growth mechanism was dominated by agglomeration, the granules were more porous, higher in surface area, more homogenous, and higher in dissolution rate constant, k, as opposed to granules grown from the large-size ranged acetaminophen powders of a given formulation having a lower, C, whose growth was dominated via consolidation and layer-by-layer mechanism and resulted in a lower dissolution rate constant, k.     

The Prediction of the Dissolution Rate Constant by Mixing Rules: The Study of Acetaminophen Batches

The purpose of this article is to promote two simple and scalable methods to accelerate the formulation development of formulated granules using acetaminophen as a model system. In method I, formulated granules made from the batch of small particle–sized acetaminophen (1) by ball milling the batch of large particle–sized acetaminophen (2), and the mixture of the two batches at equal weights (mix) gave the dissolution rate constants (k) of k₁?=?0.43 ± 0.15 minutes^?1, k₂?=?0.18 ± 0.01 minutes^?1, and k_mix?=?0.30 ± 0.03 minutes^?1 for 75 wt percent formulation; k₁?=?0.75 ± 0.01 minutes^?1, k₂?=?0.18 ± 0.01 minutes^?1, and k_mix?=?0.34 ± 0.03 minutes^?1 for 62 wt percent formulation; and k₁?=?0.28 ± 0.01 minutes^?1, k₂?=?0.16 ± 0.01 minutes^?1, and k_mix?=?0.22 ± 0.02 minutes^?1 for 30 wt percent formulation. In method II, the mixture of the formulated granules produced by mixing the formulated granules from the two batches at equal weights gave dissolution rate constants of k_mix?=?0.30 ± 0.03 minutes^?1, 0.30 ± 0.02 minutes^?1, and 0.22 ± 0.01 minutes^?1 for 75 wt percent, 62 wt percent, and 30 wt percent formulations, respectively. After fitting the three data points of k₁, k₂, and k_mix to the 10 mixing rules in materials science—series mixing rule, Hashin and Shtrikman upper bound, logarithmic mixing, Looyenga mixing rule, effective media approximation (EMA), three-point lower bound, Torquato approximation, three-point upper bound, Maxwell mixing rule, and parallel mixing rule—we found that the selection of the best suited mixing rules based on k₁, k₂, and k_mix was solely dependent on the formulations under a given operating condition and regardless of whether the system was a powder mixture or a granular mixture. The values of k₁, k₂, and k_mix in both the 75 wt percent and 30 wt percent formulations were enveloped by the parallel mixing rule and Maxwell mixing rule, whereas the values of k₁, k₂, and k_mix for the 62 wt percent formulation were encompassed by the logarithmic mixing rule, Hashin and Shtrikman upper bound, and the series mixing rule. Apparently, the best suited mixing rules could be used to predict the right proportions of either the powder mixture (Method I) or the granular mixture (Method II) for obtaining any other desired dissolution rate constant, k_mix, whose value fell in between the values of k₁ and k₂.     

Thermal stability and fire behavior of aluminum diethylphosphinate-epoxy resin nanocomposites

The flame retardancy of 2, 2-bis(4-glycidyloxyphenyl)propane (DGEBA)-aluminum diethylphosphinate (AlPi) nanocomposites (EP-AlPi/(P ? x), x = 1, 2, 3 %) was greatly enhanced by ultrasonic dispersion of nano-sized AlPi into epoxy resin. The UL 94 V-0 rating can be reached for EP-AlPi nanocomposites with a relatively low addition amount of AlPi (on the account of 8.4 wt% or phosphorus content of 2 wt%) as well as the LOI value over 37.2. The glass transition temperature (T _g) enhanced properties were investigated by DTA, which showed that: T _gs were about 5 °C higher than that of neat epoxy resin; T _g increased along with content increasing of AlPi. Based on TGA results under a non-isothermal condition, the thermal degradation kinetics of EP-AlPi/(P ? x) composites were studied by Kissinger’s, Ozawa’s, Flynn–Wall–Ozawa’s and Coast-Redfern’s methods, which suggested the conversion function f (α) = 1/2α ^?1 or f (α) = [?ln(1 ? α)]^?1 for EP-AlPi/(P ? 1 %); f (α) = [?ln(1 ? α)]^?1 for EP-AlPi/(P ? 2 %) and EP-AlPi/(P ? 3 %) during the investigated process. The epoxy resin nanocomposites obtained in this study are green functional polymers and will become flame retardant potential candidates in electronic fields such as printed wiring boards with high performance.     

Gradual Eddy-Wave Crossover in Superfluid Turbulence

We revise the theory of superfluid turbulence near the absolute zero of temperature and suggest a differential approximation model for the energy fluxes in the k-space, ε _HD(k) and ε _KW(k), carried, respectively, by the collective hydrodynamic (HD) motions of quantized vortex lines and by their individual uncorrelated motions known as Kelvin waves (KW). The model predicts energy spectra of the HD and the KW components of the system, ?_HD(k) and ?_KW(k), which experience a smooth crossover between different regimes of motion over a finite range of scales. For an experimentally relevant range of Λ≡ln?(?/a) (? is the mean intervortex separation and a is the vortex core radius) between 10 and 15 the total energy flux ε=ε _HD(k)+ε _KW(k) and the total energy spectrum ?(k)=?_HD(k)+?_KW(k) are dominated by the HD motions for k<2/?. In this region ?(k) follows the HD spectrum with constant energy flux ε?ε _HD=const.: ?(k)∝ k ^?5/3 for smaller k and tends to equipartition of the HD energy ?(k)∝ k ² for larger k. This bottleneck accumulation of the energy spectrum is milder than the one predicted before in (L’vov et al. in Phys. Rev. B 76:024520, 2007) based on a model with sharp HD-KW transition. For Λ=15, it results in a prediction for the effective viscosity ν ^?′?0.004κ (κ is the circulation quantum) which is in a reasonable agreement with its experimental value in ⁴He low-temperature experiment ≈0.003κ (Walmsley et al. in Phys. Rev. Lett. 99:265302, 2007). For k>2/?, the energy spectrum is dominated by the KW component: almost flux-less KW component close to the thermodynamic equilibrium, ?≈?_KW≈const at smaller k and the KW cascade spectrum ?(k)→?_KW(k)∝ k ^?7/5 at larger k.     

Degradation of tobramycin in aqueous solution

Abstract

The kinetics of degradation of tobramycin (Ne-De-Ka) in aqueous solution was studied as a function of pH. Tobramycin hydrolyzes in acidic solution to yield kanosamine (Ka-OH) and nebramine (Ne-De-OH) with a pseudo first-order rate constant of 2.7 × 10^?6 s^?1 in 1 N HCl at 80°C. The activation energy for the acid catalyzed hydrolysis is 32 kcal mol^?1. In basic solution, the hydrolysis products are deoxystreptamine (De-OH), nebramine (Ne-De-OH) and deoxystreptamine-kanosaminide (HO-De-Ka). The pseudo first-order rate constant for the hydrolysis in 1 N KOH is 1 × 10^?8 s^?1 at 80°C. The activation energy for the base catalyzed hydrolysis is 15 kcal mol^?1. Tobramycin is very stable towards hydrolysis at neutral pH; however, it rapidly oxidizes giving several products including De-OH, Ne-De-OH, and HO-De-Ka. In pH 7 phosphate buffer (0.01 M), the t₉₀ value is 70 hr at 80°C.     

Enhanced photocatalytic activity of Fe3O4-WO3-APTES for azo dye removal from aqueous solutions in the presence of visible irradiation

Development of highly active photocatalysts for treatment of dye-laden wastewaters is vital. The photocatalytic removal of azo dye Reactive Black 5 was investigated by Fe₃O₄-WO₃-3-aminopropyltriethoxysilane (APTES) nanoparticles in the presence of visible light. The Fe₃O₄-WO₃-APTES nanoparticles were synthesized via a facile coprecipitation method. The photocatalyst was characterized by XRD, FT-IR, SEM, EDX, VSM, UV–Vis, and pH_PZC techniques. The effects of some operational parameters such as solution pH, nanophotocatalyst dosage, initial RB5 concentration, H₂O₂ concentration, different purging gases, and type of organic compounds on the removal efficiency were studied by the Fe₃O₄-WO₃-APTES nanoparticles as a photocatalyst. Maximum phtocatalytic activity was obtained at pH 3. The photocatalytic removal of RB5 increased with increasing H₂O₂ concentration up to 5?mM. The removal efficiency declined in the presence of different purging gases and all types of organic compounds. First-order rate constant (k_obs) decreased from 0.027 to 0.0022?min^?1 and electrical energy per order (E_Eo) increased from 21.33 to 261.82 (kWh/m³) with increasing RB5 concentration from 10 to 100?mg/L, respectively. The efficiency of LED/Fe₃O₄-WO₃-APTES process for RB5 removal was approximately 89.9%, which was more effective than the LED/Fe₃O₄-WO₃ process (60.72%). Also, photocatalytic activity decreased after five successive cycles.     

Microwave dielectric properties and low temperature sintering of Ba3Ti4?x(Mg1/3Nb2/3)xNb4O21 ceramics with BaCu(B2O5) addition

Microwave dielectric ceramics of Ba₃Ti_4?x(Mg_1/3Nb_2/3)_xNb₄O₂₁ solid solutions (BTMNN-x, x?=?0–4) were prepared via the conventional solid-state reaction method. The X-ray powder diffraction analysis revealed that the BTMNN-x ceramics formed complete solid solutions with hexagonal structure. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) of BTMNN-x ceramics decreased with the increase of x, while the quality factor (Q?×?f) enhanced with increasing the substitution content. In addition, a small amount of BaCu(B₂O₅) (BCB) additive can effectively lower the sintering temperature of BTMNN ceramics. The 1.5?wt% BCB doped BTMNN-2 ceramics can be sintered at 950?°C and have good microwave dielectric properties of ε_r?=?50, Q?×?f?=?10,500?GHz and τ_f?=?18?ppm/°C, which makes it possible to be a promising candidate for mid-permittivity low temperature co-fired ceramic materials.     

Structural,dielectric and reflection analysis of ZnxMg1−xTiO3 ceramics synthesized using auto-ignition combustion method

Zinc-substituted magnesium titanate Zn_xMg_1?xTiO₃ (x?=?0.00–0.10, δx?=?0.025) solid solutions were synthesized using auto-ignition combustion process and characterized for structural, dielectric and reflection properties. X-ray diffraction analysis discovered the creation of single-phase MgTiO₃. Scanning electron microscopy (SEM) indicated the existence of low porosity closely packed grains. The study of the dielectric properties indicated that for Zn_xMg_1?xTiO₃ with x?=?0.05, a reasonably great combination of the relative permittivity (?_r) with value 16.60, loss tangent (tanδ) of 0.9?×?10^?3 and temperature coefficient of resonant frequency (τ_f) of ? 41.37 ppm/°C at 10 kHz was observed. The electromagnetic reflection analysis at microwave frequencies (12.4–26.5 GHz) revealed that composition x?=?0.05 shows maximum 90% reflection bandwidth of 5.60 GHz in the Ku frequency band and 4.59 GHz in the K frequency band. These materials with low to intermediate relative permittivity, low losses, and a minor negative temperature coefficient of resonant frequency might be utilized as a resonator, antenna, and filter substrate for next-generation communication devices.

     

Microstructure and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–MgO composite ceramics with nano-silica added

Ba(Mg_1/3Nb_2/3)O₃–MgO composite ceramics were prepared by solid-phase method. The ceramic exhibited the 1:2 ordered structure. By adding a proper amount of MgO, the permittivity decreased rapidly compared with other Ba(Mg_1/3Nb_2/3)O₃-based ceramics. The Q?×?f values of the samples were greatly improved by nano-silica. Raman spectra showed that the permittivity and Q?×?f value showed a strong correlation with Raman shift and full width at half maximum of the A_1g(O) phonon mode, respectively. The Raman shift of A_1g(O) was consistent with the variation trend of permittivity. And the full width at half maximum of the A_1g(O) phonon mode had a negative correlation with the Q?×?f value. The results showed that upon adding 1.5 wt% nano-silica to the ceramics, the ceramics sintered at 1550 °C for 5 h had the lowest Raman shift and the narrowest full width at half maximum, achieving the best microwave dielectric properties: ε_r?=?22.22, Q?×?f?=?80,436 GHz, τ_f?=?–?5.89 ppm/°C.

     

Microwave dielectric properties of Mg2SiO4–Ca1−ySm2y/3TiO3 composite ceramics

In this work, (1?x)Mg₂SiO₄–xCa_1?ySm_2y/3TiO₃ ceramics were fabricated using the conventional solid-state reaction method. When y?=?0.2, the effects of Ca_1?ySm_2y/3TiO₃ contents on microstructure, phase evolution, and microwave dielectric properties of the samples were studied. The X-ray diffraction showed that Mg₂SiO₄ and Ca_0.8Sm_0.4/3TiO₃ co-existed in composite ceramics. The dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) increased with the increasing content of Ca_0.8Sm_0.4/3TiO₃, while the quality factor (Q?×?f) decreased. At x?=?0.22, the (1?x) Mg₂SiO₄–xCa_0.8Sm_0.4/3TiO₃ ceramics sintered at 1325?°C for 3?h possessed optimal microwave dielectric properties: ε_r?~?11.89, Q?×?f value?~?32,703?GHz and τ_f?~?+?0.49?ppm/°C. As a result, the ε_r of (1?x) Mg₂SiO₄?+?xCa_1?ySm_2y/3TiO₃ ceramics could be adjusted in the range of 10.83 to 13.88 while the τ_f is near-zero.

     

Kinetics of Reactions of Np and Pu Ions with Hydrazine Derivatives: XVIII. Reaction between Np(V) and Hydroxyethylhydrazine

The Np(V) reduction with hydroxyethylhydrazine is described by the equation −d[Np(V)]/dt = k ₁[Np(V)][HOC₂H₄N₂H ₄ ⁺ ] + k ₂[Np(V)][Np(IV][H⁺]^1.8, reflecting its main and autocatalytic pathways. The rate constants are k ₁ = 0.31±0.04 l mol⁻¹ min⁻¹ and k ₂ = 4.04±0.11 l^2.8 mol^−2.8 min⁻¹ at 80°C and ionic strength μ = 4. The activation energies are E ₁ = 90±6 and E ₂ = 116±4 kJ mol⁻¹, respectively. The autocatalytic pathway is limited by the reaction between hydroxyethyldiazenium ions, HOC₂H₄N₂H ₂ ⁺ and protonated Np(V) ions. __________ Translated from Radiokhimiya, Vol. 47, No. 2, 2005, pp. 150–153. Original Russian Text Copyright ? 2005 by V. Koltunov, Baranov, G. Koltunov.     

Inclusion complexes of bendamustine with β-CD,HP-β-CD and Epi-β-CD: in-vitro and in-vivo evaluation

Abstract

The objective of the present work was to investigate the inclusion behavior of bendamustine (BM) with β-cyclodextrin and its hydrophilic derivatives (HP-β-CD and Epi-β-CD) for the enhancement of aqueous solubility, dissolution and bioavailability. The supramolecular binary complexes were prepared by three different methods, viz. physical mixture (PM), kneading (KND) and co-evaporation (COE). Phase-solubility study revealed the higher solubilizing and complexing ability of polymerized cyclodextrin (K_s?=?645?M^?1) than parent cyclodextrin (K_s?=?43?M^?1) and chemically derived cyclodextrin (K_s?=?100?M^?1). Meanwhile, the solubility of BM was significantly enhanced in phosphate buffer of pH 6.8, which was 24.5 folds greater compared with the phosphate buffer pH 4.5 and four times greater than aqueous medium. The dissolution efficiency was found to be highest for BM: Epi-β-CD complex (87%) compared to BM: HP-β-CD complex (84%), BM: β-CD (79%) and pure drug (20%). In-vivo pharmacokinetic study revealed that the bioavailability of BM was enhanced 2.55 times on complexation with Epi-β-CD using KND method. The t_1/2 of BM was increased from 34.2?min to approximately 75.7?min, allowing the absorption for longer time. The order of increase in solubility, dissolution and bioavailability of BM was KND?>?COE?>?PM?>?pure drug. Thus, the strategy of host–guest inclusion was very effective and could be successfully used in the development of suitable pharmaceutical dosage form with enhanced therapeutic activity.     

Reduction in sintering temperature and tunable microwave dielectric properties in NaCa4V5O17 via compositional engineering

Some Li⁺-substituted NaCa₄V₅O₁₇ ceramics were designed and fabricated by the solid-state reaction method. Effects of cation doping on the sintering behavior, crystal structure, microstructure, and dielectric properties were systematically studied. As expected, Li⁺ substitution effectively reduced the densification temperature of NaCa₄V₅O₁₇ ceramics and lower the relative permittivity (ε_r), but surprisingly increased the quality factor (Q×f). A composition with 20?mol% Li⁺ dopants exhibits the highest Q×f?=?66,000?±?124?GHz coupled with an ε_r of 10.4?±?0.1 and a τ_f of ? 81.3?±?1.4?ppm/°C. Such compositional dependence in dielectric properties was analyzed in terms of ionic polarization and packing fraction.

     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.