The synthesis of carbon nanowalls (CNWs) necessitated a substrate heating temperature above 600 degrees C. This study describes a low -temperature growth at 225 degrees C and characterization of CNWs synthesized by the radical injection plasma -enhanced chemical vapor deposition. To investigate the effect of temperature on the growth process, CNWs were synthesized at various temperatures ranging from 200 degrees C to 700 degrees C. The morphology of the CNWs observed with a scanning electron microscope shows that wall density increases as substrate temperature decreases. The Raman spectroscopy analysis indicates that CNWs grown at 225 degrees C have higher defect levels than those grown at higher temperatures, such as 700 degrees C. Transmission electron microscopy confirmed the presence of multiple graphene layers in the CNWs grown at 225 degrees C. The water contact angle results revealed that CNWs grown at 225 degrees C had higher hydrophobicity than those grown at higher temperatures, opening up the potential for CNW applications.

The plasma functionalization of carbon nanowalls (CNWs) is of great interest due to their potential applications in electron field emission and energy storage devices. In this study, the effects of nitrogen on the growth and properties of CNWs are thoroughly investigated by varying a wide range of the N2 concentrations in the CH4/H2/N2 discharge plasma. Nitrogen-doped carbon nanowalls (N-CNWs) were synthesized in a single-step growth using the hydrogen radical injection plasma-enhanced chemical vapor deposition technique. The formation of dense N-CNWs was found to be enhanced by nitrogen and was three times higher in the sample with a 150 sccm N2 flow rate than in the sample without N2 addition (N-0). As the N2 concentration increased from 0 to 200 sccm, the morphology of CNWs changed from relatively straight to curly-like nanowall structures. A high-resolution optical emission spectroscopic observation shows that as N2 flow increased, CN radicals became the dominant species, suppressing the formation of C2 and CH. Photoelectron spectroscopy analysis revealed the nitrogen doping effect on graphene layers at high N2 concentrations. According to Raman and Transmission Electron Microscopy analyses, the N-CNWs had more branched and thicker nanowalls with higher defect concentrations than the N-0 sample. The wettability of N-CNWs was improved, particularly for the sample with a N2 flow rate of 150 sccm.

This work presents experimental results on the synthesis of carbon nanowalls (CNWs) with predefined morphology on the surface of the nanoporous alumina membrane using two different methods, namely radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) and radical-injection (RI)-PECVD. Obtained samples were characterized by the methods of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. From the microstructure analyses of CNWs, it has been observed that there is a time dependence on the reproducibility of membrane morphology by CNWs. At the early stage of nanowalls growth, nanowalls prefer to grow around the edges of nanopores and continue to grow vertically with time. In RF-PECVD, the nanopores' size begins to shrink drastically and pores are completely covered by secondary flake-like nanowalls after 25 minutes of growth. In the case of CNWs grown using RI-PECVD, nanowalls are more vertical and self-supported. This is because of the rapid and sustained production of hydrogen radicals that prevent the secondary growth of carbon nanowalls. In addition, the influence of pores diameter and membrane thickness on the growth of RI-PECVD CNWs was revealed.

We report a novel structure of carbon nanowalls (CNWs) synthesized by a two-step growth method using a radical injection plasma-enhanced chemical vapor deposition (RI-PECVD) as combined with a capacitively coupled plasma chemical vapor deposition (CCP-CVD). Scanning electron microscopy (SEM) observation shows that the CNWs grown by this method exhibit a multi-branched morphology with an average wall distance of-20 nm and a density of-74 wall units/mu m2, which is more than 5 times higher than that of CNWs synthesized by the single-step RI-PECVD method. These branched-CNWs are crystalline and comprised of multilayer graphene as confirmed by the Raman and high-resolution transmission electron microscopy (HRTEM) analyses. A static water contact angle of 150 degrees was obtained for the multi-branched CNWs indicating a superhydrophobic property. A possible growth mechanism of the branched-CNWs was also proposed through the investigation of the time-dependent growth from the second step.

The unique structural properties of vertically aligned graphene sheets or Carbon Nanowalls (CNWs) have attracted great interests for their potential for various applications in microelectronic devices, energy storage, and catalyst support materials. During the handling or operation of the devices, tension and/or pressure are often needed. Under such conditions, CNWs must undergo compression and stress. Therefore, the deformation mechanism and evolution behavior of the CNW structures under load play a critical role in the performance and reliability of the devices. In this study, the mechanical properties of a typical maze-like CNW structure synthesized by a Radial Injection Plasma Enhanced Chemical Vapor Deposition (RI-PECVD) technique were analyzed by employing the nanoindentation method. The measured compressive strength of the CNW structure was 50 MPa with a modulus E value of 28 GPa, which is significantly higher than that of other carbon-based materials. An elastoplastic behavior of a soft material was observed in high-resolution microscopy and a mechanism of deformation for CNWs is elucidated.

•The synthesis of carbon nanowalls on the surface of a nanoporous membrane of aluminum oxide.•The shape and height of synthesized carbon nanowalls depend on the pore size and thickness of the nanoporous substrate.•It was determined that the walls of the CNWs formed on highly porous alumina substrates are thinner, that is, have fewer graphene layers due to the higher vertical growth rate.

A two-step synthesis approach was utilized to grow CaMnO3 on M-, R- and C-plane sapphire substrates. Radio-frequency reactive magnetron sputtering was used to grow rock-salt-structured (Ca, Mn)O followed by a 3-h annealing step at 800 °C in oxygen flow to form the distorted perovskite phase CaMnO3. The effect of temperature in the post-annealing step was investigated using x-ray diffraction. The phase transformation to CaMnO3 started at 450 °C and was completed at 550 °C. Films grown on R- and C-plane sapphire showed similar structure with a mixed orientation, whereas the film grown on M-plane sapphire was epitaxially grown with an out-of-plane orientation in the [202] direction. The thermoelectric characterization showed that the film grown on M-plane sapphire has about 3.5 times lower resistivity compared to the other films with a resistivity of 0.077 Ωcm at 500 °C. The difference in resistivity is a result from difference in crystal structure, single orientation for M-plane sapphire compared to mixed for R- and C-plane sapphire. The highest absolute Seebeck coefficient value is − 350 µV K−1 for all films and is decreasing with temperature.

Some oxygen defective metal oxides, such as cerium and bismuth oxides, have recently shown exceptional electrostrictive properties that are even superior to the best performing lead-based electrostrictors, e.g. lead-magnesium-niobates (PMN). Compared to piezoelectric ceramics, electromechanical mechanisms of such materials do not depend on crystalline symmetry, but on the concentration of oxygen vacancy in the lattice. In this work, we investigate for the first time the role of oxygen defect configuration on the electro-chemo-mechanical properties. This is achieved by tuning the oxygen defects blocking barrier density in polycrystalline gadolinium doped ceria with known oxygen vacancy concentration, Ce0.9Gd0.1O2-x,x= 0.05. Nanometric starting powders of ca. 12 nm are sintered in different conditions, including field assisted spark plasma sintering (SPS), fast firing and conventional method at high temperatures. These approaches allow controlling grain size and Gd-dopant diffusion, i.e. via thermally driven solute drag mechanism. By correlating the electro-chemo-mechanical properties, we show that oxygen vacancy distribution in the materials play a key role in ceria electrostriction, overcoming the expected contributions from grain size and dopant concentration.

ZnSb is one of the promising low-cost p-type thermoelectric materials for constructing waste heat recovery devices operating in the medium temperature region (250 – 400 °C). To obtain high performance, these devices require stable and low resistance contacts between thermoelectric materials and metallic electrodes. In this paper, we investigate the joining of ZnSb to Ni and Ag electrodes using a commercial solder alloy S-Bond 400 and hot-pressing technique. Ti and Cr layers are also introduced as a diffusion barrier and microstructure at the interfaces is observed by scanning electron microscopy. We found that S-bond 400 solder reacts with Ag and Ni electrodes to form different alloys at the interfaces. Cr layer was found to be broken after joining, resulting in a thicker reaction/diffusion layer at the interface, while Ti layer was preserved.

Mass diffusion controls material structuring from the atomic to the macro-scale defining properties and functionalities.

Recently, flexible thermoelectric generator (FlexTEG) modules using organic or thin film materials have gained much attention due to their potential applications for, for example, wireless sensors and wearable power. However, the performance of these modules is poor and still far from the requirement for energy harvesting. Here, the traditional semiconductor packaging technique is adapted to fabricate a large‐scale FlexTEG, for use in energy harvesting on both planar and nonplanar surfaces. The module uses high‐performance bismuth‐telluride p‐ and n‐type chips on a flexible thin plastic substrate. Using a unique isotropic design for mounting the chips, a FlexTEG module consisting of 250 p‐n pairs is successfully fabricated on a 50 × 50 mm2 flexible substrate. The output power, mechanical strength, and bending properties are investigated at different temperature gradients and bending cycles. The module exhibits a maximum output power density of 158 mW cm−2 at dT = 105 K, corresponding to an efficiency value of 1.84%, which is comparable to a conventional bulk TEG. Mechanical tests reveal that the flexible module is reliable and stable during bending. These results open great potential for applications in portable, wearable, or implantable electronic devices.

In this work, nanostructured (La0.6Sr0.4)0.99CoO3 (LSC)-Ce0.8Gd0.2O1.9 (CGO) core-shell particles were prepared by precipitating CGO nanoparticles on the surface of LSC particles under hydrothermal conditions. The as-prepared core-shell particles were sintered by spark plasma sintering (SPS) and conventional sintering, and the microstructure evolution and densification behavior were studied. Dense microstructures were reached using both sintering methods at relatively low temperatures. In the case of SPS, the core-shell architecture was partially maintained and nano-structured CGO grains were formed, while conventional sintering led to the formation of larger CGO grains. This work covers a detailed characterization of (a) the individual LSC-CGO core-shell particles and (b) the composites after densification.

In many industrial processes, a large proportion of energy is lost in the form of heat. Thermoelectric generators can convert this waste heat into electricity by means of the Seebeck effect. However, the use of thermoelectric generators in practical applications on an industrial scale is limited in part because electrical, thermal, and mechanical bonding contacts between the semiconductor materials and the metal electrodes in current designs are not capable of withstanding thermal-mechanical stress and alloying of the metal–semiconductor interface when exposed to the high temperatures occurring in many real-world applications. Here we demonstrate a concept for thermoelectric generators that can address this issue by replacing the metallization and electrode bonding on the hot side of the device by a p-n junction between the two semiconductor materials, making the device robust against temperature induced failure. In our proof-of-principle demonstration, a p-n junction device made from nanocrystalline silicon is at least comparable in its efficiency and power output to conventional devices of the same material and fabrication process, but with the advantage of sustaining high hot side temperatures and oxidative atmosphere.

For applications in energy harvesting and environmentally friendly cooling, and for power sources in remote or portable applications, it is desired to enhance the efficiency of thermoelectric mater ...

Cu2+xSnSe3 (0 <= x <= 0.08) compounds were synthesized by conventional solid-state reaction followed by spark plasma sintering (SPS) technique. Transport properties of the samples were measured as a function of temperature in the temperature range 323-773 K. As compared to Cu2SnSe3 sample, the electrical resistivity (rho) is increased for the sample with x = 0.04, thereafter a decrease is seen with further increase in copper content. Analysis of electrical resistivity indicates that small poloron hoping model is operative in the entire temperature range for all samples. The positive Seebeck coefficient (S) for the samples in the entire temperature range indicates that the majority charge carriers are holes. The highest figure of merit, ZT (= 0.32) was achieved at 773 K for the sample Cu2.06SnSe3 which is about 3 times that of Cu2SnSe3 sample. Maximum thermoelectric compatibility factor (1.28 V-1) was observed at 673 K for the sample Cu2.08SnSe3. (C) 2018 Elsevier B.V. All rights reserved.

CrN thin films with an N/Cr ratio of 95% were deposited by reactive magnetron sputtering onto (0001) sapphire substrates. X-ray diffraction and pole figure texture analysis show CrN (111) epitaxial growth in a twin domain fashion. By changing the nitrogen versus argon gas flow mixture and the deposition temperature, thin films with different surface morphologies ranging from grainy rough textures to flat and smooth films were prepared. These parameters can also affect the CrN(x )system, with the film compound changing between semiconducting CrN and metallic Cr2N through the regulation of the nitrogen content of the gas flow and the deposition temperature at a constant deposition pressure. Thermoelectric measurements (electrical resistivity and Seebeck coefficient), scanning electron microscopy, and transmission electron microscopy imaging confirm the changing electrical resistivity between 0.75 and 300 m omega cm, the changing Seebeck coefficient values between 140 and 230 mu VK-1, and the differences in surface morphology and microstructure as higher temperatures result in lower electrical resistivity while gas flow mixtures with higher nitrogen content result in single phase cubic CrN.

Thermoelectric properties of chromium nitride (CrN)-based films grown on c-plane sapphire by dc reactive magnetron sputtering were investigated. In this work, aluminum doping was introduced in CrN (degenerate n-type semiconductor) by co-deposition. Under the present deposition conditions, over-stoichiometry in nitrogen (CrN1+δ) rock-salt structure is obtained. A p-type conduction is observed with nitrogen-rich CrN combined with aluminum doping. The Cr0.96Al0.04N1.17 film exhibited a high Seebeck coefficient and a sufficient power factor at 300 °C. These results are a starting point for designing p-type/n-type thermoelectric materials based on chromium nitride films, which are cheap and routinely grown on the industrial scale.

ScN-rich (Sc,Nb)N solid solution thin films have been studied, motivated by the promising thermoelectric properties of ScN-based materials. Cubic Sc1-xNbxN films for 0 ≤ x ≤ 0.25 were epitaxially grown by DC reactive magnetron sputtering on a c-plane sapphire substrate and oriented along the (111) orientation. The crystal structure, morphology, thermal conductivity, and thermoelectric and electrical properties were investigated. The ScN reference film exhibited a Seebeck coefficient of −45 μV/K and a power factor of 6 × 10−4 W/m K2 at 750 K. Estimated from room temperature Hall measurements, all samples exhibit a high carrier density of the order of 1021 cm−3. Inclusion of heavy transition metals into ScN enables the reduction in thermal conductivity by an increase in phonon scattering. The Nb inserted ScN thin films exhibited a thermal conductivity lower than the value of the ScN reference (10.5 W m−1 K−1) down to a minimum value of 2.2 Wm−1 K−1. Insertion of Nb into ScN thus resulted in a reduction in thermal conductivity by a factor of ∼5 due to the mass contrast in ScN, which increases the phonon scattering in the material.

We demonstrate an advanced approach using state of the art in situ transmission electron microscopy (TEM) to understand the interplay between nanostructures and thermoelectric (TE) properties of high-performance Mg-doped Zn4 Sb3 TE systems. By using the technique, microstructure and crystal evolutions of TE material have been dynamically captured as a function of temperature from 300 K to 573 K. On heating, we have clearly observed precipitation and growth of a Zn-rich secondary phase as nanoinclusions in the matrix of primary Zn4 Sb3 phase. Elemental mapping by STEM-EDX spectroscopy reveals enrichment of Zn in the secondary Zn6 Sb5 nanoinclusions during the thermal processing without decomposition. Such nanostructures strongly enhances phonon scattering, resulting in a decrease in the thermal conductivity leading to a zT value of 1.4 at 718 K.