Temperature stability,low loss and defect relaxation of MgO-TiO2 microwave dielectric ceramics modified by Ca0.8Sr0.2TiO3

Temperature-stable and low-loss microwave dielectrics based on the MgO-TiO₂ system with nominal formation Mg_n+1Ti_nO_3n+1 (n = 5, MT) were prepared via the conventional solid-state reaction method. Ca_0.8Sr_0.2TiO₃ (CST) was chosen as a τ_f compensator for matrix MT to form the composite ceramics (1-x)Mg₆Ti₅O₁₆-xCa_0.8Sr_0.2TiO₃ (0.10 ≤ x ≤ 0.26, MT-CST). The effects of CST additions on the phase composition, defect relaxation behavior, and microwave dielectric properties of MT were investigated. It revealed that undoped MT was basically consisted of MgTiO₃ as a major phase and Mg₂TiO₄ as a minor phase, and such two phases coexisted well with CST additions. Interestingly, τ_f could be tuned close to zero (?1.28 ppm/°C) for the MT-CST ceramics at x = 0.22, accompanied with a high Q×f value ~ 74,200 GHz and a proper ε_r ~ 20.25 (9.90 GHz). These materials possessed a good potential for applications in microwave components and devices. Meanwhile, significant relaxation phenomena were observed in all the MT-CST samples using dielectric spectroscopy and thermally stimulated depolarization current (TSDC) techniques. The oxygen-vacancy-related defects, shown as ($T{i}_{Ti}^{\prime }$)-($V_O^{??}$) dipoles and $V_O^{??}$, were the main types of defects in MT-CST, which was responsible for the relaxation behavior; meanwhile, the defect concentrations increased with the increase of CST content, thus resulting in the increase of dielectric loss at low and high frequencies.     

Giant negative electrocaloric effect and energy storage response in 0.94(K0.5Na0.5)NbO3-0.06SrMnO3 nanocrystalline ceramics

We report the electrocaloric (EC) effect investigation on lead-free 0.94(K_0.5Na_0.5)NbO₃-0.06SrMnO₃ nanocrystalline ceramics by an indirect thermodynamic method using Maxwell's relations. The maximum value of the negative EC effect ($\mathrm{\Delta }{\mathrm{T}}_{\mathit{max}}=?1.66\mathrm{K}$) was observed at 459?K near the transition temperature at the field of 50?kV/cm. The corresponding EC responsivity was calculated to be $?3.32\times {10}^{?7}\mathrm{K}\mathrm{m}/\mathrm{V}$ under 50?kV/cm at 459?K whereas the coefficient of performance (COP) and recoverable energy density (${\mathrm{W}}_{\mathit{rec}}$) were found to be 1.03 and $0.17\mathrm{J}/{\mathrm{cm}}^3$, respectively under 50?kV/cm at 443?K. The observed values of negative EC effect, and EC responsivity are larger than any other lead-free ferroelectric ceramics with good COP and ${\mathrm{W}}_{\mathit{rec}}$ value. The results are interesting to improve the cooling efficiency and energy storage for device application.     

Sintering temperature effect on microstructure,electrical properties and temperature stability of MnO-modified KNN-based ceramics

Lead-free 0.955K_0.5Na_0.5NbO₃-0.045Bi_0.5Na_0.5ZrO₃?+?0.6%MnO (KNN-0.045BNZ?+?Mn0.6) ceramics have been prepared by a conventional solid-state sintering method in air. All the samples sintered at different temperatures possess a coexisting phase boundary (CPB) between rhombohedral (R) phase and tetragonal (T) phase. The increase of sintering temperature (T_s) increases the phase fraction of T phase in CPB region. Mn²⁺, Mn³⁺ and Mn⁴⁺ ions coexist in all the KNN-0.045BNZ?+?Mn0.6 ceramics sintered at 1110?°C to 1190?°C. High sintering temperature can induce a transformation from ${\mathrm{M}\mathrm{n}}_{\mathrm{N}\mathrm{b}}^{\text{'}\text{'}}$ defects to ${\mathrm{M}\mathrm{n}}_{\mathrm{N}\mathrm{b}}^{\text{'}}$ defects. The samples with fine grain show stable octahedral structure. The KNN-0.045BNZ?+?Mn0.6 ceramics with fine grain possess excellent temperature stability of $d_{33}^{*}$ due to the wide phase transition region. The increase of sintering temperature induces the (R-T) phase transition temperature to move to room temperature.     

Dielectric,ferroelectric and piezoelectric properties of (1-x)(Bi0.5Na0.5)0.935Ba0.065Ti -x(LiSbO3) solid solutions

Compositions in the solid solution series (1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065Ti–x(LiSbO₃), (BNBT-LS) (x = 0, 0.01, 0.02, 0.03, 0.04) have been fabricated via solid state reaction route. Room temperature x-ray diffraction traces of all samples were found to reveal coexistence of the two structures; rhombohedral and tetragonal. The micrographs recorded from the Field Emission Scanning Electron Microscope, provided an overall dense and single phase microstructure for the systems. The dielectric anomalies corresponding to the maximum relative dielectric constant, ε_r, persistently broadened and shifted to low temperatures as a function of increase in LiSbO₃ (LS) content. The ferroelectric $\mathrm{P}?\mathrm{E}$ loops became steadily narrow with similar LS modification showing a continual decline in remnant polarization $({\mathrm{P}}_{\mathrm{r}})$ and coercive field $({\mathrm{E}}_{\mathrm{c}})$. These phenomena clearly indicated the onset of a growing relaxor-like behavior. The maximum normalized unipolar strain $\left({\mathrm{S}}_{\max }/{\mathrm{E}}_{\max }={\mathrm{d}}_{33}^{*}=400\mathrm{pm}/\mathrm{V}\right)$ under the field 50 kV/cm was recorded for the system with x = 0.02.     

Gd3+ doping induced enhanced upconversion luminescence in Er3+/Yb3+ co-doped transparent oxyfluoride glass ceramics containing NaYF4 nanocrystals

In this article, transparent oxyfluoride glass ceramics containing $\mathrm{\beta }$-NaYF₄ nanocrystals were successfully prepared via Gd³⁺ doping. Compared to conventional non-doped glasses, the thermal treatment temperature required for the precipitation of $\mathrm{\beta }$-NaYF₄ nanocrystals can be lowered with the doping of Gd³⁺. Furthermore, under the same thermal treatment condition, more $\mathrm{\beta }$-NaYF₄ nanocrystals were precipitated in Gd³⁺ doped ones, which greatly improves the luminescence efficiency of rare earth doped glass ceramics. Possible mechanism for the Gd³⁺ doping induced enhanced upconversion luminescence phenomenon was proposed, based on thorough structural and optical characterizations. The results revealed that the doping of Gd³⁺ ions could decrease the crystallization activation energy and promote the formation of Y-F-Na bonding, which helps the precipitation of $\mathrm{\beta }$-NaYF₄ nanocrystals. Consequently, a large enhancement in upconversion luminescence was achieved. Moreover, the strategy can be successfully applied to the development of other glass ceramic systems for various optoelectronic applications.     

Supersonic atmospheric plasma sprayed ZrO2 and ZrB2-SiC/ZrO2 coatings on Cf/Mg composites for anticorrosion

To improve the corrosion resistance of the carbon fiber reinforced magnesium matrix composites (C_f/Mg composites), ZrO₂ and ZrB₂-SiC/ZrO₂ composite coatings were prepared by supersonic atmospheric plasma spraying (SAPS) on C_f/Mg composites. The microstructure and phase composition of the coatings before and after the corrosion test were investigated. Open circuit potential and potentiodynamic polarization tests were measured at room temperature. Results revealed that the corrosion current density ($i_{\mathit{corr}}$) of the ZrO₂ coated C_f/Mg composites decreased by one order while the ZrB_2-SiC/ZrO₂ coated C_f/Mg composites reduced by two orders. Compared with C_f/Mg composites, the corrosion potential ($E_{\mathit{corr}}$) of the ZrO₂ and ZrB₂-SiC/ZrO₂ coated C_f/Mg composites increased by 220.5?mV and 1021.8?mV respectively, indicating that the ZrB₂-SiC/ZrO₂ composite coatings greatly improve the corrosion resistance of C_f/Mg composites. The uniform distribution of the SiC particles with small grain size in ZrB₂ is responsible for the densification of the coating. The ZrB₂-SiC/ZrO₂ composite coatings provide a barrier for the substrate to impede the entry of Cl^- in the corrosion solution, thus exhibiting a better corrosion resistance than the ZrO₂ coating.     

Critical influence of different diamagnetic ions on electromagnetic properties of BaFe12O19

The correlation between the electromagnetic properties of BaFe_12-xD_xO₁₉ (0.1?x?1.2) solid solutions (D = Al³⁺, In³⁺ and Ga³⁺) and the concentration of diamagnetic ions was investigated. The changes of electromagnetic properties of the investigated samples were explained using neutron powder diffraction and Mossbauer spectroscopy data. The diamagnetic ions with different ionic radii and electronic configurations were chosen. The transmission spectra of all the samples demonstrated a deep minimum in the frequency range 20–65?GHz which was associated with the natural ferromagnetic resonance. The resonance frequency $f_{\mathit{res}}$ and amplitude were changed by the substitution type and level. With increasing the substitution concentration, the resonance frequency increases from 51?GHz to 61?GHz and decreases from 50.5?GHz to 27?GHz for Al- and In-substituted samples, respectively. More complicated concentration dependences of resonance characteristics were obtained for Ga-substituted samples in a narrow range of 49–50.5?GHz. The value of $f_{\mathit{res}}$ had a minimum around x?=?0.6 and further increased to 50.5?GHz for x?=?1.2. It was concluded that the intrasublattice interactions were responsible for tailoring the magneto crystalline anisotropy and resonance parameters. More detailed investigations would require the development of phenomenological model on the basis of the electronic structure using Goodenough-Kanamori approaches.     

Comparative study on dielectric behavior in fresh and aged Na0.5Bi0.5(Ti,Fe,W)O3 thin films

Na_0.5Bi_0.5(Ti_0.90Fe_0.02W_0.08)O₃ (NBTFW) thin film was deposited on the ITO/glass substrate by chemical solution deposition. Microstructure, insulating and dielectric performances were measured and compared in fresh and aged samples. Both samples crystallize into phase-pure perovskite structures and possess smooth surfaces without any cracks. Lower leakage current density and relative dielectric constant can be obtained in aged NBTFW film compared to those for fresh one. At low voltage or high frequency, the fresh film exhibits a typical butterfly pattern of normalized dielectric constant–electric field curve, while the aged film shows an abnormal characteristic with two normal double–peaks shape. The related aging mechanism is discussed to be associated with the realignment of ${\mathrm{V}}_{\mathrm{O}}^{\cdot \cdot }-{\mathrm{Fe}}_{\mathrm{Ti}}^{\prime }$ defect dipoles along the spontaneous polarization via the migration of mobile oxygen vacancies.     

Effect of Ti3AlC2 precursor on the electrochemical properties of the resulting MXene Ti3C2 for Li-ion batteries

The presented work compared the etching behavior between combustion synthesized Ti₃AlC₂ (SHS-Ti₃AlC₂) and pressureless synthesized Ti₃AlC₂ (PLS-Ti₃AlC₂). Because the former had a more compact structure, it was harder to be etched than PLS-Ti₃AlC₂ under the same conditions. When served as anode material for Li-ion batteries, SHS-Ti₃C₂ showed much lower capacity than PLS-Ti₃C₂ at 1?C (52.7 and 87.4?mAh?g^?1, respectively) due to the smaller d-spacing. Furthermore, Potentiostatic Intermittent Titration Technique (PITT) was used to determine the Li-ion chemical diffusion coefficient (${\mathrm{D}}_{{\mathrm{Li}}^{+}}$) of Ti₃C₂ in the range of 10^?10 ??10^?9 cm² s^?1, indicating that Ti₃C₂ could exhibit an excellent diffusion mobility for Li-ion.     

Structural and spectroscopic properties of Yb3+ doped borophosphate glasses for IR laser application

Ytterbium (Yb³⁺) doped borophosphate (BPYb) glasses have been prepared and studied by means of their physical, optical, structural and spectroscopic properties aiming laser performance parameters. The main structural groups, ${\mathrm{PO}}_3^{2?}$, ${\mathrm{P}}_2{\mathrm{O}}_7^{4?}$ and BO₄ of the borophosphates were analyzed through their FTIR-ATR and Raman spectra. A linear increment of the absorption coefficient and of the integrated emission intensity were observed with increase of Yb³⁺ ions. The absorption and emission cross-sections have been evaluated and reported. Decay lifetimes were found to decrease with the increase of Yb³⁺ ions. The evaluated laser performance parameters (I_sat and I_min), figure of merit and cross-sections (absorption and emission) suggest that 0.5?mol% of Yb³⁺ doped borophosphate glass is the most appropriate candidate for solid state infrared (IR) laser applications.