Effect of La3+, Ag+ and Bi3+ doping on thermoelectric properties of SrTiO3: First-principles investigation

First principles calculations were applied to study the thermoelectric properties of La³⁺-, Ag⁺- and Bi³⁺- doped SrTiO₃. With the exception of Sr_0.9La_0.1TiO₃, the band gaps of Sr_0.8La_0.1Ag_0.1TiO₃, Sr_0.8La_0.1Bi_0.1TiO₃ and Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃ were higher than that of SrTiO₃. La³⁺, Ag⁺, and Bi³⁺ doping can cause an increase in electrical conductivity and power factor, and a decrease in thermal conductivity, which improves the ZT. The thermal conductivities of SrTiO₃, Sr_0.9La_0.1TiO₃, Sr_0.8La_0.1Ag_0.1TiO₃, Sr_0.8La_0.1Bi_0.1TiO₃ and Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃ successively decreased, while power factor and ZT increased. Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃, in particular, has the smallest thermal conductivity (2.237 W/m/K), the highest power factor (1.18 mW/(mK²)) and ZT (0.597) at 1200 K, 2.195 times larger than that of SrTiO₃ (0.272). In addition, the solid state reaction method was applied to prepare dense ceramics of 10 wt% Bi-doped and (Bi, Ag)-codoped Sr_0.9La_0.1TiO₃. It is demonstrated that (Bi, Ag)-codoped Sr_0.9La_0.1TiO₃ have improved power factors, thermal conductivities and ZT values. The calculations and experimental results are consistent. This work demonstrates a method of co-doping Bi³⁺, Ag⁺, and La³⁺ to enhance the thermoelectric performance of SrTiO₃.     

Enhanced thermoelectric performances in BiCuSeO oxyselenides via Er and 3D modulation doping

BiCuSeO is a kind of promising oxide-based thermoelectric material with special natural superlattice structure. Herein, the effects of Er doping and modulation doping on the thermoelectric properties of BiCuSeO have been investigated. The results indicate that Er doping can tune the Fermi level, widen the band gap, reduce the energy offset between the heavy band and the light band, and then greatly improve the electrical conductivity, power factor and ZT. Modulation doping based on Er doping can increase carrier concentration and power factor significantly while maintaining high carrier mobility. Meanwhile, due to the special heterostructure of the modulation doping sample, the phonon scattering is greatly enhanced, and the corresponding lattice thermal conductivity is significantly reduced. Finally, the maximum ZT value reaches 0.99 for the modulation doped Bi_0.92Er_0.08CuSeO bulk at 873?K, which was ~1.8 times as that of the pristine BiCuSeO sample.     

The structure and magnetic properties in Dy3+ doped SmCrO3 ceramics

The Dy³⁺ doped SmCrO₃ polycrystalline ceramics are prepared by solid state method. The structure and magnetic properties are investigated. All samples show orthorhombic structure with space group Pnma. Three magnetic transitions are detected in Dy³⁺ doped SmCrO₃ samples, which arise from the Cr³⁺-Cr³⁺ interactions and the spin reorientation of Cr³⁺, respectively. Both field cooled (FC) and zero field cooled (ZFC) exchange bias (EB) effects are observed in the prepared Sm_1-xDy_xCrO₃ (x = 0 − 0.5) samples below the spin reorientation temperature (T_SR), and the EB field (H_EB) increases dramatically below T_SR. With the increase of the doping level, the H_EB is depressed. Three anomalous variations of magnetic entropy change (ΔS_M) derived from the isothermal magnetization are observed, which are consistent with the magnetic transitions. Compared with the ΔS_M after ZFC processes, the anomalous variations of ΔS_M at ~25 K almost disappear after FC processes due to the enhanced unidirectional anisotropy, and no obvious influence is observed for the other two anomalous variations after FC processes.     

Luminescent Er3+ doped transparent alumina ceramics

We report on successful preparation of Er³⁺ doped transparent alumina (0.1–0.17 at.%) exhibiting visible light photoluminescence using wet shaping method and hot isostatic pressing. The effects of dopant amount, type of doping powder and powder pre-treatment on final microstructure, real in-line transmittance and photoluminescence characteristics were studied.The real in-line transmittance ranged between 28 and 56%, depending on processing parameters. The transparency decreased with increased amount of dopant. The decrease is dependent on the type of doping powder and its pre-treatment.The photoluminescence spectra measured in both visible and NIR region showed typical emission bands due to the presence of Er³⁺ ions. The decay profiles of the ⁴S_3/2 → ⁴I_15/2 transition were fitted with a 2-exponential function, with faster component in the range of 360–700 ns and slower component around 1.6-2.4 μs. The intensity of emissions and lifetime of the ⁴S_3/2 level decrease significantly with increasing concentration of Er³⁺ ions.     

Effect of Sm3+ doping on ferroelectric,energy storage and photoluminescence properties of BaTiO3 ceramics

Rare earth doped ferroelectric ceramics have attracted much attention due to their great potential application for novel multifunctional optical-electro devices. Herein, the x% mol Sm³⁺ doped BaTiO₃ (BTO:xSm³⁺) ceramics were fabricated by the conventional solid-state reaction method. The Sm³⁺ ions composition dependent phase structure, ferroelectric, energy storage and photoluminescence properties were systematically investigated. With the increase of Sm³⁺ ions composition, the remanent polarization decreases dramatically from 15.705 μC/cm² (BTO) to 7.132 μC/cm² (BTO:3.0%Sm³⁺), but the energy storage density and efficiency increase greatly with a relative change of 79.76% and 31.13%, respectively. Furthermore, Sm³⁺ doping causes the transformation from the tetragonal to pseudo-cubic phase for BTO ceramics at room temperature, resulting in a broader temperature transition range from the ferroelectric to paraelectric phase and a lower Curie temperature. Particularly, the pure BTO and BTO:xSm³⁺ ceramics show great thermal stability for energy storage properties. In addition, under the excitation of 408 nm near-ultraviolet light, the BTO:xSm³⁺ ceramics exhibit the strongest orange-red emission peak around 596 nm with a large relative tunability of intensity by 88.97%. The results suggest that the BTO:xSm³⁺ ceramics are suitable for the design of optoelectronic devices.     

The effect of B site doping of Nb5+ and aging process on the properties of BNKT-BT lead-free piezoelectric ceramics

This research explored the influence of Nb⁵⁺ doping on the electrical properties and microstructure of [0.852(Bi_0.5Na_0.5)-0.11(Bi_0.5K_0.5)-0.038Ba]Ti_(1-x)Nb_xO₃ (designated as BNKT-BT). The BNKT-BT samples were fabricated and the XRD results showed that the doping of Nb⁵⁺ ions induced a transformation from the coexistence of both rhombohedral and tetragonal phase to a single pseudo cubic phase. Also, the doping of Nb⁵⁺ ions caused the temperature of ferroelectric state-ergodic relaxor state transition to decrease below the room temperature (RT). The SEM observations showed that the Nb⁵⁺ doping had a minor effect on the grain size of BNKT-BT samples. In addition, the samples achieved a large electrostrain of 0.43% (d*₃₃ = S_max/E_max = 614 pm/V) under 7 kV/mm at RT. Moreover, the aging process employed at 80 °C for two weeks in air could effectively increase the electrostrain from 0.38% to 0.41% under 6 kV/mm (d*₃₃ = 683 pm/V) and reduce the strain hysteresis from 57% to 34%. These results indicated that the B site doping, combined with the aging process, could provide an effective way to enhance the electrostrain performance of lead-free piezoelectric ceramics.     

The effect of Dy3+ doping on the thermoluminescence properties of Ba2SiO4

The thermoluminescence (TL) properties of barium silicate phosphor, Ba₂SiO₄:%3Dy³⁺ synthesized by using hydrothermal method were investigated and presented in detail. The crystallographic structure of Ba₂SiO₄:%3Dy³⁺ was determined by conventional x-ray diffraction technique and the results showed that the sample was grown in orthorhombic phase with Pmcn (62) space group (PDF: 01-077-0150). The excitation spectra of Ba₂SiO₄:Dy³⁺ were measured in the wavelength range of 220-400 nm and the spectra showed that there were several excitation bands in the sample. The CIE chromaticity coordinates were also calculated from emission spectra for Dy³⁺-doped Ba₂SiO₄. In order to calculate the kinetic parameters of the sample the additive dose, peak shape and computerized glow curve deconvolution methods were used. It was found that Ba₂SiO₄:Dy³⁺ was composed of five general order TL glow peaks. The fading characteristics of the sample were also studied over a period time. At the end of the planned storage times, the normalized TL peak area of Ba₂SiO₄:Dy³⁺ reduced 60% of its original value.     

Effect of MgO doping on the structure and optical properties of YAG transparent ceramics

The paper studies the features of Mg²⁺ ions as sintering aid for reactive solid-state sintering of YAG transparent ceramics. Phase composition, microstructure and optical properties of YAG ceramics, doped by 0 ÷ 0.15 wt.% MgO, were investigated. Solubility limit of Mg²⁺ ions in YAG crystal lattice was found to be in the range of 0.06 ÷ 0.1 wt.% of MgO additive. Substitution mechanism of Mg²⁺ in ceramic YAG was identified by comparison of XRD data and ab initio calculation. It was shown that within the solubility limit Mg²⁺ ions most likely substitute Al³⁺ sites. Doping by MgO above solubility limit led to precipitation of spinel secondary phases. It was found that doping by Mg²⁺ ions increases concentration of oxygen vacancies in YAG lattice that effectively promote sintering. The optimal concentration range of MgO sintering aid that allow to achieve YAG transparent ceramics was defined as 0.03 ÷ 0.06 wt.%.     

Electro-mechano-optical properties of the Er3+ modified Bi0.5Na0.4K0.1TiO3 versatile ceramics

A series of Bi_0.5-xEr_xNa_0.4K_0.1TiO₃ (BNKT-xEr) ceramics were designed and fabricated, the dopant effects on dielectric, piezoelectric and photoluminescence properties were studied. The results show that the piezoelectric property of BNKT can be enhanced by a trace amount of Er dopant, which is also reflected in the large linear electrostrain (S_uni = 0.29%, under 55 kV/cm) achieved in BNKT-0.0025 Er. On the other hand, higher Er content can produce excellent dielectric temperature stability, with △?/?_{150 °C} < ±15% over temperature range of 90～510 °C. Meanwhile, all BNKT-xEr ceramics exhibit good photoluminescence properties, which may open new applications of these multifunctional ceramics.     

The effect of Lu3+ doping upon YAG:Ce phosphor ceramics for high-power white LEDs

Intense green emission is extremely significant to the color rendering index (CRI) of white LEDs. Various green-emitting YLuAG:Ce phosphor ceramics were successfully prepared by vacuum sintering. The effects of Lu³⁺ doping on structure and luminescence property were investigated in detail. In comparison with YAG:Ce, YLuAG:Ce ceramics own smaller grain size, better luminescence performance and higher thermal stability. The photoluminescence (PL) intensity of YLuAG:Ce ceramics increases by 23.6 % due to the “light scattering enhanced effect”. Furthermore, the Ce³⁺ emission is obviously blue-shifting from 533 nm to 519 nm, and the intensity of YLuAG:Ce ceramics reduces only about 8.9 % at 250 °C, showing better thermal stability (vs 11.1 % of YAG:Ce). The LE of LED packaged by YLuAG:Ce ceramic is up to 148.88 lm/W when the doping Lu³⁺ y is 2.1. The above results show that tailored YLuAG:Ce phosphor ceramic is a potential green-emitting color converter for high-power LEDs (hp-LEDs).     

Microstructural,structural and dielectric properties of Er3+-modified BaTi0.85Zr0.15O3 ceramics

The (micro)structural and electrical properties of undoped and Er³⁺-doped BaTi_0.85Zr_0.15O₃ ceramics were studied in this work for both nominal Ba²⁺ and Ti⁴⁺ substitution formulations. The ceramics were produced from solid-state reaction and sintered at 1400 °C for 3 h. For those materials prepared following the donor-type nominal Ba_1?xEr_x(Ti_0.85Zr_0.15)O₃ composition, especially, Er³⁺ however showed a preferential substitution for the (Ti,Zr)⁴⁺ lattice sites. This allowed synthesis of a finally acceptor-like, highly resistive Ba(Ti,Zr,Er)O_3?δ-like system, with a solubility limit below but close to 3 cat.% Er³⁺. The overall phase development is discussed in terms of the amphoteric nature of Er³⁺, and appears to mainly or, at least, partially also involve a minimization of stress effects from the ion size mismatch between the dopant and host cations. Further results presented here include a comparative analysis of the behavior of the materials’ grain size, electrical properties and nature of the ferroelectric-to-paraelectric phase transition upon variation of the formulation and Er³⁺ content.     

Effect of BiNbO4 doping on the dielectric properties of BaTiO3 ceramics

In this paper, the effect of BiNbO₄ doping on the structural characteristics and dielectric properties were investigated systematically. With the increase in the BiNbO₄ content, a systematic structural characteristics change from ferroelectric tetragonal to cubic phase. Dielectric properties changed from classical ferroelectric behavior to dispersive relaxor‐like behavior, further to linear behavior. Because of the created defect dipoles, the long‐range dipolar interaction was interrupted and the weak couplings were formed. Energy storage density reached the maximum of 0.797 J/cm³ with energy efficiency of 92.5% in 0.9BT‐0.1BN. The nonlinearity was suppressed obviously, which could enhance the energy storage density in lead‐free relaxor materials.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Enhanced high‐temperature thermoelectric performance of CdO ceramics with randomly distributed micropores

CdO ceramics with randomly distributed micropores were synthesized by the conventional solid‐state reaction method via adding polymethyl methacrylate (PMMA) as the void forming agent. The introduction of microsized pores can significantly affect both electrical and thermal transport properties of the CdO matrix, and possible underlying mechanisms are discussed. Due to the dramatically reduced thermal conductivity, a highest ZT value of ~0.51 has been achieved in the porous CdO at about 1000 K, increased by about 52% as compared to that of the dense sample without PMMA addition. This work demonstrates that introducing microsized pores is a very simple, low‐cost, and efficient strategy to improve the high‐temperature thermoelectric performance of CdO ceramics.     

Antimony doping effect on barium titanate structure and electrical properties

Nanopowders of pure and antimony doped barium titanate (BaTiO₃-BT) were synthesized by polymeric precursors method based on Pechini process. Obtained powders were pressed and sintered at 1300 °C for 8 h. XRD analysis showed the formation of cubic crystal structure in all nanopowders and tetragonal in BT ceramics. The influence of antimony concentration on structure change, grain size reduction and microstructure development was analyzed. Dielectric behavior of pure and antimony doped ceramics was studied as a function of temperature and frequency. The significant dielectric properties modification as a consequence of doping with different antimony concentration was noticed. The electrical resistivity measurements pointed out that antimony concentration influenced also on materials change from insulator to semiconductor.     

Microstructure and thermoelectric properties of La0.1Dy0.1SrxTiO3 ceramics

La_0.1Dy_0.1Sr_xTiO₃ (x=0.80, 0.78, 0.75, 0.70) powders were synthesized via a sol-gel method, followed by sintering at 1550 °C in a reducing atmosphere of 5 vol% hydrogen in nitrogen. The microstructure and thermoelectric properties of the Sr-deficient La and Dy co-doped SrTiO₃ were investigated. The result of XRD revealed that La_0.1Dy_0.1Sr_xTiO₃ consisted of SrTiO₃ with a cubic crystal structure as the main phase and of a small amount of Dy₂Ti₂O₇ as the second phase. All the Sr-deficient samples exhibited a step-like microstructure. As the nominal Sr deficient content increased, the electrical conductivity of the Sr-deficient La_0.1Dy_0.1Sr_xTiO₃ ceramics enhanced due to the increasing Sr and oxygen vacancies, the absolute value of the Seebeck coefficient increased a little, and the thermal conductivity decreased to ~3.0 W m⁻¹ K⁻¹, leading to a high ZT value of 0.19 for La_0.1Dy_0.1Sr_0.75TiO₃ at 500 °C.     

Effects of Er3+ and/or Cr3+ doping on crystallization activation energy and fluorescence properties of transparent ZnGa2O4 glass-ceramics

Er³⁺ and/or Cr³⁺ doped transparent ZnGa₂O₄ glass-ceramics were successfully obtained by one-step heat treatment. The results showed that Er³⁺ ions can enrich around ZnGa₂O₄ crystal to reduce the crystallization activation energy and promote the growth of ZnGa₂O₄ crystal. Cr³⁺ ions may successfully occupy the Ga³⁺ sites in the ZnGa₂O₄ lattice but will increase crystallization activation energy and inhibit the growth of the ZnGa₂O₄ crystal. Before and after crystallization, the coordination-field intensity of Cr³⁺ ions increased from 2.17 to 2.86, resulting in the peak position of its emission spectra moving from 850 to 688 nm. By excitation at 378 nm, the precursor glass co-doped with Er³⁺ and Cr³⁺ ions only showed the characteristic emission peaks belonging to Er³⁺ ions. After heat treatment, the characteristic emission peaks belonging to Er³⁺ and Cr³⁺ ions existed simultaneously, and the emission color changed from green to yellow. By excitation at 980 nm, there were only characteristic emission peaks belonging to Er³⁺ ions of the Er³⁺/Cr³⁺ co-doped glasses before and after heat treatment. The results showed that the Er³⁺ and/or Cr³⁺ doped ZnGa₂O₄ glass-ceramics have adjustable luminescence ability and show potential application value in the field of luminescence display.     

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba²⁺ doping on the thermoelectric properties of the p-type Bi_1–xBa_xCuSeO (0?≤?x?≤?0.21) fabricated by spark plasma sintering. The substitution of Ba²⁺ for Bi³⁺ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi_0.86Ba_0.14CuSeO at 500?°C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500?°C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba²⁺ for Bi³⁺.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.