Dual-activator luminescence of LuAG:Mn4+/Tb3+ phosphor for optical thermometry

Mn⁴⁺ and Tb³⁺ singly doped and Mn⁴⁺/Tb³⁺ codoped lutetium aluminum garnet (Lu₃Al₅O₁₂, or simply LuAG) phosphors were synthesized and investigated for the application of optical thermometry. X-ray powder diffraction and luminescence spectroscopy measurements were performed on all samples to analyze their crystal phases and optical properties. In particular, temperature-dependent luminescence of the LuAG:Mn⁴⁺/Tb³⁺ sample was measured at the temperature range of 270–420 K. The results showed that the luminescence intensity of Mn⁴⁺ has gone through a remarkable decline while the luminescence of Tb³⁺ has an only insignificant change with the rise of temperature which leads to a dramatic decrease in the fluorescence intensity ratio (FIR) between the two activator Mn⁴⁺ and Tb³⁺. Further analysis showed that the LuAG:Mn⁴⁺/Tb³⁺ sample used for temperature sensing has a high relative sensitivity with maximum value of 4.3% K⁻¹ at 333 K. Our research indicated that this LuAG:Mn⁴⁺/Tb³⁺ material is a promising candidate for FIR-type optical temperature sensing.     

Structural,optical and temperature dependent photoluminescence properties of Cr3+-activated LaGaO3 persistent phosphor for optical thermometry

Cr³⁺ doped LaGaO₃ phosphor was prepared by hydrothermal reaction method with post-annealing treatment. XRD pattern showed the pure orthorhombic phase of LaGaO₃ at an annealing temperature of 1000 °C. TEM image showed the particles in the range 40-120 nm. The bandgap energy and Urbach tail increased in the doped sample as compared to the undoped sample as estimated from UV–visible diffuse reflectance spectra. PL excitation spectra showed peaks in UV, blue and orange regions. The emission spectra showed broadband with peaks in the NIR region due to emission from ⁴T₂ and ²E states. The intermediate strength of the crystal field has been calculated from the estimated spectroscopic parameter. The average lifetime was found to be in the ms range. Afterglow decay was also recorded. From the low-temperature PL, the zero phonon line, stokes shift energy, vibrational energy and Huang-Rhys parameter were calculated. With rising the temperature, PL emission peak intensity and lifetime values decreased and FWHM increased because of increased numbers of electrons in ⁴T₂ state and increasing non-radiative transition. Temperature-dependent peak intensity ratios and lifetime values were utilized for temperature sensing applications in below room temperature and above room temperature. The results indicate the possibility of present phosphor to be used as optical nanothermometer.     

Mode disorder in elliptical hole PCFs

In the elliptical-hole photonic crystal fibers, when the central defect air hole is large enough, the modes will be disordered. The order of the HE₁₁-like mode is investigated in detail based on the full vector supercell overlapping method. The degeneracy will be enhanced at the frequency where the modes begin to be disordered.     

Fast pressureless solid-state reaction sintering of highly infrared transparent MgAlON ceramics from MgAl2O4 and AlON powders by two-step heating

A two-step heating strategy was proposed to fabricate transparent MgAlON ceramics by solid-state reaction of MgAl₂O₄ and AlON powders via pressureless sintering. By dwelling 60 min at 1700 ℃ followed by 150 min at 1880 ℃, highly infrared transparent MgAlON ceramics with transmittance up to 80.4 % were successfully fast prepared. The phase transformation and microstructure evolution during heating from 1400 ℃ to 1800 ℃ and dwelling at 1700 ℃ for 0–90 min was thoroughly studied to reveal the solid-state reaction and densification mechanism of MgAlON by two-step heating. Surprisingly, it was found that the grown grains could break during dwelling at 1700 ℃. This secondary massive fragmentation of grown grains resulted in the minimized grain size and improved moveability of grains, which in turn prompted fast and high densification with pore free in the following sintering step. The grain breakage at 1700 ℃ could be attributed to the decomposition of AlON and formation of MgAlON.     

Luminescent properties of Li(Ga1-xCrx)5O8 (LGCO) phosphors

In this work, we investigate the synthesis of LiGa₅O₈ ceramic powders through a polyvinyl alcohol-based sol-gel technique and their optical properties when doped with high Cr³⁺ concentrations (5, 25 and 50 mol% with respect to the Ga³⁺ sites). The results indicate that the main crystalline phase, LiGa₅O₈, is obtained after calcining the samples at 1000 °C/2 h in a static air atmosphere. Via X-ray photoelectron spectroscopy, Cr is confirmed to exist in its trivalent oxidation state and the evaluation of the optical properties is performed via photoluminescence excited from visible to vacuum ultraviolet energy range and with X-ray excited optical luminescence, indicating the typical Cr³⁺ emission at the near-infrared energy range. The crystal field and Racah parameters are calculated and the influence of Cr³⁺ concentration in the host material indicates luminescence suppression/quenching and a redshift for a higher amount of these ions.     

LED-pumped intense red luminescence based on Ba2LaTaO6: Mn4+ double perovskite phosphor

Non-rare earth Mn⁴⁺ ion-doped red oxide phosphors have great potential for applications in warm white light-emitting diodes (wLEDs) due to their low cost and stable physicochemical properties. Herein, a series of Ba₂LaTaO₆ (BLTO): Mn⁴⁺ phosphors were successfully synthesized by the high-temperature solid-state method. The theoretical values of the band gap calculated by the density functional theory are close to the experimental values obtained by the absorption spectroscopy. In addition, the phosphors have a broad excitation band in the wavelength range of 280–550 nm and emit red light at the peak wavelength of 681 nm under excitation. The concentration quenching of the BLTO: Mn⁴⁺ phosphor was caused by dipole-dipole interactions. The activation energy and the average decay lifetimes of the samples were calculated. Meanwhile, the effects of synthesis temperature and Li⁺ ion doping on the luminescence performance of the samples were also investigated. Satisfactorily, the color purity and internal quantum efficiency of the phosphor reached 98.3% and 26.8%, respectively. Further, the samples were prepared as red-light components for warm wLEDs. The correlated color temperature, color rendering index, and luminous efficiency of the representative devices driven by 60 mA current were 5190 K, 83.3, and 81.59 lm/W, respectively. This work shows that the BLTO: Mn⁴⁺ red phosphor with excellent luminescence performance can be well applied to warm wLEDs.     

钒氧化物薄膜的溅射法制备及电学性质研究

目的通过真空镀膜工艺,获得电学性能优良的钒氧化物薄膜。方法采用直流反应磁控溅射法,通过改变镀膜时部分实验条件,包括沉积气氛中氧含量、衬底温度以及衬底材质,进而控制薄膜组成成分。利用椭偏仪、XRD和变温四探针仪检测所镀钒氧化物薄膜的厚度、晶体结构及电学性能。结果衬底温度在350~400℃之间,镀膜室氧气含量在40%~50%之间时,反应溅射所得的钒氧化物薄膜具有较高的温度敏感性,此外得益于(0001)面的α-Al_2O_3与VO_2有相近的晶格参数,在此衬底上沉积出的钒氧化物薄膜电阻在30~100℃区间内有一个数量级的线性变化。结论确定了最佳的钒氧化物薄膜镀制条件范围,同时发现蓝宝石衬底相比于硅片和玻璃片更适合外延生长VO_2薄膜。按此种方法所得样品具有优良的电学性能,可以应用到热敏电阻等研究领域当中。     

Characterisation of the system Li–Na0.90Fe0.90Ti1.10O4 by electrochemical techniques

The electrochemical insertion of lithium has been studied in order to determine, from the potential–composition curves, the domains of single-phase (or solid solution) and two-phase regions in the Li–Na_0.90Fe_0.90Ti_1.10O₄ (Li–NFTO) phase diagram. For this, the step potential electrochemical spectroscopy technique was applied to analyse the Li–NFTO system over a potential range from 3.1 to 1 V vs. lithium. In this voltage range 0.9 lithium ions/formula can be intercalated. Between 0≤x≤0.38, where the variable x is referred as x in Li_xNFTO, a solid solution is obtained. This process takes place in the potential range from 3 to 1.7 V. On the other hand, for x values greater than 0.38 a two-phase domain is developed, which finishes with the complete transformation of Li_0.38NFTO to a compound of formal composition Li_0.90NFTO. This second reduction falls into a lower potential range (1.7–1.3 V).     

Self-tuning of PID-type fuzzy logic controller coefficients via relative rate observer

In this study, a new method is proposed for tuning the coefficients of PID-type fuzzy logic controllers (FLCs). The new method adjusts the input scaling factor corresponding to the derivative coefficient and the output scaling factor corresponding to the integral coefficient of the PID-type FLC using a fuzzy inference mechanism in an on-line manner. The fuzzy inference mechanism that adjusts the related coefficients has two inputs, one of which is called “normalized acceleration” and the other one is the classical “error”. The “normalized acceleration” gives the “relative rate” information about the fastness or slowness of the system response. An appropriate rule-base is generated for the adaptation of the derivative coefficient of the PID-type FLC using these two input variables. The integral coefficient is then updated as the reciprocal of the derivative coefficient. The robustness and effectiveness of the new self-tuning algorithm have been compared with the other related tuning methods proposed in the literature through simulations. The simulations are done on a second-order system with varying parameters and time delay.     

The influence of the transformation of electronic structure and micro-structure on improving the thermoelectric properties of zinc antimonide thin films

Measurements of electronic structure, microstructure and thermoelectric properties of zinc antimonide thin films prepared by direct current magnetron co-sputtering were carried out. The as-deposited zinc antimonide thin film had a very high resistivity similar to insulating ceramics, which was due to a low binding energy of both zinc and antimony, with the electron scattering increases and impedes the current transport. With the increase in annealing temperature, the films became more crystalline and the thermoelectric properties were also improved. The resistivity of the film decreased rapidly with its crystallinity when the annealing temperature was above 350 °C. The Seebeck coefficients of the thin films were positive, indicating that the films were P-type. The Seebeck coefficient of those samples increased with increasing annealing temperature. The thin film annealed at 400 °C has an optimal power factor of 1.87 × 10⁻³ Wm⁻¹ K⁻² with a Seebeck coefficient of 300 μVK⁻¹ and a resistivity of 4.82 × 10⁻⁵ Ωm at 573 K.