Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the ... more Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the radio frequency magnetron sputtering method. The structure evolution with annealing temperature of the predominantly (002)-oriented Li-doped ZnO (LZO) films after in-situ post-annealing process is determined. The largest values of the piezoelectric coefficient (d33) and the remnant polarization (Pr) (22.85 pm/V and 0.655 μC/cm2, respectively) are obtained for LZO films post-annealed at 600 °C, which can be attributed to the predominant (002)-oriented crystalline structure, the release of intrinsic residual compressive stress, and less non-lattice oxygen.
Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray phot... more Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu2+/Eu3+ and Ti4+/Ti3+ in EuTiO3 and Eu0.7Ba0.3TiO3, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co2+/Ni2+ in Eu0.7Ba0.3TiO3 resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti3+ numbers and an increase in Eu3+ numbers. On the other hand, Ba2+ doping led to an increased lattice parameter due to its larger ionic size than Eu2+, whereas the Co2+/Ni2+ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu0.7Ba0.3TiO3 exhibited a clear ferroelectricity, which persisted in the Co2+/Ni2+ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu0.7Ba0.3TiO3 remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co...
Abstract BiCuSeO epitaxial films were grown on (001)/(110) SrTtO3 (STO) substrates by RF magnetro... more Abstract BiCuSeO epitaxial films were grown on (001)/(110) SrTtO3 (STO) substrates by RF magnetron sputter deposition. A sputter power over 40 W was needed to transfer target composition to substrate, leading to a fast deposition rate over 17.5 nm/min. The films grown on (001) STO at 100−150 °C showed a dominant [110] orientation instead of [001], which was what expected in view of the smallest lattice misfit and perfect match in symmetry between (001) BiCuSeO and (001) STO. This was caused by slow film growth along c-axis that could not follow the fast deposition rate, whereas a fair lattice misfit, combined with strong bonding along directions vertical to c-axis, entails the growth along [110] at low temperature. Preferred [001] orientation only occurred at high temperature (>350 °C) when the kinetic process was accelerated. However, high temperature and the fast deposition rate imposed by the required sputter power led to the growth of multiple orientations. Epitaxial growth of (001) BiCuSeO film on (001) STO was achieved after the deposition rate was reduced to 0.67 nm/min by a periodically opening and closing shutter installed in front of sputter target. Interestingly, (001) BiCuSeO epitaxial film could be grown on (110) STO at a fast deposition rate of 15 nm/min. Such films on (110) STO had to be grown at higher temperature (∼500 °C). In contrast to (001) STO, (110) STO did not had good lattice match with any BiCuSeO orientation other than [001], so fast kinetic process only promoted the growth of single [001] orientation.
BiFeO 3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from ch... more BiFeO 3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from chemical solution via hydrothermal route at 200 °C. However, for many applications, ceramic samples of reasonable bulk density (>80%) have to be sintered at temperature over 700 °C, which in this case results in a significant reduction in resistivity due to increased amounts of Fe 2+. Interestingly, doping of a few percent Sb minimized such a problem and the sintered Sb:BFO ceramics retained a similarly high resistivity as samples cold-pressed from the chemical-solution synthesized powders. However, for cold-pressed samples, Sb:BFO actually had higher conductivity than undoped BFO. Temperature-dependent conductivity showed that cold-pressed samples of both undoped and Sb doped BFO had the similar activation energy of 1.0 eV, typical for electrons trapped in oxygen vacancies. After sintering, the activation energy of Sb:BFO remained almost unchanged, but the activation energy of undoped BFO changed to 0.4 eV, which is associated to electron hopping between Fe 2+ /Fe 3+. X-ray photoelectron spectroscopy (XPS) showed a significant increase in Fe 2+ /Fe 3+ ratio from 6.6/93.4 to 25.7/74.3 in undoped BFO after sintering, while for 1% Sb doped BFO the increase was much milder from 10.9/89.1 to 14.1/85.9. XPS also showed that Sb had single +3 oxidation state before sintering, but after sintering a fairly large portion of Sb 5+ occurred. So, charge compensation for oxygen vacancies in undoped BFO was achieved dominantly by reduction of Fe 3+ to Fe 2+ , while in Sb:BFO it was achieved more by cation vacancies. Highlights Pure and Sb-doped BiFeO 3 were synthesized by hydrothermal method at 200 °C. As-prepared samples showed typical hysteresis loops of ideal ferroelectrics. Sintering the hydrothermal powders resulted in a great decrease in resistivity. Sb doping helped to retain a similarly high resistivity in the sintered samples. Sb doping kept oxygen vacancies rather than Fe 2+ ions as the main defects.
Abstract La and Y doped SrTiO3 (STO) thin films were grown on (100) oriented pure STO single crys... more Abstract La and Y doped SrTiO3 (STO) thin films were grown on (100) oriented pure STO single crystal substrates by the RF magnetron sputter technique. The La/Y doped layer is intended for use as bottom electrode for epitaxial growth of various functional oxide films. The results showed that one of the crucial deposition parameters to achieve the epitaxial growth of La/Y doped STO films with a pure phase and good electrical conductivity was sputter power, which must exceed 120 W. The conductivity of the grown films increased with the increase in sputter power and the La0.1Sr0.9TiO3 (LSTO) films deposited at 200 W had the highest conductivity of 292 S/cm. This was ascribed to the increased percentage of Ti3+ ions in the sputtered films, which was confirmed by the X-ray photoelectron spectroscopy. LSTO films deposited at 130 W were (100) oriented but the increased sputter power promoted the growth of a second texture, i.e. (110). On the other hand, Y0.08Sr0.92TiO3 (YSTO) films were able to keep the unique (100) texture in the films deposited over a range of sputter powers up to 200 W, although the conductivity of YSTO was lower, which was 98.0 S/cm for the films sputtered at 200 W.
Polycrystalline samples with nominal composition FeTe1−x S x (x = 0.00-0.30) were synthesized via... more Polycrystalline samples with nominal composition FeTe1−x S x (x = 0.00-0.30) were synthesized via solid state reaction method with intermittent grinding in argon gas flow. Xray diffraction (XRD) patterns revealed the tetragonal structure (space group P4/nmm) of the samples with the presence of impurities Fe3O4 and FeTe2. By substitution with S, the a and c lattice parameters shrink probably due to the smaller ionic radius of S2− compared to Te2−. Scanning electron microscopy images showed that the samples developed plate-like grains with increasing S substitution. Substitution of Te with S suppresses the structural transition of the parent compound FeTe as shown by both the temperature dependence of resistance and magnetic moment measurements. All of the S-substituted samples showed a rapid drop of resistance at around 9-10 K but zero resistance down to 4 K was not observed. In addition, negative magnetic moment corresponds to diamagnetism was detected in the samples for x = 0.25 and 0.30 suggesting the coexistence of magnetic and superconducting phase in these samples. The magnetization hysteresis loops measured at room temperature showed ferromagnetic behavior for the pure and S substituted samples. However, the magnetization, rentivity and coercivity decreased with S content.
Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO) þ BiFeO 3 (BFO) composite films were co-sputtered on (001) SrTiO 3 ... more Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO) þ BiFeO 3 (BFO) composite films were co-sputtered on (001) SrTiO 3 (STO) substrates, which were confirmed to be epitaxial by both 1D and 2D X-ray diffractions. Conductive LaNiO 3 (LNO) was sputtered on STO as bottom electrode for magnetoelectric (ME) measurement across film. The LNO layer was epitaxial and composed of nanocolumns perpendicular to substrate, which are helpful in reducing "substrate clamp" effect because of the flexibility of nanocolumns. NZFO þ BFO films cosputtered on LNO/STO were also epitaxial but both out-of-plane and in-plane textures of BFO were degraded with full width at half maximum of 1.974 and 2.55 , respectively, compared to 1.163 and 1.71 in films grown on bare STO. There were apparent tetragonal distortions in NZFO and BFO due to compressive in-plane strains imposed by epitaxial growth of larger NZFO and BFO lattices on smaller LNO lattice. Transmission electron microscopy (TEM) showed that the composite films were compact with clear boundaries between the NZFO and BFO phases, which were uniformly distributed with the ratio of about 35% NZFO and 65% BFO. Cross-sectional TEM revealed that crystallographic planes were very well aligned between BFO and LNO, but NZFO planes tilted 7.5 due to large lattice misfit. When measured vertically from LNO to top surface of composite film, large ME voltage coefficient at zero bias field was observed, which was 911 mVcm À1 Oe À1 at the frequency of 8 kHz. This was ascribed to the large heteroepitaxial strains in composite films.
Multiferroic composite films of Ni0.5Zn0.5Fe2O4 (NZFO) + BiFeO3 (BFO) were deposited on Si and La... more Multiferroic composite films of Ni0.5Zn0.5Fe2O4 (NZFO) + BiFeO3 (BFO) were deposited on Si and LaNiO3 (LNO) buffered Si substrates at 700 °C, by co-sputtering from two individual targets of the components.
Rare-earth iron garnets (RIG, R ¼ Y, Tb and Lu) were sintered at 1350 C. Their crystal structures... more Rare-earth iron garnets (RIG, R ¼ Y, Tb and Lu) were sintered at 1350 C. Their crystal structures were refined by the Rietveld method, which showed that the oxygen coordination polyhedra were highly distorted. X-ray photoelectron spectroscopy revealed a similar Fe 2þ /Fe 3þ ratio (~27/73) in different RIGs. The origins of dielectric responses were identified by cross examination of the permittivity, modulus and impedance presentations. Fitting experimental data with the Debye-type and Maxwell-Wagner models revealed that the dielectric relaxation at room temperature was dominated by the Debye-type process but as temperature increased, the Maxwell-Wagner effect gradually took over. The static permittivity was 750, 785 and 2653 for YIG, TbIG and LuIG, respectively. The particularly large permittivity of LuIG arose from a large difference between the distortions of FeO 6 and FeO 4 in the structure and therefore, a large dipole moment was created when electron hopping between the octahedral and tetrahedral sites took place.
Bi 1Àx Ca x CuSeO (x ¼ 0e0.3) was synthesized at 650 C in an air-tight system flowing with pure a... more Bi 1Àx Ca x CuSeO (x ¼ 0e0.3) was synthesized at 650 C in an air-tight system flowing with pure argon. The Ca doping resulted in an increase in the thermoelectric figure of merit (ZT) as the consequence of increased carrier concentration. X-ray photoelectron spectroscopy (XPS) was carried out to check the oxidation states in Bi 1Àx Ca x CuSeO. The results indicated that in addition to the expected Bi 3þ and Cu 1þ , there existed Bi 2þ and Cu 2þ in the undoped BiCuSeO, whereas in the Ca-doped BiCuSeO, Bi 4þ , Cu 3þ and Cu 2þ were observed. The Ca dopant was confirmed to be in the 2þ oxidation state. Two broad peaks centered at 54.22 and 58.59 eV were recorded in the vicinity around the binding energy of Se 3d. The former is often observed in the Se-containing intermetallics while the latter is often found in the Secontaining oxides, indicating that along with the expected SeeCu bonding, a bonding between Se and O may also exist. Based on the XPS results, the charge compensation mechanisms were proposed for Bi 1Àx Ca x CuSeO, which may shed some light on the origins of charge carriers. BiCuSeO based oxides have recently be discovered to have a large ZT comparable to the best alloys currently in use, because of the large Seebeck coefficient and small thermal conductivity. However, their electrical conductivity is lower compared to the best thermoelectrics. This work may provide some hints for the further improvement of ZT in BiCuSeO based oxides.
Abstract Crednerite CuMnO 2 ceramic has been investigated for the first time in this study as a p... more Abstract Crednerite CuMnO 2 ceramic has been investigated for the first time in this study as a potential thermoelectric material based on its low band gap and layer structure. Undoped CuMn 1+ x O 2 (0≤ x ≤0.2) samples with a delafossite structure were prepared via solid-state reactions and sintered at 1353 K for 2.5 h in argon. The phase, microstructure, and thermoelectric properties of the CuMn 1+ x O 2 (0≤ x ≤0.2) samples were discussed by adjusting the sample composition. The crystallinity and microstructure of sintered bulks were analyzed using X-ray diffraction and scanning electron microscopy, respectively. The thermoelectric properties of the samples were studied from room temperature to 573 K. CuMn 1+ x O 2 sintered bulks showed a relative density of 91% and a layered structure. The composition affected the phases and thermoelectric properties of sintered bulks. Bulks with the crednerite CuMnO 2 phase were only obtained at x =0.1 or 0.143. The crednerite CuMn 1+ x O 2 (0≤ x ≤0.2) samples were p-type semiconductors. The Seebeck coefficient ( S ) increased and the electrical conductivity ( σ ) decreased with Mn content up to x =0.1. Excess Mn content ( x >0.143) decreased the Seebeck coefficient and increased electrical conductivity. The power factor (PF) of the CuMn 1+ x O 2 (0≤ x ≤0.2) samples improved due to the significant increase in the Seebeck coefficient. The thermal conductivity ( κ ) decreased with increasing temperature. The lowest κ value (6.79 W m −1 K −1 ) was found for the CuMn 1.1 O 2 sample at 573 K. The dimensionless figure of merit ZT values of the undoped CuMn 1+ x O 2 bulks are too small for these bulks to be candidates for thermoelectric materials due to weak electrical conductivity.
Well crystalized CIGS films were deposited at below 300 C by PLD. The chalcopyrite-phase f... more Well crystalized CIGS films were deposited at below 300 C by PLD. The chalcopyrite-phase films were obtained without further selenization. Increasing repetition rate from 5 Hz to 1 kHz increased film crystallinity. Low rate suppressed Se loss while high rate suppressed Ga loss in the films. P-N junctions made with 1 kHz grown films showed single ideality factor of 1.3.
Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the ... more Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the radio frequency magnetron sputtering method. The structure evolution with annealing temperature of the predominantly (002)-oriented Li-doped ZnO (LZO) films after in-situ post-annealing process is determined. The largest values of the piezoelectric coefficient (d33) and the remnant polarization (Pr) (22.85 pm/V and 0.655 μC/cm2, respectively) are obtained for LZO films post-annealed at 600 °C, which can be attributed to the predominant (002)-oriented crystalline structure, the release of intrinsic residual compressive stress, and less non-lattice oxygen.
Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray phot... more Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu2+/Eu3+ and Ti4+/Ti3+ in EuTiO3 and Eu0.7Ba0.3TiO3, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co2+/Ni2+ in Eu0.7Ba0.3TiO3 resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti3+ numbers and an increase in Eu3+ numbers. On the other hand, Ba2+ doping led to an increased lattice parameter due to its larger ionic size than Eu2+, whereas the Co2+/Ni2+ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu0.7Ba0.3TiO3 exhibited a clear ferroelectricity, which persisted in the Co2+/Ni2+ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu0.7Ba0.3TiO3 remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co...
Abstract BiCuSeO epitaxial films were grown on (001)/(110) SrTtO3 (STO) substrates by RF magnetro... more Abstract BiCuSeO epitaxial films were grown on (001)/(110) SrTtO3 (STO) substrates by RF magnetron sputter deposition. A sputter power over 40 W was needed to transfer target composition to substrate, leading to a fast deposition rate over 17.5 nm/min. The films grown on (001) STO at 100−150 °C showed a dominant [110] orientation instead of [001], which was what expected in view of the smallest lattice misfit and perfect match in symmetry between (001) BiCuSeO and (001) STO. This was caused by slow film growth along c-axis that could not follow the fast deposition rate, whereas a fair lattice misfit, combined with strong bonding along directions vertical to c-axis, entails the growth along [110] at low temperature. Preferred [001] orientation only occurred at high temperature (>350 °C) when the kinetic process was accelerated. However, high temperature and the fast deposition rate imposed by the required sputter power led to the growth of multiple orientations. Epitaxial growth of (001) BiCuSeO film on (001) STO was achieved after the deposition rate was reduced to 0.67 nm/min by a periodically opening and closing shutter installed in front of sputter target. Interestingly, (001) BiCuSeO epitaxial film could be grown on (110) STO at a fast deposition rate of 15 nm/min. Such films on (110) STO had to be grown at higher temperature (∼500 °C). In contrast to (001) STO, (110) STO did not had good lattice match with any BiCuSeO orientation other than [001], so fast kinetic process only promoted the growth of single [001] orientation.
BiFeO 3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from ch... more BiFeO 3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from chemical solution via hydrothermal route at 200 °C. However, for many applications, ceramic samples of reasonable bulk density (>80%) have to be sintered at temperature over 700 °C, which in this case results in a significant reduction in resistivity due to increased amounts of Fe 2+. Interestingly, doping of a few percent Sb minimized such a problem and the sintered Sb:BFO ceramics retained a similarly high resistivity as samples cold-pressed from the chemical-solution synthesized powders. However, for cold-pressed samples, Sb:BFO actually had higher conductivity than undoped BFO. Temperature-dependent conductivity showed that cold-pressed samples of both undoped and Sb doped BFO had the similar activation energy of 1.0 eV, typical for electrons trapped in oxygen vacancies. After sintering, the activation energy of Sb:BFO remained almost unchanged, but the activation energy of undoped BFO changed to 0.4 eV, which is associated to electron hopping between Fe 2+ /Fe 3+. X-ray photoelectron spectroscopy (XPS) showed a significant increase in Fe 2+ /Fe 3+ ratio from 6.6/93.4 to 25.7/74.3 in undoped BFO after sintering, while for 1% Sb doped BFO the increase was much milder from 10.9/89.1 to 14.1/85.9. XPS also showed that Sb had single +3 oxidation state before sintering, but after sintering a fairly large portion of Sb 5+ occurred. So, charge compensation for oxygen vacancies in undoped BFO was achieved dominantly by reduction of Fe 3+ to Fe 2+ , while in Sb:BFO it was achieved more by cation vacancies. Highlights Pure and Sb-doped BiFeO 3 were synthesized by hydrothermal method at 200 °C. As-prepared samples showed typical hysteresis loops of ideal ferroelectrics. Sintering the hydrothermal powders resulted in a great decrease in resistivity. Sb doping helped to retain a similarly high resistivity in the sintered samples. Sb doping kept oxygen vacancies rather than Fe 2+ ions as the main defects.
Abstract La and Y doped SrTiO3 (STO) thin films were grown on (100) oriented pure STO single crys... more Abstract La and Y doped SrTiO3 (STO) thin films were grown on (100) oriented pure STO single crystal substrates by the RF magnetron sputter technique. The La/Y doped layer is intended for use as bottom electrode for epitaxial growth of various functional oxide films. The results showed that one of the crucial deposition parameters to achieve the epitaxial growth of La/Y doped STO films with a pure phase and good electrical conductivity was sputter power, which must exceed 120 W. The conductivity of the grown films increased with the increase in sputter power and the La0.1Sr0.9TiO3 (LSTO) films deposited at 200 W had the highest conductivity of 292 S/cm. This was ascribed to the increased percentage of Ti3+ ions in the sputtered films, which was confirmed by the X-ray photoelectron spectroscopy. LSTO films deposited at 130 W were (100) oriented but the increased sputter power promoted the growth of a second texture, i.e. (110). On the other hand, Y0.08Sr0.92TiO3 (YSTO) films were able to keep the unique (100) texture in the films deposited over a range of sputter powers up to 200 W, although the conductivity of YSTO was lower, which was 98.0 S/cm for the films sputtered at 200 W.
Polycrystalline samples with nominal composition FeTe1−x S x (x = 0.00-0.30) were synthesized via... more Polycrystalline samples with nominal composition FeTe1−x S x (x = 0.00-0.30) were synthesized via solid state reaction method with intermittent grinding in argon gas flow. Xray diffraction (XRD) patterns revealed the tetragonal structure (space group P4/nmm) of the samples with the presence of impurities Fe3O4 and FeTe2. By substitution with S, the a and c lattice parameters shrink probably due to the smaller ionic radius of S2− compared to Te2−. Scanning electron microscopy images showed that the samples developed plate-like grains with increasing S substitution. Substitution of Te with S suppresses the structural transition of the parent compound FeTe as shown by both the temperature dependence of resistance and magnetic moment measurements. All of the S-substituted samples showed a rapid drop of resistance at around 9-10 K but zero resistance down to 4 K was not observed. In addition, negative magnetic moment corresponds to diamagnetism was detected in the samples for x = 0.25 and 0.30 suggesting the coexistence of magnetic and superconducting phase in these samples. The magnetization hysteresis loops measured at room temperature showed ferromagnetic behavior for the pure and S substituted samples. However, the magnetization, rentivity and coercivity decreased with S content.
Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO) þ BiFeO 3 (BFO) composite films were co-sputtered on (001) SrTiO 3 ... more Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO) þ BiFeO 3 (BFO) composite films were co-sputtered on (001) SrTiO 3 (STO) substrates, which were confirmed to be epitaxial by both 1D and 2D X-ray diffractions. Conductive LaNiO 3 (LNO) was sputtered on STO as bottom electrode for magnetoelectric (ME) measurement across film. The LNO layer was epitaxial and composed of nanocolumns perpendicular to substrate, which are helpful in reducing "substrate clamp" effect because of the flexibility of nanocolumns. NZFO þ BFO films cosputtered on LNO/STO were also epitaxial but both out-of-plane and in-plane textures of BFO were degraded with full width at half maximum of 1.974 and 2.55 , respectively, compared to 1.163 and 1.71 in films grown on bare STO. There were apparent tetragonal distortions in NZFO and BFO due to compressive in-plane strains imposed by epitaxial growth of larger NZFO and BFO lattices on smaller LNO lattice. Transmission electron microscopy (TEM) showed that the composite films were compact with clear boundaries between the NZFO and BFO phases, which were uniformly distributed with the ratio of about 35% NZFO and 65% BFO. Cross-sectional TEM revealed that crystallographic planes were very well aligned between BFO and LNO, but NZFO planes tilted 7.5 due to large lattice misfit. When measured vertically from LNO to top surface of composite film, large ME voltage coefficient at zero bias field was observed, which was 911 mVcm À1 Oe À1 at the frequency of 8 kHz. This was ascribed to the large heteroepitaxial strains in composite films.
Multiferroic composite films of Ni0.5Zn0.5Fe2O4 (NZFO) + BiFeO3 (BFO) were deposited on Si and La... more Multiferroic composite films of Ni0.5Zn0.5Fe2O4 (NZFO) + BiFeO3 (BFO) were deposited on Si and LaNiO3 (LNO) buffered Si substrates at 700 °C, by co-sputtering from two individual targets of the components.
Rare-earth iron garnets (RIG, R ¼ Y, Tb and Lu) were sintered at 1350 C. Their crystal structures... more Rare-earth iron garnets (RIG, R ¼ Y, Tb and Lu) were sintered at 1350 C. Their crystal structures were refined by the Rietveld method, which showed that the oxygen coordination polyhedra were highly distorted. X-ray photoelectron spectroscopy revealed a similar Fe 2þ /Fe 3þ ratio (~27/73) in different RIGs. The origins of dielectric responses were identified by cross examination of the permittivity, modulus and impedance presentations. Fitting experimental data with the Debye-type and Maxwell-Wagner models revealed that the dielectric relaxation at room temperature was dominated by the Debye-type process but as temperature increased, the Maxwell-Wagner effect gradually took over. The static permittivity was 750, 785 and 2653 for YIG, TbIG and LuIG, respectively. The particularly large permittivity of LuIG arose from a large difference between the distortions of FeO 6 and FeO 4 in the structure and therefore, a large dipole moment was created when electron hopping between the octahedral and tetrahedral sites took place.
Bi 1Àx Ca x CuSeO (x ¼ 0e0.3) was synthesized at 650 C in an air-tight system flowing with pure a... more Bi 1Àx Ca x CuSeO (x ¼ 0e0.3) was synthesized at 650 C in an air-tight system flowing with pure argon. The Ca doping resulted in an increase in the thermoelectric figure of merit (ZT) as the consequence of increased carrier concentration. X-ray photoelectron spectroscopy (XPS) was carried out to check the oxidation states in Bi 1Àx Ca x CuSeO. The results indicated that in addition to the expected Bi 3þ and Cu 1þ , there existed Bi 2þ and Cu 2þ in the undoped BiCuSeO, whereas in the Ca-doped BiCuSeO, Bi 4þ , Cu 3þ and Cu 2þ were observed. The Ca dopant was confirmed to be in the 2þ oxidation state. Two broad peaks centered at 54.22 and 58.59 eV were recorded in the vicinity around the binding energy of Se 3d. The former is often observed in the Se-containing intermetallics while the latter is often found in the Secontaining oxides, indicating that along with the expected SeeCu bonding, a bonding between Se and O may also exist. Based on the XPS results, the charge compensation mechanisms were proposed for Bi 1Àx Ca x CuSeO, which may shed some light on the origins of charge carriers. BiCuSeO based oxides have recently be discovered to have a large ZT comparable to the best alloys currently in use, because of the large Seebeck coefficient and small thermal conductivity. However, their electrical conductivity is lower compared to the best thermoelectrics. This work may provide some hints for the further improvement of ZT in BiCuSeO based oxides.
Abstract Crednerite CuMnO 2 ceramic has been investigated for the first time in this study as a p... more Abstract Crednerite CuMnO 2 ceramic has been investigated for the first time in this study as a potential thermoelectric material based on its low band gap and layer structure. Undoped CuMn 1+ x O 2 (0≤ x ≤0.2) samples with a delafossite structure were prepared via solid-state reactions and sintered at 1353 K for 2.5 h in argon. The phase, microstructure, and thermoelectric properties of the CuMn 1+ x O 2 (0≤ x ≤0.2) samples were discussed by adjusting the sample composition. The crystallinity and microstructure of sintered bulks were analyzed using X-ray diffraction and scanning electron microscopy, respectively. The thermoelectric properties of the samples were studied from room temperature to 573 K. CuMn 1+ x O 2 sintered bulks showed a relative density of 91% and a layered structure. The composition affected the phases and thermoelectric properties of sintered bulks. Bulks with the crednerite CuMnO 2 phase were only obtained at x =0.1 or 0.143. The crednerite CuMn 1+ x O 2 (0≤ x ≤0.2) samples were p-type semiconductors. The Seebeck coefficient ( S ) increased and the electrical conductivity ( σ ) decreased with Mn content up to x =0.1. Excess Mn content ( x >0.143) decreased the Seebeck coefficient and increased electrical conductivity. The power factor (PF) of the CuMn 1+ x O 2 (0≤ x ≤0.2) samples improved due to the significant increase in the Seebeck coefficient. The thermal conductivity ( κ ) decreased with increasing temperature. The lowest κ value (6.79 W m −1 K −1 ) was found for the CuMn 1.1 O 2 sample at 573 K. The dimensionless figure of merit ZT values of the undoped CuMn 1+ x O 2 bulks are too small for these bulks to be candidates for thermoelectric materials due to weak electrical conductivity.
Well crystalized CIGS films were deposited at below 300 C by PLD. The chalcopyrite-phase f... more Well crystalized CIGS films were deposited at below 300 C by PLD. The chalcopyrite-phase films were obtained without further selenization. Increasing repetition rate from 5 Hz to 1 kHz increased film crystallinity. Low rate suppressed Se loss while high rate suppressed Ga loss in the films. P-N junctions made with 1 kHz grown films showed single ideality factor of 1.3.
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Papers by Xiaoding Qi