Effect of microstructure on the thermoelectric properties of CSD-grown Bi2Sr2Co2Oy thin films

Three Bi2Sr2Co2Oy thin films with different microstructures have been prepared by chemical solution deposition on LaAlO 3(001) through varying the annealing temperature.With the decrease in the annealing temperature,both the size and c-axis alignment degree of grains in the film decrease as well,leading to an increase in the film resistivity.In addition,the decrease in the annealing temperature also results in a slight increase in the Seebeck coefficient due to the enhanced energy filtering effect of the small-grain film.The nanostructured Bi2Sr2Co2Oy film with an average grain size of about 100 nm shows a power factor comparable to that of films with larger grains.Since the thermal conductivity of the nanostructured films can be depressed due to the enhanced phonon scattering by grain boundary,a higher figure of merit is expected in Bi2Sr2Co2Oy thin film with grains in nanometer size.     

Tunable Ba_0.5 Sr_0.5 TiO₃ film bulk acoustic resonators using SiO₂ /Mo Bragg reflectors

Tunable and switchable Ba 0.5 Sr 0.5 TiO 3 film bulk acoustic resonators(FBARs) based on SiO 2 /Mo Bragg reflectors are explored,which can withstand high temperature for the deposition of Ba x Sr 1 x TiO 3(BST) films at 800 C.The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect.The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing,while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias(< 45 V) while downwards at higher dc bias.The calculated relative tunability of shifts at series resonance frequency is around 2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias,which can be comparable to AlN FBARs.This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum(Mo) as the high acoustic impedance layer in a Bragg reflector,which not only provides excellent acoustic isolation from the substrate,but also improves the crystallinity of BST films withstanding higher deposition temperature.     

Charge ordering modulations in a Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi 0.4 Ca 0.6 MnO 3 film with a thickness of 110 nm at 103 K.Six different types of superlattice structures are observed using the selected-area electron diffraction(SAED) technique,while three of them match well with the modulation stripes in high-resolution transmission electron microscopy(HRTEM) images.It is found that the modulation periodicity and direction are completely different in the region close to the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface from those in the region a little further from the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface,and the possible reasons for this are discussed.Based on the experimental results,structural models are proposed for these localized modulated structures.     

Synthesis and thermoelectric properties of Mn-doped AgSbTe₂ compounds

Polycrystalline p-type Ag 0.9 Sb 1.1 x Mn x Te 2.05(x = 0.05,0.10,and 0.20) compounds have been prepared by a combined process of melt-quenching and spark plasma sintering.The sample composition of Ag 0.9 Sb 1.1 x Mn x Te 2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag 2 Te phase in the matrix by adding insufficient Te,which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect.All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis.After the treatment of spark plasma sintering,only the sample with x = 0.20 has a small amount of MnTe 2 impurities.The thermal analysis indicates that a tiny amount of Ag 2 Te phase exists in all these samples.The presence of the MnTe 2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity.In contrast,the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect.A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K,showing promising thermoelectric properties in the medium temperature range.     

Temperature-dependent rectifying and photovoltaic characteristics of an oxygen-deficient Bi_2Sr_2Co_2O_y/Si heterojunction

A Bi2Sr2Co2Oy/Si heterojunction is obtained by growing a layer of p-type oxygen-deficient Bi2Sr2Co2Oy film on a commercial n-type silicon wafer by pulsed laser deposition. Its rectifying and photovoltaic properties are studied in a wide temperature range from 20 K to 300 K. The transport mechanism under the forward bias can be attributed to a trap- filled space-charge-limited current conduction mechanism. Under the irradiation of a 532-nm continuous wave laser, a clear photovoltaic effect is observed and the magnitude ofphotovoltage increases as the temperature decreases, The results demonstrate the potential application of a Bi2SrzCo2Oy-based heterojunction in the photoelectronic devices.     

Interface-related switching behaviors of amorphous Pr_0.67Sr_0.33MnO₃-based memory cells

The resistive switching properties in amorphous Pr0.67Sr0.33MnO3 films deposited by pulsed laser deposition are investigated.Reproducible and bipolar counter-8-shape and 8-shape switching behaviours of Au/Pr0.67Sr0.33MnO3 /F:SnO2 junctions are obtained at room temperature.Dramatically,the coexistence of two switching polarities could be reversibly adjusted by an applied voltage range.The results allocated those two switching types to areas of different defect densities beneath the same electrode.The migration of oxygen vacancies and the trapping effect of electrons under an applied electric field play an important role.An interface-effect-related resistance switching is proposed in an amorphous Pr0.67Sr0.33MnO3-based memory cell.     

Magnetic properties of a Pt/Co₂ FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co 2 FeAl(CFA)/MgO multilayers are studied to understand perpendicular magnetic anisotropy(PMA) of half-metallic full-Heusler alloy films.PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope.It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA.The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature.At the intersection points,the decreasing slope of the saturation magnetization(M s) changes because of the conversions.The dependence of M s on the annealing temperature and MgO thickness is also studied.     

High temperature thermoelectric properties of highly c-axis oriented Bi2Sr2Co2Oy thin films fabricated by pulsed laser deposition

High-temperature thermoelectric transport property measurements have been performed on the highly c-axis oriented Bi2Sr2Co20v thin films prepared by pulsed laser deposition on LaA1Oa （001）. Both the electric resistivity p and the seebeck coefficient S of the film exhibit an increasing trend with the temperature from 300 K-1000 K and reach up to 4.8 m. cm and 202 V/K at 980 K, resulting in a power factor of 0.85 mW/mK which are comparable to those of the single crystalline samples. A small polaron hopping conduction can be responsible for the conduction mechanism of the film at high temperature. The results demonstrate that the Bi2Sr2Co2Oy thin film has potential application has high temperature thin film thermoelectric devices,     

Formations and morphological stabilities of ultrathin CoSi₂ films

In this paper we investigate the formations and morphological stabilities of Co-silicide films using 1-8-nm thick Co layers sputter-deposited on silicon(100) substrates.These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450℃ to 850℃.For a Co layer with a thickness no larger than 1 nm,epitaxially aligned CoSi2 films readily grow on silicon(100) substrate and exhibit good morphological stabilities up to 600℃.For a Co layer thicker than 1 nm,polycrystalline CoSi and CoSi2 films are observed.The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon(100) substrate.The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.     

Thermal properties of high-k Hf_1-xSi_xO₂

Classical atomistic simulations based on the lattice dynamics theory and the Born core-shell model are performed to systematically study the crystal structure and thermal properties of high-k Hf1-xSixO2 . The coefficients of thermal expansion, specific heat, Grneisen parameters, phonon densities of states and Debye temperatures are calculated at different temperatures and for different Si-doping concentrations. With the increase of the Si-doping concentration, the lattice constant decreases. At the same time, both the coefficient of thermal expansion and the specific heat at a constant volume of Hf1-xSixO2 also decreases. The Grneisen parameter is about 0.95 at temperatures less than 100 K. Compared with Si-doped HfO2 , pure HfO2 has a higher Debye temperature when the temperature is less than 25 K, while it has lower Debye temperature when the temperature is higher than 50 K. Some simulation results fit well with the experimental data. We expect that our results will be helpful for understanding the local lattice structure and thermal properties of Hf1-xSixO2 .     

10BaF₂:NaF,Na₃AlF₆/TiO₂ composite as a novel visible-light-driven photocatalyst based on upconversion emission

A rare-earth free upconversion luminescent material, 10BaF 2 :NaF, Na 3 AlF 6 , is synthesized by a hydrothermal method. The study of fluorescent spectrum indicates that it can convert visible light (550 nm–610 nm) into ultraviolet light (290 nm–350 nm), and two emission peaks at 304 nm and 324 nm are observed under the excitation of 583 nm at room temperature. Subsequently, 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 composite photocatalyst is prepared and its catalytic activity is evaluated by the photocatalytic reduction of CO 2 under visible light irradiation (λ > 515 nm). The results show that 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 is a more effective photocatalyst for CO 2 reduction than pure TiO 2 , their corresponding methanol yields are 179 and 0 μmol/g-cat under the same conditions. Additionally, the mechanism of photocatalytic reduction of CO 2 on 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 is proposed.     

Evolution of the 251 cm^-1 infrared phonon mode with temperature in Ba_0.6K_0.4Fe₂As₂

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

Low-voltage antimony-doped SnO₂ nanowire transparent transistors gated by microporous SiO₂-based proton conductors

A battery drivable low-voltage transparent lightly antimony(Sb)-doped SnO 2 nanowire electric-double-layer(EDL) field-effect transistor(FET) is fabricated on an ITO glass substrate at room temperature.An ultralow operation voltage of 1 V is obtained on account of an untralarge specific gate capacitance(～2.14 μF/cm 2) directly bound up with mobile ions-induced EDL(sandwiched between the top and bottom electrodes) effect.The transparent FET shows excellent electric characteristics with a field-effect mobility of 54.43 cm 2 /V · s,current on/off ration of 2 × 10 4,and subthreshold gate voltage swing(S = dV gs /d(log I ds)) of 140 mV/decade.The threshold voltage V th(0.1 V) is estimated which indicates that the SnO 2 namowire transistor operates in an n-type enhanced mode.Such a low-voltage transparent nanowire transistor gated by a microporous SiO 2-based solid electrolyte is very promising for battery-powered portable nanoscale sensors.     

Amorphous-crystalline dual-layer structures resulting from metastable liquid phase separation in(Fe₅₀Co₂₅B₁₅Si₁₀)₈₀Cu₂₀ melt-spun ribbons

(Fe50Co25B15Si10)80Cu20 ribbons are prepared by using the single-roller melt-spinning method.A dual-layer structure consisting of a(Fe,Co)-rich amorphous phase and a Cu-rich crystalline phase forms due to metastable liquid phase separation before solidification.The magnetic hysteresis loops of the as-quenched and annealed samples are measured at room temperature.It is indicated that the coercivity of the ribbon is almost zero in the as-quenched state.The crystallization leads to the increase of coercivity and decrease of saturation magnetization.     

Improved multiferroic properties of La-doped 0.6BiFeO₃-0.4SrTiO₃ solid solution ceramics

The 0.6(Bi1-xLax)FeO 3-0.4SrTiO 3(x = 0,0.1) multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization(Pr) and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6(Bi0.9La0.1)FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Substrate-induced stress in silicon nanocrystal/SiO₂ multilayer structures

A Raman frequency upshift in the nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in the Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO2 (3 nm) layers are deposited by high-vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N2 atmosphere at 1100°C for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high growth temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in the thermal expansion coefficient between the substrate and the Si/SiO2 SL film. Such a substrate-induced stress indicates a new method for tuning the optical and electronic properties of Si nanocrystals for strained engineering.     

微波水热法制备铋铕共掺杂氧化钇磷光粉

用微波水热法制备化学计量比为(Y_0.94-x,Eu_0.06, Bi_x)₂O₃(x=0, 0.01~0.06 )的铋铕共掺杂氧化钇磷光粉。用XRD、SEM、EDS、荧光光谱及HRTEM等进行表征。结果表明:该系统由于铋的加入使(211)晶面的生长受到抑制;在激发波长346 nm时,由于铋的掺入使发光增强;随铋掺杂量的增加,其发光先增强后减弱,并在x=0.03时有最大值;因此该系统可作为320~375 nm的近紫外(如白光LED及高压汞灯)激发用磷光粉。该系统在激发波长为254 nm时,铋的加入使发光强度减弱;因此该系统不适合用于低压汞灯。     

Transport properties of a binary mixture of CO₂-N₂ from the pair potential energy functions based on a semi-empirical inversion method

The potential energy surface of a CO 2 –N 2 mixture is determined by using an inversion method, together with a new collision integral correlation [J. Phys. Chem. Ref. Data 19 1179 (1990)]. With the new invert potential, the transport properties of CO2–N2 mixture are presented in a temperature range from 273.15 K to 3273.15 K at low density by employing the Chapman–Enskog scheme and the Wang Chang–Uhlenbeck–de Boer theory, consisting of a viscosity coefficient, a thermal conductivity coefficient, a binary diffusion coefficient, and a thermal diffusion factor. The accuracy of the predicted results is estimated to be 2% for viscosity, 5% for thermal conductivity, and 10% for binary diffusion coefficient.     

The phase transition,hygroscopicity, and thermal expansion properties of Yb_2-xAl_xMo₃O₁₂

Materials with the formula Yb 2-xAlxMo3O12(x =0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8) were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffrac- tion, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3O12 solid solutions crystallize in a single monoclinic phase for 1.7 ≤ x ≤ 2.0 and in a single orthorhombic phase for 0.0 ≤ x ≤ 0.4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhombic phase transition temperature of Al2Mo3O12 can be reduced by partial substitution of Al 3+ by Yb3+, and the Yb2-x AlxMo3O12 (0.0 < x ≤ 2.0) materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of Al3+ for Yb3+ in Yb2Mo3O12 decreases its hygroscopicity, and the linear thermal expansion co- efficients after complete removal of water species are measured to be 9.1×10 6 /K, 5.5×10 6 /K, 5.74×10 6 /K, and 9.5 × 10 6 /K for Yb1.8 Al0.2 (MoO4)3 , Yb1.6Al0.4 (MoO4 )3, Yb0.4 Al1.6 (MoO4)3 , and Yb 0.2Al1.8 (MoO4)3 , respectively.     

Preparing Cu₂ZnSnS₄ films using the co-electrodeposition method with ionic liquids

Cu₂ZnSnS4（CZTS） films are successfully prepared by co-electrodeposition in aqueous ionic solution and sulfurized in elemental sulfur vapor ambient at 400 C for 30 min using nitrogen as the protective gas.It is found that the CZTS film synthesized at Cu/（Zn+Sn）=0.71 has a kesterite structure,a bandgap of about 1.51 eV,and an absorption coefficient of the order of 10 4 cm 1.This indicates that the co-electrodeposition method with aqueous ionic solution is a viable process for the growth of CZTS films for application in photovoltaic devices.