High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates(REjTaO7 and REjNbO7)have been considered as promising candidate thermal barrier coating(TBC)materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia(YSZ).However,the low Vickers hardness and toughness are the main shortcomings of RE;TaO-and REjNbOr that limit their applications as TBC materials.To increase the hardness,high entropy(Yu3Ybu3Er/3)sTaOr,(Y13YbnErns)NbO,and(Sm1/6Eu1/6Y 1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7 are designed and synthesized in this study.These high entropy ceramics exhibit high Vickers hardness(10.912.0 GPa),close thermal expansion coefficients to that of single-principal-component RE3TaO,and RE;NbO,(7.9×10^-6-10.8×10-6 C-1 at room temperature),good phase stability,and good chemical compatibility with thermally grown Al2O3,which make them promising for applications as candidate TBC materials.

High Dielectric Constant and Low-loss Dielectric Ceramics of Sr_5 LnTi_3Ta_7O_(30)(Ln=La,Nd,Sm and Y)

Polycrystalline Sr5LnTi3Ta7O30 （Ln=La, Nd, Sm and Y） ceramics were prepared as single-phase materials through conventional solid-state ceramics method. The structure was characterized by X-ray diffraction method and scanning electron microscopy （SEM）. The dielectric properties were measured from room temperature to 400℃. All compounds are paraelectric phases adopting the filled tetragonal tungsten bronze （TB） structure at room temperature. At 1 MHz their dielectric constant （ετ） varied from 109 to 139, dielectric loss changed from 0.003 3 to 0.005 8, and the temperature coefficients of the dielectric constant （τε） moved from -710 to -880×10^-6℃^-1.

Dielectric Properties and the Phase Transition of Pure and Cerium Doped Calcium-Barium-Niobate

The complex dielectric constant of pure and cerium doped calcium-barium-niobate (CBN) was studied at frequencies 20 Hz ≤ f ≤ 1 MHz in the temperature range 300 K ≤ T ≤ 650 K and compared with the results for the well known ferroelectric relaxor strontium-barium-niobate (SBN). By the analysis of the systematically taken temperature and frequency dependent measurements of the dielectric constant the phase transition characteristic of the investigated materials was evaluated. From the results it must be assumed that CBN shows a slightly diffuse phase transition without relaxor behavior. Doping with cerium yields a definitely different phase transition characteristic with some indications for a relaxor type ferroelectric material, which are common from SBN.

Morphology-controlled synthesis and photocatalytic properties of K_(1.9)Na_(0.1)Ta_2O_6·2H_2O

The controllable synthesis of tantalate K1.9Na0.1Ta2O6·2H2O has been successfully achieved by a two-step technique,namely,the molten salt and hydrothermal methods,at a low temperature. By simply varying the KOH concentration in the hydrothermal process,K1.9Na0.1Ta2O6·2H2O particles with spherical,cuboctahedral,and durian-like morphologies were synthesized. The photocatalytic activity of the obtained samples for the degradation of rhodamine B was studied under ultraviolet light,which indicates that the photocatalytic properties of the samples are highly dependent on their morphologies. The K1.9Na0.1Ta2O6·2H2O nanospheres,with rough surfaces and the highest specific surface area,exhibit the best performance. The present work provides a unique approach for the controlled synthesis of tantalate photocatalysts,which are difficult to achieve through other synthetic approaches.

The Luminescence Properties of M' type of YTaO_4∶Eu

A series of Eu 3+ doped of Mtype of yttrium orthotantalate Y 1-x Eu x TaO 4( x =0-0.30) was sythesized. The influence of activator concentration on luminescence properties was studied in detail. The strong evidence from excitation spectra, emission spectra and diffusive reflection spectra of this system shows that there is energy transfer from TaO 3- 4 to Eu 3+ via the charge transfer and spectral overlap.

Growth Defects in Cubic KTa_(1-x)Nb_xO_3 Crystal

Potassium tantalate niobate （KTa1-xNbxO3, KTN） crystals with different dimensions and quality situations were grown by Czochralski method. Crystal growth process and morphology properties of KTN are presented in this paper. It was found that some defects, such as bubble, inclusion, crack, dislocation etc., can all appear if the crystal is grown in an improper condition. The character and formation mechanism of such defects in macro growth are discussed. We consider that the CO2, which was not released absolutely during the sintering process and dissolved in the melt, led to bubbles. The composition of the inclusion caused by high pulling and rotation rates is KTN polycrystalline. The crack and dislocation in KTN crystal mainly come from improper temperature field. Etching and high-resolution X-ray diffraction （HRXRD） experiment results indicate that the central area is the defects concentrated.

STUDY ON THE TANTALIZING ON THE SURFACE OF TITANIUM ALLOY BY NET-SHAPE CATHODE GLOW DISCHARGING

A new net-shape cathode sputtering target which has a simple structure and a hig h sputtering was put forward. The multiple-structure made of alloying and coatin g layers of tantalum was achieved on the surface of TC4 (Ti6Al4V) using this met hod in double glow surface alloying process. The tantalized samples were investi gated by SEM, XRD and electrochemical corrosion method .Results show the complic ated tissue of pure tantalizing layer and diffusion layer was successfully forme d on the surface of TC4 with the method of net-shape cathode glow discharge, whi ch further improved the corrosion-resistance of TC4 and formed good corrosion-re sistant alloys.

Ferroelectric solid solution Li1-xTa1-xWxO3 as potential photocatalysts in microbial fuel cells:Effect of the W content

Microbial fuel cells(MFCs)are bio-electrochemical systems that can directly convert the chemical energy contained in an effluent into bioelectricity by the action of microorganisms.The performance of these devices is heavily impacted by the choice of the material that forms the cathode.This work focuses on the assessment of ferroelectric and photocatalytic materials as a new class of non-precious catalysts for MFC cathode construction.A series of cathodes based on mixed oxide solid solution of LiTaO_3with WO_3formulated as Li_(1-x)Ta_(1-x)W_xO_3(x=0,0.10,0.20 and0.25),were prepared and investigated in MFCs.The catalyst phases were synthesized,identified and characterized by DRX,PSD,MET and UV–Vis absorption spectroscopy.The cathodes were tested as photoelectrocatalysts in the presence and in the absence of visible light in devices fed with industrial wastewater.The results revealed that the catalytic activity of the cathodes strongly depends on the ratio of substitution of W^(6+)in the LiTaO_3matrix.The maximum power densities generated by the MFC working with this series of cathodes increased from60.45 mW·m^(-3)for x=0.00(LiTaO_3)to 107.2 mW·m^(-3)for x=0.10,showing that insertion of W^(6+)in the tantalate matrix can improve the photocatalytic activity of this material.Moreover,MFCs operating under optimal conditions were capable of reducing the load of chemical oxygen demand by 79%(COD_(initial)=1030 mg·L^(-1)).

Pressure-and temperature-dependent luminescence from Tm^(3+) ions doped in GdYTaO_(4)

Luminescent properties of Tm^(3+)-doped GdYTaO_(4) are studied for exploring their potential applications in temperature and pressure sensing.Two main emission peaks from ^(3)H_(4)→^(3)H_(6) transition of Tm^(3+)are investigated.Intensity ratio between the two peaks evolves exponentially with temperature and has a highest sensitivity of 0.014 K^(−1) at 32 K.The energy difference between the two peaks increases linearly with pressure increasing at a rate of 0.38 meV/GPa.Intensity ratio between the two peaks and their emission lifetimes are also analyzed for discussing the pressure-induced variation of the sample structure.Moreover,Raman spectra recorded under high pressures indicate an isostructural phase transition of GdYTaO_(4) occurring at 4.46 GPa.

Mapping the Compositions of Zinc Tantalate for Optimum Photocatalytic Performance in Degradation of Organic Pollutants

This work aims at mapping the compositions of zinc tantalate for optimum photocatalytic performance in degradation of organic pollutants. Three zinc tantalates, low-temperature form ZnTa2O6 （LT-ZnTa2O6）, high-temperature form ZnTa2O6 （HT-ZnTa2O6）, and Zn3Ta2O8 were prepared by solid state method. Photocatalytic activities of these zinc tantalates were tested for the degradation of methyl orange under UV irradiation and compared with Sr2Ta2O7, an efficient catalyst previously reported. It is found that the photocatalytic activity of these tantalates follows such a sequence： LT-ZnTa2O6 〉 Sr2Ta2O7 〉 HT-ZnTa2O6 〉 Zn3Ta2O8, in which LT-ZnTa2O6 shows an optimum activity at least twice higher than Sr2Ta2O7. This photocatalytic performance was revealed to primarily originate from the formation of ·OH radicals as indicated by photo- luminescence measurements. The synergistic effects of chemical compositions, crystal structure, and band structure on photocatalytic performances were discussed.

Growth and Raman Study of Phase Transitions of the KTa0.63Nb0.37O3 Crystal

The potassium tantalate niobate KTa_（0.63）Nb_（0.37）O_3 single crystal with large size and good quality was synthesized by Czochralski method.The crystal composition and structure were determined by electric probe microanalysis（EPMA） and X-ray diffraction（XRD） technique,respectively.The sequence of phase transitions of the as-grown crystal was investigated by Raman scattering technique.What features in the phonon spectrum corresponding to each phase transition,namely cubic to tetragonal（C-T）,tetragonal to orthorhombic（T-R） and orthorhombic to rhombohedral（O-R）,is discussed.Following the features the C-T phase transition point measured is in the vicinity of –40 ℃,and the difference from theoretical value was discussed;T-O phase transition point is at around –130 ℃,while the O-R phase transition point was not obtained due to the band overlapping and experiment limit.

Development of a novel Eu^(3+)-doped tantalate red-emitting phosphor for w-LEDs application

Two novel phosphors LiBa_(4(1-x))Eu_(4x)Ta_(3)O_(12)(H-LBTO:xEu^(3+)) and Li_(0.25)Ba_(1-x)Eu_(x)Ta_(0.75)O_(3)(C-LBTO:xEu^(3+))were prepared successfully by a molten salt method.The transformation between these two structures was realized by changing the sintering temperature or changing the Eu^(3+) ions concentration,which was also demonstrated by the X-ray diffraction(XRD),scanning electron microscopy(SEM),diffuse reflectance spectra(DRS),and photoluminescence excitation(PLE) analyses,Both the sintering temperature and the Eu^(3+)ions doping concentration have significant impact on the formation of the crystal phase.All these phosphors sintered at 1023 K exhibit two major luminescence lines at 594 and 614 nm under nearUV light of 395 nm excitation,corresponding to Eu^(3+)ions typical transitions of ^(5)D_(0)→^(7)F_(1) and ^(5)D_(0)→^(7)F_(2).The optimum concentration of Eu^(3+) ions is 9 mol% for C-LBTO:xEu^(3+) samples and the quenching interaction type is the nearest-neighbor ion interaction.The thermal stability of the C-LBTO:0.09Eu^(3+)sample was investigated in detail and the device application further suggests that C-LBTO:0.09Eu^(3+) can be used as a red phosphor for near-UV excited w-LEDs in lighting.

Orange-red emitting Sr_3LaTa_3O_(12):Sm^(3+) phosphors with perovskite structure and high thermal stability for w-LEDs

A novel layered perovskite tantalate phosphor Sr_(3)LaTa_(3)O_(12):Sm^(3+)(SLTO:Sm^(3+)) with orange-red emission was obtained for the first time via the solid-phase synthetic method. The phase purity, surface morphology,element distribution and luminescent properties of the SLTO:xSm^(3+)(x= 0.01 mol%-0.30 mol%) phosphors were investigated. Under 408 nm excitation, the optimum doped SLTO:0.10Sm^(3+) phosphor emits orangered light at 598 nm with the highest emission peak(4G_(5/2)→6H_(7/2)). The critical energy transfer distance is 1.687 nm. The prepared SLTO:0.10Sm^(3+)phosphor has excellent thermal stability with temperature quenching temperature(T_(0.5)> 500 K) and high activation energy(E_(a)= 0.25 e V). Through calculation, the chromaticity coordinates of all samples are concentrated in the orange-red area, and the color purity reaches 99%. The fabricated white light-emitting diode(w-LED) has a good correlated color temperature(5132 K), a high R_(a)(89), and the CIE chromaticity coordinates(0.340, 0.327). Consequently, the superiority of orange-red-emitting tantalate SLTO:Sm^(3+) phosphors for w-LEDs is demonstrated.

Development of red phosphor Li_(8)CaLa_(2)Ta_(2)O_(13):Eu^(3+)for WLEDs and anti-counterfeiting applications

A set of novel red phosphors Li_(8)CaLa_(2)Ta_(2)O_(13):Eu^(3+)(LCLTO:xEu^(3+))were successfully prepared using a solid-state reaction method.The properties of the prepared samples,including phase purity,elemental composition,and morphology,were systematically investigated using X-ray diffraction,scanning electron microscopy,and diffuse reflectance spectroscopy analyses.The 610 nm maximum emission peak is attributed to the^(5)D_(0)→^(7)F_(2)transition of Eu^(3+)ion under 394 nm irradiation.Among all the LCLTO:xEu^(3+)phosphors,LCLTO:0.6Eu^(3+)showed the strongest emission intensity because of the concentration quenching effect of the electric dipole-dipole in-teraction among the Eu^(3+)ions,which was also demonstrated by the decay curves.Remarkably,the emission intensity of the optimal LCLTO:0.6Eu^(3+)phosphor,which exhibited a high internal quantum efficiency of 49.30%and excellent color purity of 96.79%,was approximately 2.29 times higher than that of commercial Y203:Eu^(3+)red phosphors.The thermal stability of the LCLTO:0.6Eu^(3+)sample with good color stability was meticulously inves-tigated.The fabricated white-light-emitting diode(WLED)exhibited a superior color-rendering index of Ra=82 and chromaticity coordinates of(0.3260,0.3639),suggesting that LCLTO:0.6Eu^(3+)has potential applicability in developing efficient and high-quality WLEDs.Moreover,the prepared LCLTO:0.6Eu^(3+)/PDMS composite film demonstrated exceptional flexural resistance and chemical stability,indicating considerable promise for practical anti-counterfeitingapplications.

High-entropy perovskite RETa_(3)O_(9) ceramics for high-temperature environmental/thermal barrier coatings

Four high-entropy perovskite(HEP)RETa_(3)O_(9) samples were fabricated via a spark plasma sintering(SPS)method,and the corresponding thermophysical properties and underlying mechanisms were investigated for environmental/thermal barrier coating(E/TBC)applications.The prepared samples maintained low thermal conductivity(1.50 W·m^(−1)·K^(−1)),high hardness(10 GPa),and an appropriate Young’s modulus(180 GPa),while the fracture toughness increased to 2.5 MPa·m^(1/2).Nanoindentation results showed the HEP ceramics had excellent mechanical properties and good component homogeneity.We analysed the influence of different parameters(the disorder parameters of the electronegativity,ionic radius,and atomic mass,as well as the tolerance factor)of A-site atoms on the thermal conductivity.Enhanced thermal expansion coefficients,combined with a high melting point and extraordinary phase stability,expanded the applications of the HEP RETa_(3)O_(9).The results of this study had motivated a follow-up study on tantalate high-entropy ceramics with desirable properties.

Mechanical and thermal properties of RETaO4(RE=Yb,Lu,Sc)ceramics with monoclinic-prime phase

As candidate thermal/environmental barrier coatings(T/EBCs),the structure characteristics and comprehensive properties of monoclinic-prime(m')RETaO4(RE=Yb,Lu,Sc)with excellent Al2O3/SiO2 chemical compatibility are studied.Excellent thermal insulation protection will be provided by m'RETaO4 due to their low thermal conductivity(~1.6 Wm^-1 K^-1,900℃)and prominent thermal radiation resistance,which is much better than those of YSZ(~2.5 Wm^-1 K^-1,1000℃)and La-12 Zr2O7(~2.0 Wm^-1 K,900℃).The thermal expansion coefficients(TECs)are 3.0–8.0×10^-6 K^-1(200-1200℃),which is suitable for T/EBCs applications.Furthermore,absence of phase transition and extraordinary chemical compatibility with Al2O3/SiO2 up to 1500℃indicate the potential application prospect.The documented governing mechanisms of m'RETaO4 properties will enable researchers to promote their application in the future investigation.

Influence of HfO2 alloying effect on microstructure and thermal conductivity of HoTaO4 ceramics

HfO2 alloying effect has been applied to optimize thermal insulation performance of HoTaO4 ceramics.X-ray diffraction,Raman spectroscopy,and X-ray photoelectron spectroscopy are employed to decide the crystal structure.Scanning electronic microscopy is utilized to detect the influence of HfO2 alloying effect on microstructure.Current paper indicates that the same numbers of Ta5+and Ho3+ions of HoTaO4 are substituted by Hf4+cations,and it is defined as alloying effect.No crystal structural transition is introduced by HfO2 alloying effect,and circular pores are produced in HoTaO4.HfO2 alloying effect is efficient in decreasing thermal conductivity of HoTaO4 and it is contributed to the differences of ionic radius and atomic weight between Hf4+ions and host cations(Ta5+and Ho3+).The least experimental thermal conductivity is 0.8 W·K–1·m–1 at 900℃,which is detected in 6 and 9 mol%-HfO2 HoTaO4 ceramics.The results imply that HfO2–HoTaO4 ceramics are promising thermal barrier coatings(TBCs)due to their extraordinary thermal insulation performance.

Mechanical property enhancements and amorphous thermal transports of ordered weberite-type RE_(3)Nb/TaO_(7) high-entropy oxides

A_(3)BO_(7)-type(A=rare earth(RE),B=Nb or Ta)oxides have been studied as protective coating materials because of their low thermal conductivity;however,their hardness,toughness,and stiffness are insufficient,particularly for members with webeirte-type structures.In this work,we have synthesized two high-entropy oxides(HEOs)of weberite-type RE niobates/tantalates(RE_(3)Nb/TaO_(7)),i.e.,(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))_(3)NbO_(7)(7HEOs-Nb)and(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))3(Nb_(1/2)Ta_(1/2))O_(7)(7HEOs-NbTa),to overcome the mechanical deficiencies.The short-and long-range ordered arrangements of RE cations in the A-site and Nb/Ta cations in the B-site were identified by the X-ray diffraction(XRD),scanning electron microscopy equipped with energy-dispersive spectrometry(EDS),and transmission electron microscopy.The enhancements in hardness(H=9.4 GPa)and fracture toughness(KIC=2.0 MPa·m^(1/2))were realized by grain refinement,solid solution strengthening,and high stiffness(K).The exceptional phase stability at 25-1500℃,amorphous thermal conductivity(k=1.5-1.7 W·m^(−1)·K^(−1) at 25-900℃),and high thermal expansion coefficients(TEC>11.0×10^(−6) K^(−1) at 1500℃)further supported their potential application as protective coating materials.

High-entropy ferroelastic(10RE_(0.1))TaO_(4) ceramics with oxygen vacancies and improved thermophysical properties

The primary purpose of this work is to optimize the thermophysical properties of rare-earth tan-talate ceramics using the high-entropy effect.Here,the high-entropy rare-earth tantalate ceramic(Y_(0.1)Nd_(0.1)Sm_(0.1)Gd_(0.1)Dy_(0.1)Ho_(0.1)Er_(0.1)Tm_(0.1)Yb_(0.1)Lu_(0.1))TaO_(4)((10RE_(0.1))TaO_(4))is synthesized successfully.The lat-tice distortion and oxygen vacancy concentration are characterized firstly in the rare-earth tantalates.Notably,compared with single rare-earth tantalates,the thermal conductivity of(10RE_(0.1))TaO_(4) is reduced by 16%-45%at 100℃ and 22%-45%at 800℃,and it also presents lower phonon thermal conductivity in the entire temperature range from 100 to 1200℃.The phonon thermal conductivity(1.0-2.2 W m^(-1) K^(-1),100-1200℃)of(10RE_(0.1))TaO_(4) is lower than that of the currently reported high-entropy four-,five-and six-component rare-earth tantalates.This is the result of scattering by the ferroelastic domain,lattice distortion associated with size and mass disorder,and point defects,which target low-,mid-and high-frequency phonons.Furthermore,(10RE_(0.1))TaO_(4),as an improved candidate for thermal barrier coatings materials(TBCs),has a higher thermal expansion coefficient(10.5×10^(-6)K^(-1) at 1400℃),lower Young’s modulus(123 GPa)and better high-temperature phase stability than that of single rare-earth tantalates.

A potential red-emitting phosphor Ca_(2)YTaO_(6):Eu^(3+):Luminescence properties,thermal stability and applications for white LEDs

A novel red-emitting phosphor tantalate Ca_(2)YTaO_(6):Eu^(3+)was synthesized by a solid-state reaction.The purity and surface morphology of the phosphors were characterized.The Ca_(2)YTaO_(6):Eu^(3+)phosphors show a sharp emission peak at 612 nm under near-ultraviolet(n-UV) at 395 nm because of the ^(5)D0→^(7)F_(2) transition of Eu^(3+).The optimal Eu^(3+)doping concentration in Ca_(2)YTaO_(6) is 40 mol% and the critical energy-transfer distance of Eu^(3+)ions was calculated to be 0.9 nm.The emission spectra of Ca_(2)YTaO_(6):Eu_(3+)from 300 to 480 K were investigated.The thermal-quenching temperature(T_(0.5)) of Ca_(2)YTaO_(6):Eu^(3+)is above 480 K.The color purity of Ca_(2)YTaO_(6):40 mol%Eu^(3+)is as high as 99.8%.The luminescence lifetime of Ca_(2)YTaO_(6):40 mol%Eu^(3+)was also discussed.The high color purity and high thermal stability of Eu^(3+)-doped Ca_(2)YTaO_(6) phosphors contribute to its application value in white lightemitting diodes(w-LEDs).