Polycrystalline Pellet Research Articles

Ultralow Thermal Conductivity and Very High Seebeck Coefficient in Two-Dimensional TeSe2 Semiconductor.

Emerging chalcogenide-based two-dimensional (2D) materials possess various unique yet fully explored properties and are thus considered promising candidates for next-generation optoelectronic and energy conversion applications. Here, TeSe2 crystals with interesting thermoelectric features were synthesized using a simple solid-state reaction. High-resolution transmission electron microscopy reveals that TeSe2 stabilizes in a 2D atomic structure with helical chains, resembling 2D tellurene. The thermoelectric properties were measured from 2 to 390 K in a polycrystalline pellet, showing an ultralow thermal conductivity below 0.25 W m-1 K-1 and a very high positive Seebeck coefficient of up to 865 μV K-1. Particularly, the thermal conductivity shows a hysteresis effect upon temperature cycling, which is tentatively explained as cracks opening and partially closing. Optical measurements indicate that TeSe2 is a semiconductor with two bandgaps at 1.43 and 1.65 eV. These results highlight that TeSe2 is an intriguing 2D semiconductor with complex thermoelectric properties, which provides a platform to further study the interplay of emerging 2D structure, thermal, and electronic properties.

Thermoelectric properties of La1.96Sr0.04CuO4 for energy harvesting and storage applications

AbstractThe polycrystalline pellet of La2‐xSrxCuO4 with x = 0.04, has been sintered using the solid‐state reaction process. X‐ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) have been carried out for microstructural characterizations. The transport properties (electrical and thermal) of La2‐xSrxCuO4 for x = 0.04 have been investigated in the temperature range between 15 and 300 K with the aid of a closed‐cycle refrigerator (CCR). The DC four‐probe resistivity measurement confirms a decrease in resistivity with increasing temperature below 200 K. However, above 200 K, an increase in resistivity with temperature (metallic behaviour) has been noticed due to charge carrier localization. The Hall measurement conveys hole‐type conduction and phonon‐dominated mobility with carrier concentration ~1018/cm3, which lie somewhere between non‐degenerate and degenerate limit. The Seebeck coefficient also confirms hole type conduction and its temperature dependence can be explained in the light of the model applicable for mixed‐valence heavy fermions systems. The steady‐state method for thermal conductivity measurement has been employed. The plot of thermal conductivity (λ) as a function of temperature (T) shows a hump‐like structure at around 65 K, which may be attributed to an increase in the quasi‐particle mean free path. Further, the electronic and lattice (phononic) contributions to thermal conductivity have been separated using electrical conductivity data with the help of the Wiedemann‐Franz law. Our analysis confirms the phononic dominance in the thermal conductivity of the investigated sample. The figure of merit (ZT) estimated from the experimental data has a maximum value of 0.086 at 290 K and hence shows its inherent energy harvesting properties that can further be stored in a battery or capacitor.

The First Ring Enlargement Induced Large Piezoelectric Response in a Polycrystalline Molecular Ferroelectric.

Inorganic ferroelectrics have long dominated research and applications, taking advantage of high piezoelectric performance in bulk polycrystalline ceramic forms. Molecular ferroelectrics have attracted growing interest because of their environmental friendliness, easy processing, lightweight, and good biocompatibility, while realizing the considerable piezoelectricity in their bulk polycrystalline forms remains a great challenge. Herein, for the first time, through ring enlargement, a molecular ferroelectric 1-azabicyclo[3.2.1]octonium perrhenate ([3.2.1-abco]ReO4 ) with a large piezoelectric coefficient d33 up to 118 pC/N in the polycrystalline pellet form is designed, which is higher than that of the parent 1-azabicyclo[2.2.1]heptanium perrhenate ([2.2.1-abch]ReO4 , 90 pC/N) and those of most molecular ferroelectrics in polycrystalline or even single crystal forms. The ring enlargement reduces the molecular strain for easier molecular deformation, which contributes to the higher piezoelectric response in [3.2.1-abco]ReO4 . This work opens up a new avenue for exploring high piezoelectric polycrystalline molecular ferroelectrics with great potential in piezoelectric applications.

An electrically conductive metallocycle: densely packed molecular hexagons with π-stacked radicals

Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni6(NDI-Hpz)6(dma)12(NO3)6]·5DMA·nH2O (PMC-hexagon) (NDI-Hpz = N,N′-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H2O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2–2.1) × 10−4 S cm−1 (pressed pellet), which is superior to that of most NDI-based conductors including metal–organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.

Disorder and epitaxial strain control of metamagnetic transition, large saturation magnetization, and magneto-terahertz properties of YMn0.5Cr0.5O3 polycrystals and thin films

The multiferroic YMn0.5Cr0.5O3 (YMCO) compound possesses magnetic phases that are debatable for their origin in intrinsic and extrinsic attributes. To extract the contribution of the polycrystalline disorder as well as determine the effect of structural modifications and epitaxial strain on the magnetic properties, we have formed YMCO bulk in polycrystalline pellet and powder forms, and epitaxial thin films with a wide range of compressive and tensile strains. The temperature and field dependent magnetization of polycrystals discards the possibility of magnetization reversal and unveils the presence of multidomain states with soft and hard characters in powder form. The epitaxial YMCO films possess an extraordinary structural control of their magnetic properties, as the tuning of compressive strain results in an increase of 4–5 factors of magnitude in the saturation magnetic moment. This strain engineering further allows a tuning of the magnetic property from spin frustration/short-range ordering to the long-range ordering, which also induces a rare phenomenon of sharp step-like metamagnetic transition. The implementation of magneto-terahertz time-domain spectroscopy on this system does not show the manifestation of any magnon/electromaganon resonance typical of a magnetoelectric phase, thus suggesting either a very weak or lack of coupling between magnetic and electric ordered parameters in polycrystalline YMCO. This study resolves some longstanding issues on the magnetic and magnetoelectric phases in addition to the epitaxial control of large magnetization and metamagnetic transition being potentially relevant in several aspects of spintronics applications.

Rational Electronic and Structural Designs Advance BiCuSeO Thermoelectrics

AbstractIn this work, a record high thermoelectric figure‐of‐merit ZT of 1.6 ± 0.2 at 873 K in p‐type polycrystalline Bi0.94Pb0.06CuSe1.01O0.99 by a synergy of rational band manipulation and novel nanostructural design is reported. First‐principles density functional theory calculation results indicate that the density of state at the Fermi level that crosses the valence band can be significantly reduced and the measured optical bandgap can be enlarged from 0.70 to 0.74 eV by simply replacing 1% O with 1% Se, both indicating a potentially reduced carrier concentration and in turn, an improved carrier mobility and a boosted power factor up to 9.0 µW cm−1 K−2. Meanwhile, comprehensive characterizations reveal that under Se‐rich condition, Cu2Se secondary microphases and significant lattice distortions triggered by Pb‐doping and Se‐substitution can be simultaneously achieved, contributing to a reduced lattice thermal conductivity of 0.4 W m−1 K−1. Furthermore, a unique shear exfoliation technique enables an effective grain refinement with higher anisotropy of the polycrystalline pellet, leading to a further improved power factor up to 10.9 µW cm−1 K−2 and a further reduced lattice thermal conductivity of 0.30 W m−1 K−1, which gives rise to record high ZT.

Comparative structural and optical studies on pellet and powder samples of BaTiO3 near phase transition temperature

In order to investigate the possible effect of inter-granular strain on the physical properties of Barium titanate (BaTiO3), comparative studies have been carried out on the polycrystalline pellet and its corresponding powder samples. For this purpose, the polycrystalline pellet sample of BaTiO3 has been prepared via conventional solid-state reaction route and powder is obtained by crushing the part of the prepared BaTiO3 pellet. The comparative room temperature structural studies, temperature dependent optical and Raman spectroscopy measurements have been carried out on the prepared pellet and powder samples. Room temperature X-ray diffraction and Raman analysis confirms the presence of extra amount of strain in pellet sample compared with that of the powder sample. Temperature dependent Raman analysis also suggests the difference in the high temperature tetragonal to cubic transition temperature in both cases. Temperature dependent optical absorption properties measured in terms of Urbach energy (EU), Urbach focus (E0) and Urbach relaxation clearly indicates the significant change in these quantities for both types of samples on BaTiO3. Present results strongly reveal the difference in structural, optical and vibrational properties of BaTiO3 especially across phase transition in pellet and its corresponding powder which clearly shows the importance of inter-granular strain at the grain boundaries.

Low-field microwave absorption in Zn1−x(Mn:Fe(Ni))xO (x = 0.02) system: hysteresis, line shapes and powdering effects

We studied the low-field microwave absorption (LFMA) in polycrystalline pellet and powdered (0.1–0.2 µm particle size) samples with a nominal composition of Zn1 − x(Mn:Fe(Ni))xO (x = 0.02). LFMA signals are stronger in the case of Mn:Fe co-doped ZnO, as compared to that of Mn:Ni co-doped ZnO. While the bulk samples show hysteresis, it disappears in the case of powdered samples. Further the line shapes of LFMA were modified with powdering, leading to small saturation fields for LFMA. This indicates that the microwave absorption in these powders is more sensitive than in the pellet form. We interpret these results in terms of interparticle–interfacial pinning. In this work, we clearly establish the low-field tunability of microwave absorption in the Zn1 − x(Mn:Fe(Ni))xO (x = 0.02) system, which is good for the applications as microwave absorbers with small field tunability as a functionality. This means LFMA can give inputs for the right choice of material for field-tunable microwave absorber design.

Intermittent Contact Alternating Current Scanning Electrochemical Microscopy: A Method for Mapping Conductivities in Solid Li Ion Conducting Electrolyte Samples

Intermittent contact alternating current scanning electrochemical microscopy (ic-ac-SECM) has been used to determine the electrochemical response to an ac signal of several types of materials. A conductive gold foil and insulating Teflon sheet were first used to demonstrate that the intermittent contact function allows the topography and conductivity to be mapped simultaneously and independently in a single experiment. Then a dense pellet of an electronically insulating but Li-ion conducting garnet phase, Al-substituted Li7La3Zr2O12 (LLZO), was characterized using the same technique. The polycrystalline pellet was prepared by classical ceramic sintering techniques and was comprised of large (~150 μm) grains. Critical information regarding the contributions of grain and grain boundary resistances to the total conductivity of the garnet phase was lacking due to ambiguities in the impedance data. In contrast, the use of the ic-ac-SECM technique allowed spatially resolved information regarding local conductivities to be measured directly. Impedance mapping of the pellet showed that the grain boundary resistance, while generally higher than that of grains, varied considerably, revealing the complex nature of the LLZO sample.

Anisotropic expansion and amorphization of Ga2O3 irradiated with 946 MeV Au ions

The structural response of β-Ga2O3 to irradiation-induced electronic excitation was investigated. A polycrystalline pellet of this material was irradiated with 946MeV Au ions and the resulting structural modifications were characterized using in situ X-ray diffraction analysis at various ion fluences, up to 1×1013cm−2. Amorphization was induced, with the accumulation of the amorphous phase following a single-impact mechanism in which each ion produces an amorphous ion track along its path. Concurrent with this phase transformation, an increase in the unit cell volume of the material was observed and quantified using Rietveld refinement. This unit cell expansion increased as a function of ion fluence before saturating at 1.8%. This effect is attributed to the generation of defects in an ion track shell region surrounding the amorphous track cores. The unit cell parameter increase was highly anisotropic, with no observed expansion in the [010] direction. This may be due to the structure of β-Ga2O3, which exhibits empty channels of connected interstitial sites oriented in this direction.

(Invited) Rate-Determining Step for Oxygen Reduction Reaction on Oxide Cathode in SOFC and Its Interpretation Based on Band Energy Diagram

It is well known that the rate-determining step (RDS) of highly reactive oxide cathodes like La1-xSrxCoO3 (LSC), La1-xSrxCo1-yFeyO3 (LSCF), and Ba1-xSrxCo1-yFeyO3 (BSCF), for solid oxide fuel cells (SOFCs) is the surface reaction of oxygen reduction reaction (ORR), shown as 1/2O2(g) + Vö -> Oo + 2h, while its elementary step is still under discussion to date. The authors focus on the surface reactivity, defined as interfacial conductivity, σe, which is inverse of area-specific resistance, using dense thin film cathode. In the present study, σe was estimated using conventional triple-electrode cells composed of thin dense cathodes with additional thin surface modified layer, either polycrystalline pellet of Gd-doped CeO2 (GDC) or single crystalline yttria-stabilized zirconia (YSZ) with GDC barrier layer, and porous platinum anode. Various type of cathodes of LaCoO3 (LC) and LSC with variable (La+Sr)/Co ratios as well as pure and BaPrO3 coated PrOx, which shows stoichiometric composition of Pr6O11 at low temperatures below ≈480˚C in high oxygen pressure regime. The compact cathode films of LC/LSC with typically 500 nm thick coated with surface modification layer of 0, 10, 20 and 40nm are sputtered at RT, followed by a post heat-treatment at 600 ˚C. PrOx is selected to examine the influence of electronic band structure by comparison with LC and LSC systems, since PrOx is known as a mixed Vö and electron conductor in contrast to Vö and electron hole dominance in nonstoichiometric LC and LSCs. The measurements were made from 400 to 600˚C in pure O2 gas flow and the oxygen partial pressure (Po2) dependence of σe was examined at 600˚C from 10-2 to 1 atm. All the impedance data show three distinct semicircles in the impedance spectra, and specific types of resistance, corresponding to the electrolyte impedance, REl, middle frequency impedance, RMF, and low frequency impedance, RLF were extracted for further analyses. Although the origin of RMF is not well understood, it is reasonable to assume as the impedance that occurs out of cathode region; presumably an interfacial impedance between cathode and electrolyte. Although RMF of all the samples does not show the Po2 dependence, RLF clearly exhibits 1/2 power dependence for all the cathode materials from LC/LSC to PrOx system. The σe values of PrOx were 3-5 and 1-2 orders of magnitude lower than LSC and LC, respectively, which were greatly improved by adding merely 1nm thick surface layer of BaPrO3, presumably due to increased surface basicity. The rate equation with square root dependence of Po2 can be interpreted that the RDS is the reaction between the peroxide ion (O2 2-) adsorbed on the oxide cathode surface and Vö diffusing from cathode/electrolyte interface. Therefore, σe is proportional to the probability for bi-dentate immobile O2 2- to meet with Vö, expressed by the concentration product of [O2 2-][Vö]. Poor performance of PrOx can be attributed to a low [O2 2-] due to low basicity. The ab initio molecular dynamics were employed to observe the surface reaction of oxygen molecules on NiO with and without Vö at the surface. NiO was selected as a model system and GGA functional without spin correlation were employed. O2 molecules are adsorbed on NiO surface in step-by-step manner from mono-dentate O2 - to bi-dentate O2 2-. The increase of coordination number of O2 immediately increase the number of donated electron, maintaining one electron per coordination as average, and no further electron donation occurs without a help of Vö. Upon meeting of O2 2- and Vö, O2 2- splits and the third electron is donated to decompose O2 2- into OoX and O- ad. The observation above justify our conclusion from the experiments that the reaction between O2 2- and Vö is the RDS for ORR. A combination of O1s XAS and XPS measurements were made to estimate the electronic structure of oxide cathode materials to understand the energy level alignment between adsorbed O2 2- and cathode oxide surface. A higher EF of cathode material contributes to easy transfer of electrons, which is assumed to be represented, as a zeroth approximation, by the energy separation between EF and the maximum occupied partial DOS of O2p orbital in the valence band, ΔE, which corresponds to the basicity of the oxides. The σe for ORR observed shows excellent proportionality with ΔE, suggesting an important role of O2 2- in the cathodic ORR.

Eu3+ probe ion for rare-earth dopant site structure in sol–gel derived LiYF4 oxyfluoride glass–ceramic

Sol–gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass–ceramic containing Eu3+-doped LiYF4 nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass–ceramic Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably as Eu–O center and/or dimer centers in low symmetry (C2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization.

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

Raman spectroscopy and differential thermal analysis (DTA) and thermogravimetric analysis have been carried out on Cu-doped KH2PO4 (Cu-KDP). X-ray diffraction powder data reveal that the structure of the KDP crystal does not change with the additive Cu2+ ion. DTA analysis and Raman study of Cu-KDP as a function of temperature reveal that this compound undergoes two phase transitions at about Ttr =453 and 473 K. The electrical conductivity measurements on polycrystalline pellet of Cu-KDP (5) are performed from room temperature (RT) up to 495 K. Only one phase transition is observed at 470 K. The activation energy in the migration is 0.42 eV in the temperature range from RT to 470 K. For temperature above 470 K, the activation energy of the superprotonic phase is 1.87 eV.

Lateral Resolution of an Infrared Visible Optical Sum Frequency Generation Confocal Microscope

We have demonstrated experimentally that an infrared visible sum frequency generation (SFG) confocal microscope with an objective lens of numerical aperture 0.45 has a lateral resolution of 0.48±0.06 µm. As samples for the demonstration, we used a ZnS(100) wafer with a structure fabricated by a focused ion beam method and a ZnS polycrystalline pellet. The result was consistent with the theoretical resolution of a confocal microscope.

Transport properties of 2H-NbSe2: Effect of Ga-intercalation

We have prepared compounds of 2H-NbSe2 and 2H-GaxNbSe2 (x=5% and 10%) at 700 °C. 2H-NbSe2 single crystal exhibits superconducting and charge density wave (CDW) transition at 7.4 K and 35 K, respectively, whereas in polycrystalline pellet of poor residual resistance ratio (RRR) value, the CDW is absent. In Ga-intercalated compounds, the rapid decrease of superconducting transition temperature was noticed. Here we report the occurrence of two-band model in which the temperature dependence of mean free path length ‘l(ε)’ is the main factor for overall behavior of resistivity and thermopower.

Power dependent photoluminescence of ZnO

The effect of excitation power on the photoluminescence (PL) of three types of ZnO samples, including a polycrystalline pellet, thin film, and nanowires, was investigated. The intensity ratio of the defect to band edge emission as well as the overall spectral line shape of the defect emission was strongly affected by the excitation power. A blueshift of the defect emissions at high excitation powers was observed, indicating that donor-acceptor transitions are responsible for the defect emissions. The power dependent PL also suggests that comparisons of defect concentrations among ZnO samples may be possible only if the PL spectra are measured under the same excitation power.

Reactivity of H 2O gas with the surface of polycrystalline Li 2O pellet

Ultraviolet/X-ray photoelectron spectroscopy was performed for a polycrystalline pellet sample of Li 2O, in order to elucidate the interaction of H 2O with the Li 2O surface. By H 2O-gas exposures of 0–13 L (1 L = 1.33 × 10 −4 Pa s), a variation was induced in the electronic structure, corresponding to the formation of surface –OH via the dissociative adsorption of H 2O. No clear qualitative difference arose either in the electronic structure or in the reactivity to H 2O between the polycrystalline pellet and the Li 2O (1 1 1) single crystal. The variation in the contribution of –OH to the total intensity of the O 1s peak was investigated as a function of the amount of H 2O-gas exposure, and the result was discussed in conjunction with an adsorption/absorption model using a Monte Carlo simulation. It was indicated that not only adsorption, but also absorption up to a few surface layers occurred with a high probability even in the annealed sample.

Pulsed-laser deposition of NiMnSb thin films at moderate temperatures

Thin films of the half-Heusler alloy NiMnSb, which is predicted by band-structure calculations to be a half-metallic ferromagnet, were grown on various substrates (Si and InAs) using pulsed-laser deposition. The growth process occurred at moderate substrate temperatures (190–210 °C), the lowest ever reported for producing crystalline films, which are necessary for making devices. We have identified a set of experimental conditions that allow the growth of highly crystalline thin films with nanoscale smooth surfaces. The morphology of the films was examined by atomic force microscopy and scanning electron microscopy. The structural, magnetic and electrical properties of the films are consistent with those of the polycrystalline pellet of NiMnSb used as target.

Structural, Chemical, and Transport Properties of a New Clathrate Compound: Cs8Zn4Sn42

The synthesis and structural, chemical, and transport properties of a new clathrate compound with elemental composition Cs8Zn4Sn42 is reported. This compound is cubic with space group Pm3n and a lattice parameter of 12.1226(19) A. Single-crystal X-ray diffraction and structural refinement indicate that Cs atoms reside in the polyhedral cavities formed by the tetrahedrally bonded network of Zn and Sn atoms. The Cs atom in the larger polyhedra (tetrakaidecahedra) exhibit greater and more anisotropic thermal motion than those of the smaller polyhedra (dodecahedra). This “rattling” of the Cs contributes to the low thermal conductivity of this compound, obtained on a polycrystalline pellet. In addition, Zn is ordered in this compound. Electronic properties and the potential of this type of Sn-clathrate for thermoelectric applications are also discussed.

A Small-Angle X-ray Scattering Apparatus for Studying Biological Macromolecules in Solution

This paper describes the development of a simple laboratory-based small-angle X-ray scattering apparatus for the study of biological macromolecules in solution. The instrument is based on a two-circular-aperture collimation design combined with a conventional rotating-anode Cu Ka X-ray source, a graphite monochromator and a multiwire area detector. The geometry of the collimator, the beam-stop-to-detector distance and the thickness of the platinum foil of the defining aperture have been optimized to reduce background scattering. The effective Q range is from 0.01 to 0.33 Å−1, where Q = (4π sin θ)/λ is the magnitude of the scattering vector, 2θ is the scattering angle and λ is the wavelength of the X-rays. The length of the collimator, the pinhole sizes and the helium-flushed sample-to-detector path can be easily changed depending on the resolution and intensity requirements of an experiment. The diffraction pattern of a polycrystalline pellet of ammonium sulfate mounted about 2.5 cm in front of the beam stop and 40 cm in front of the detector is used to monitor changes in the incident-beam intensity as well as the differences in absorption of X-rays by the sample solutions and the solvents, to ensure correct background subtractions. Data collection is controlled by a computer through a parallel DMA (direct memory access) I/O module. Data collection and reduction software has been developed. The typical data collection time is about 2 h for a 5 mg ml−1 10 kDa protein dissolved in an aqueous solution. Examples of applications of this small-angle X-ray scattering instrument to studying protein size and conformation changes are presented.