Transparent sol-gel glass ceramics containing β-NaYF4:Yb3+/Er3+ nanocrystals: Structure,upconversion luminescent properties and optical thermometry behavior

Transparent bulk glass ceramics (GCs) containing $\beta ?\mathit{NaY}{\mathrm{F}}_4:{\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ upconversion nanocrystals were successfully prepared via a new sol-gel route for the first time. The structure, composition and morphology of the as-fabricated glass ceramics are characterized by X-ray diffraction ($\mathit{XRD}$), scanning electron microscopy ($\mathit{SEM}$) and transmission electron microscopy ($\mathit{TEM}$), which confirm the segregation of β-NaYF₄ nanocrystals in silica glass matrix with the maintenance of their crystalline phase and microstructure. More significantly, intense upconversion (UC) emissions can be realized for ${\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ co-doped glass ceramics by profiting from low-phonon-energy environment of erbium ions in β-NaYF₄ nanophase. Furthermore, temperature-dependent UC emission performance of the present GC is systematacially investigated to explore their potential application in optical thermometry. Obviously, owing to intense UC emissions of $\beta ?\mathit{NaY}{\mathrm{F}}_4:{\mathit{Yb}}^{3+}/{\mathit{Er}}^{3+}$ nanocrystals and high transparency, superior chemical/mechanical stability of oxide glassy matrix, the as-fabricated GCs exhibit good temperature sensing performance and good thermostability for precise temperature detecting. It is expected that the preliminary research can give a reference for designing new transparent bulk GCs and may exploit a valid method for developing high-performance optical temperature sensors.     

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.     

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.     

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.     

Mn-stabilized zirconia ceramics: Phase transformation and mixed ionic-electronic conductivity

Cubic-phased Mn-stabilized zirconia (${\mathrm{Zr}}_{1?\mathrm{x}}{\mathrm{Mn}}_{\mathrm{x}}{\mathrm{O}}_{2?\delta }$) was fabricated by a co-precipitation calcination method and sintering at $1400°\mathrm{C}$ in Ar. Within the solid solubility of Mn in the ZrO₂ matrix, substitution solid solutions were gained without any other second phase. Bivalency Mn ions play an important part in the grain growth, densification, and phase stabilization mechanism. The intragrain, intergrain, and total conductivities were measured by AC impedance and four-probe DC methods. Electrical conductivity of Mn-stabilized ZrO₂ ceramics increased with both increasing temperature and Mn concentration. ${\mathrm{Zr}}_{0.8}{\mathrm{Mn}}_{0.2}{\mathrm{O}}_{2?\delta }$ ceramic with the cubic structure possessed the highest conductivity of 0.2 and 4.15?S/m at $600°\mathrm{C}$ and $1000°\mathrm{C}$, respectively. The activation energy decreased with increasing the Mn concentration, which was most likely due to the intergrain electronic conductivity. The single-cubic-phased structure, high electrical properties and low activation energy render this system potentially useful as electrode or electrolyte materials.     

Friction coefficient and mass of silver agglomerates in the transition regime

The dynamic shape factor and the exponents, $\eta$ and $D_{\mathit{fm}}$, which characterize the power law dependence of friction coefficient on the number of primary spheres and the mass on the mobility diameter, have been determined for silver agglomerates using the differential mobility-aerosol particle mass (DMA-APM) analyzer method. This method provides characterization of nearly monodisperse agglomerates and is able to analyze thousands of particles over a 10 min period. A quantitative uncertainty analysis finds that the calibration of the APM is the major source of uncertainty and that the combined uncertainties are about 6–7% for the dynamic shape factor and about 3% for the exponents $\eta$ and $D_{\mathit{fm}}$. The dynamic shape factor obtained based on free molecular analysis is larger than the measured results. The observed decrease in $\eta$ by about 15% with increasing agglomerate size compared to almost constant values for the model predictions suggests a flow interaction between the primary particles not included in the models which are based on free molecular dynamics. An empirical equation is given for the N dependence of the ratio of the measured friction coefficient to a free molecular expression based on a computer simulation. Model predictions indicate that $\eta$ is independent of agglomerate size while $D_{\mathit{fm}}$ is sensitive to agglomerate size. Experimentally, it appears the opposite is true: the dependence of $\eta$ on particle size is greater than for $D_{\mathit{fm}}$. The near constancy for the measured $D_{\mathit{fm}}$ results from the decreasing value in $\eta$ being compensated by the slip correction term in the expression relating $d_m$ to the friction coefficient.     

Effect of oriented defect-dipoles on the ferroelectric and piezoelectric properties of CuO-doped (K0.48Na0.52)0.96Li0.04Nb0.805Ta0.075Sb0.12O3 ceramics

The orientation characteristics of the defect dipoles formed by acceptor ions (${\mathrm{Cu}}_{\mathrm{Nb}}^{″\prime }$) and oxygen vacancies ($V_{\mathrm{O}}^{??}$) in CuO-doped (K_0.48Na_0.52)_0.96Li_0.04Nb_0.805Ta_0.075Sb_0.12O₃ piezoceramics were investigated. The ferroelectric and piezoelectric properties of the ceramics were obviously affected by the defect dipoles which are oriented along with the domains when poled under a dc electrical field of 3.5?kV/mm at the temperature near T_C. The poled ceramics with 1.0–3.0?mol% CuO displayed strong asymmetric P-E loops and a large internal bias field (E_i), 3.89–4.57?kV/cm, when polarized at the temperature near T_C. The enhanced piezoelectric coefficient d₃₃, 186–192?pC/N, was obtained for the ceramics poled near T_C when 0.02?≤?x?≤?0.03. The ceramics with oriented defect dipoles showed a relatively good thermal stability of d₃₃ until 180?°C.