Melt growth and characterization of Mg2Si bulk crystals

We have grown Mg₂Si bulk crystals by the vertical Bridgman method using a high-purity Mg (6N-up) source. The grown crystals were single-phase Mg₂Si and had well-developed grains (1-5 mm³). Laue observations and SEM-EDX observations confirmed that crystalline quality in the grains was single crystal with stoichiometric composition. Electron concentration of the single crystalline specimens grown from 6N-up-Mg was 4.0 × 10¹⁵ cm^− 3 at room temperature (RT). This value is more than one order of magnitude lower than that of specimens grown from 4N-Mg [(5-7) × 10¹⁶ cm^− 3]. The Hall mobility of 14,500 cm²/Vs was observed at 45 K in the crystals grown from 6N-up-Mg. We also found that Al impurity plays an important role in the crystals grown from a low-purity Mg source. From the optical absorption measurement, we estimated that the indirect energy gap was about 0.66 eV at 300 K and about 0.74 eV at 4 K.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Thermoelectric properties of the bismuth-antimony-telluride and the antimony-telluride films processed by electrodeposition for micro-device applications

Thermoelectric properties of the electrodeposited bismuth-antimony-telluride (Bi-Sb-Te) and the antimony-telluride (Sb-Te) films were characterized. The electrodeposited Bi-Sb-Te films exhibited the Seebeck coefficients of 21-71 µV/K and a maximum power factor of 1.2 × 10^− 4 W/K²-m. The Sb-Te films electrodeposited at potentials of 10 mV-30 mV were amorphous with compositions close to the Sb₂Te₃ stoichiometry. The electrodeposited Sb-Te films exhibited the Seebeck coefficients larger than 250 µV/K due to their noncrystallinity. The amorphous Sb-Te films of the stoichiometric composition exhibited a maximum power factor of 57 × 10^− 4 W/K²-m.     

Thermal and mechanical properties of novel substrate crystal Mg0.4Al2.4O4

Mg_0.4Al_2.4O₄ single crystal was grown by the Czochralski method. The measured specific heat values are 0.804-1.06 J g^− 1 K^− 1 in the temperature range from 298.15 to 573.15 K. The calculated thermal conductivity components are 11.37, 11.47 and 10.77 W m^− 1 K^− 1 along the [111], [004] and [22?0] direction at 298.15 K. The Vickers microhardness values are 1328-1414 kg mm^− 2. These experimental results show that Mg_0.4Al_2.4O₄ crystal is a promising substrate for GaN-based LEDs.     

Growth of homogeneous polycrystalline Si1-xGex and Mg2Si1-xGex for thermoelectric application

Homogeneous polycrystalline Si_1-xGe_x were grown using a Si(seed)/Ge/Si(feed) sandwich structure under the low temperature gradient less than 0.4 °C/mm. It was found that the composition of the Si_1-xGe_x was controlled by the growth temperature. The homogeneous Mg₂Si_1-xGe_x was synthesized by heat treatment of the homogeneous Si_1-xGe_x powders under Mg vapor. The Mg₂Si_1-xGe_x sample with the relative density of 95% was synthesized by spark plasma sintering technique. The resistivity and the Seebeck coefficient of the Si, Ge, Si_1-xGe_x and Mg₂Si_1-xGe_x samples were evaluated as a function of temperature. It indicated that Seebeck coefficients of the Si_1-xGe_x and Mg₂Si_1-xGe_x samples were higher than those of Si and Ge. Moreover, the Seebeck coefficient of Mg₂Si_0.7Ge_0.3 sample was higher than that of Mg₂Si_0.5Ge_0.5 and Si_0.5Ge_0.5 samples.     

Thermal treatment effects on interfacial layer formation between ZrO2 thin films and Si substrates

This paper describes the growth condition of stoichiometric ZrO₂ thin films on Si substrates and the interfacial structure of ZrO₂ and Si substrates. The ZrO₂ thin films were prepared by rf-magnetron sputtering from Zr target with mixed gas of O₂ and Ar at room temperature followed by post-annealing in O₂ ambient. The stoichiometric ZrO₂ thin films with smooth surface were grown at high oxygen partial pressure. The thick Zr-free SiO₂ layer was formed with both Zr silicide and Zr silicate at the interface between ZrO₂ and Si substrate during the post-annealing process due to rapid diffusion of oxygen atoms through the ZrO₂ thin films. After post annealing at 650-750 °C, the multi-interfacial layer shows small leakage current of less than 10⁻⁸ A/cm² that is corresponding to the high-temperature processed thermal oxidized SiO₂.     

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

Abstract

High-quality thermoelectric La_0.2Sr_0.8TiO₃ (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO₃(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10^?4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO₃ single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.     

Possibility of Mg- and Ca-based intermetallic compounds as new biodegradable implant materials

Mg- or Ca-based intermetallic compounds of Mg₂Ca, Mg₂Si, Ca₂Si and CaMgSi are investigated as possible new candidates for biodegradable implant materials, attempting to improve the degradation behavior compared to Mg and Ca alloys. The reactivity of Ca can be indeed reduced by the formation of compounds with Mg and Si, but its reactivity is still high for applications as an implant material. In contrast, Mg₂Si shows a higher corrosion resistance than conventional Mg alloys while retaining biodegradability. In cytotoxicity tests under the severe condition conducted in this study, both pure Mg and Mg₂Si showed relatively high cytotoxicity on preosteoblast MC3T3-E1. However, the cell viability cultured in the Mg₂Si extract medium was confirmed to be better than that in a pure Mg extract medium in all the conditions investigated with the exception of the 10% extract medium, because of the lower corrosion rate of Mg₂Si. The cytotoxicity derived from the Si ion was not significantly detected in the Mg₂Si extract medium in the concentration level of ~ 70 mg/l measured in the present study. For aiming the practical application of Mg₂Si as an implant material, however, its brittle nature must be improved.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

Characterization of MgxZn1 − xO thin films grown on sapphire substrates by metalorganic chemical vapor deposition

Wurtzite-structure Mg_xZn_1 − xO materials with five different compositions of x from 0 to 0.14 were grown on sapphire substrates by metalorganic chemical vapor deposition. It was found that increasing Mg content in the Mg_xZn_1 − xO not only increased the band gap energy of the film but was also beneficial to the epitaxial growth of p-type Mg_xZn_1 − xO without using any doping sources. In addition, the combined ultraviolet photoluminescence (PL) and Raman scattering spectra were measured with PL-Raman signals obtained together, showing a blue-shift of PL band and variation of resonant Raman multi-order longitudinal optical phonon modes with an increase of Mg content.     

Thermoelectric properties of perovskite oxides La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>3</Subscript> prepared by polymerlized complex method

P-type perovskite oxides La_1−x Sr _x CoO₃ (0 ≤ x ≤ 0.2) have been prepared using a polymerlized complex method and sintering. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the oxides were studied from room temperature to 773 K. The ln(σT)−1/T relationships revealed small-polaron hopping mechanism for the higher Sr contents. Large Seebeck coefficients were observed in lightly Sr-doped samples. Sr doping greatly reduced the Seebeck coefficient and enhanced the electrical and thermal conductivity of the samples. The temperature-induced spin-state transition of Co³⁺ ions strongly influenced the transport properties. The highest ZT value found in this series of oxides was 0.046 at 300 K for x = 0.1.     

Epitaxial growth and properties of cubic Zn0.7Mg0.3O films on TiN-buffered Si(100) substrates by in situ pulsed laser deposition

A novel cubic Zn_0.7Mg_0.3O film on silicon substrate is conducted by KrF excimer pulsed-laser ablation system. By introducing a thin TiN buffer, layer-by-layer growth of cubic Zn_0.7Mg_0.3O film epilayer has been realized. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the Zn_0.7Mg_0.3O films were strongly affected by the TiN buffer layer. The Zn_0.7Mg_0.3O film obtained at an optimal buffer layer exhibited high quality and good surface. For the metal-insulator-metal (MIM) structure of Pt/Zn_0.7Mg_0.3O (200 nm)/TiN (20 nm)/Si (400 μm) prepared at the optimal growth conditions achieved a very low leak current density of ∼10⁻⁶ A cm⁻² at an electric field of 9 × 10⁵ V cm⁻¹ and the permittivity (?_r) of about 8.1, agreed well with that of acquired MgO film and MgO single crystal.     

Enhanced thermoelectric properties of NaCo2O4 by adding ZnO

The primary phase present in the as-sintered Na(Co_1 − xZn_x)₂O₄ (0 ≤ x ≤ 0.1) bodies was the solid solution of the constituent oxides with a bronze-type layered structure. The electrical conductivity of the Na(Co_1 − xZn_x)₂O₄ samples significantly increased with an increase in ZnO content. The sign of the Seebeck coefficient for all samples was positive over the whole temperature range (723-1073 K), i.e., p-type conduction. The power factor of Na(Co_0.95Zn_0.05)₂O₄ showed an outstanding power factor (1.7 × 10^− 3Wm^− 1 K^− 2) at 1073 K. The power factor was above four times superior to that of ZnO-free NaCo₂O₄ (0.4 × 10^− 3Wm^− 1 K^− 2). This originates from an unusually large Seebeck coefficient (415 μVK^− 1) accompanied with high conductivity (127Ω^− 1 cm^− 1) at 1073 K.     

Structural, electrical and optical properties of transparent Zn1 − xMgxO nanocomposite thin films

Zn_1 − xMg_xO thin films of various Mg compositions were deposited on quartz substrates using inexpensive ultrasonic spray pyrolysis technique. The influence of varying Mg composition and substrate temperature on structural, electrical and optical properties of Zn_1 − xMg_xO films were systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70 mol%. AFM images of the Zn_1 − xMg_xO films (x = 0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zn_1 − xMg_xO. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zn_1 − xMg_xO decreases with corresponding increase in Mg content, which result in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16 eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side.     

Effect of compositional variations on charge compensation of AlO4 and BO4 entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass

This paper presents the structural and crystallization study of a rare-earth-rich aluminoborosilicate glass that is a simplified version of a new nuclear glass proven to be a potential candidate for the immobilization of highly concentrated radioactive wastes that will be produced in the future. In this work, we studied the impact of changing the nature of alkali (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) or alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) cations present in glass composition on glass structure (by ²⁷Al and ¹¹B nuclear magnetic resonance spectroscopy) and on its crystallization tendency during melt cooling at 1 K/min (average cooling rate during industrial process). From these composition changes, it was established that alkali cations were preferentially involved in charge compensation of (AlO₄)⁻ and (BO₄)⁻ entities in the glassy network comparatively to alkaline-earth cations. Whatever the nature of alkali cations, glass compositions containing calcium gave way to the crystallization of an apatite silicate phase bearing calcium and rare-earth (RE) cations (Ca₂RE₈(SiO₄)₆O₂, RE = Nd or La) but melt crystallization tendency during cooling strongly varied with the nature of alkaline-earth cations.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

The influence of ambient conditions on properties of MgxZn1−xO films by sputtering

The Mg_xZn_1−xO films were prepared in different Ar-O₂ mixture ambience by magnetron sputtering. According to the X-ray diffraction (XRD) patterns and the energy dispersive X-ray spectroscopy (EDS) results, it was found that the Mg contents in the films varied with the different ratios of O₂/O₂+Ar, and the crystal quality of the films improved with the increasing of Mg contents. Meanwhile, the ultraviolet and visible (UV-vis) absorption spectroscopy indicated that the band gap of the films also increased. Moreover, it could be seen that the photoluminescence (PL) spectrum was different from that of undoped Zinc oxide (ZnO) films or the results in other reports on the Mg_xZn_1−xO films: there was no blueshift effect happening for the near-band-edge (NBE) emission in Mg_xZn_1−xO films with different Mg contents.     

钠钙玻璃上Mg2Si薄膜的制备及其电学性质

采用磁控溅射技术和退火工艺在钠钙玻璃衬底上制备了Mg_2Si半导体薄膜,研究了Mg膜厚度对Mg_2Si薄膜结构及其电学性质的影响。在钠钙玻璃上分别溅射两组相同厚度(175nm)的P-Si和N-Si膜,然后在其上溅射不同厚度Mg膜(240nm、256nm、272nm、288nm、304nm),低真空退火4h制备一系列Mg_2Si半导体薄膜。通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)、霍尔效应测试仪对Mg_2Si薄膜的晶体结构、表面形貌、电学性质进行表征与分析。结果表明:采用磁控溅射技术在钠钙玻璃衬底上成功制备出以Mg_2Si(220)为主的Mg_2Si薄膜。随着沉积Mg膜厚度的增加,Mg_2Si衍射峰逐渐增强,薄膜表面更连续,电阻率逐渐减小,霍尔迁移率逐渐降低,载流子浓度逐渐增加。此外,Si膜导电类型和Mg膜厚度共同影响Mg_2Si薄膜的导电类型。溅射N-Si膜时,Mg_2Si薄膜的导电类型随着Mg膜厚度的增加由P型转化为N型;溅射P-Si膜时,Mg_2Si薄膜的导电类型为P型。可以控制制备的Mg_2Si半导体薄膜的导电类型,这对Mg_2Si薄膜的器件开发有着重要的指导意义。     

Preparation and thermoelectric properties of nc-Si:(Al2O3 + SiO2) composite film

A composite film of nanocrystalline Si (nc-Si) embedded in (Al₂O₃ + SiO₂) has been prepared on a quartz substrate by thermally evaporating a 400 nm thick Al film on a quartz substrate and annealing in air at 580 °C for 1 h. During annealing, the Al reacts with the SiO₂ of the quartz substrate and produces nc-Si, which is embedded in the (Al₂O₃ + SiO₂) film. The average size of nc-Si is ~ 22 nm and the thickness of the nc-Si:(Al₂O₃ + SiO₂) composite film is ~ 810 nm. It is found that the prepared film is thermoelectric with a Seebeck coefficient of − 624 μV/K at 293 K and − 225 μV/K at 413 K.     

Electrical transport and structural study of CuCr1 − xMgxO2 delafossite thin films grown by pulsed laser deposition

The growth and properties of delafossites CuCr_1 − xMg_xO₂ thin films are examined. These films are grown by pulsed laser deposition. As a class of materials delafossites have received recent interest since some members show p-type behavior. While not considered true wide-bandgap materials due to a narrow indirect bandgap that fails to adsorb light due to a forbidden same parity transition, optical transparencies greater than 40% in the visible can be observed. In order to be useful for transparent device applications, CuCr_1 − xMg_xO₂ films are needed with low resistivity and high optical transparency. Epitaxial films of CuCr_1 − xMg_xO₂ were grown on c-sapphire, examining the effects of oxygen pressure and growth temperature on film properties. Films were realized with resistivity of ~ 0.02 Ω-cm and optical transparency of 40% in the visible. The formation of a problematic secondary minority spinel phase of (Cu,Mg)Cr₂O₄ is discussed. While conductivity increases substantially with Mg doping, the incidence of the spinel phase increases as well.