Effect of Doping and Defect Structures on Thermo Physical Properties of Thermoelectric Materials

PhDDevelopment of thermoelectric materials to date has focused on materials that can operate at lower temperatures. However; there is now an increased need to develop materials for higher temperature applications. In this research, medium to high temperature oxide and non-oxide thermoelectric materials were fabricated and characterized. For oxide thermoelectric materials, La4Ti4O14 and Sr4Nb4O14 were chosen. These compounds are members of the homologous A4B4O14 series and possess perovskite-like layered structure (PLS). PLS compounds have low thermal conductivity due to a layered structure compared to the perovskite materials (e.g. SrTiO3). These atomic scale layers help to reduce the thermal conductivity of PLS compounds. Doping in PLS materials also creates atomic scale disorders. The effect of acceptor-donor doping and oxidation-reduction on the thermal conductivity of PLS ceramics were investigated in relation to mass contrast and compositional non-stoichiometry. High resolution TEM and XPS revealed that acceptor doping of La4Ti4O14 produced nanoscale intergrowth regions of n=5 layered phase inside n=4 layered phase, while donor doping produced nanoscale intergrowth regions of n=3 layered structure. As a result of these nanoscale intergrowths, the thermal conductivity value reduced by ~ 20% compared to the theoretical value. Pure La4Ti4O14 has a thermal conductivity value of ~ 1.1 W/m.K which dropped to a value of ~ 0.98 W/m.K in Sr doped La4Ti4O14 and ~ 0.93 W/m.K in Ta doped La4Ti4O14. Pure Sr4Nb4O14 has a thermal conductivity value of ~ 1.05 W/m.K which dropped to ~ 0.6 W/m.K after La doping. The factors influencing the thermal conductivity of PLS compounds were also discussed.For non-oxide ceramics, CoSb3 was chosen due to its cage-like structure and ideal for the application of Phonon Glass Electron Crystal Concept. The cage like structure gives room to engineer its electrical and thermal properties without affecting the other. For the first time, CoSb3 stuffed with Yb and substituted with Te (YbyCoSb3-xTex) was synthesized by mechanical alloying and spark plasma sintering. The electrical and thermal properties were characterized for pure and doped material. A Seebeck coefficient value of ~ 160 μV/K was obtained at ~ 600-800 K for Yb0.075CoSb2.85Te0.15. The electric resistivity dropped from ~ 1000 μΩm for pure CoSb3 to ~ 9 μΩm for Yb0.075CoSb2.85Te0.15. Lattice thermal conductivity was significantly reduced to a very low value of 1.17 W/m.K by the addition of Yb atoms into CoSb2.85Te0.15 without significantly affecting its Seebeck coefficient and electrical resistivity. This value is comparable to those produced by the costly processing of nanostructured materials. A zT value of ~ 0.70 was obtained at 600 K. This research has shown that by engineering the defect chemistry of thermoelectric materials, it is possible to significantly reduce their thermal conductivity without compromising their electrical properties.Financial support from QMUL and Nanoforce Technology Limited, U.

Fabrication of Piezoelectric Composites Using High-Temperature Dielectrophoresis

In this paper, we present a method to create a highly sensitive piezoelectric quasi 1–3 composite using a thermoplastic material filled with a piezoelectric powder. An up-scalable high-temperature dielectrophoresis (DEP) process is used to manufacture the quasi 1–3 piezoelectric polymer-ceramic composites. For this work, thermoplastic cyclic butylene terephthalate (CBT) is used as a polymer matrix and PZT (lead zirconium titanate) ceramic powder is chosen as the piezoelectric active filler material. At high temperatures, the polymer is melted to provide a liquid medium to align the piezoelectric particles using the DEP process inside the molten matrix. The resulting distribution of aligned particles is frozen upon cooling the composite down to room temperature in as little as 10 min. A maximum piezoelectric voltage sensitivity (g33) value of 54 ± 4 mV·m/N is reported for the composite with 10 vol% PZT, which is twice the value calculated for PZT based ceramics

Factors Affecting the Piezoelectric Performance of Ceramic-Polymer Composites: A Comprehensive Review

Over the past few decades, piezoelectric materials have emerged as one of the powerful platforms for energy harvesting applications. Nowadays, they offer sustainable solutions for high-performance, low-power electronic devices required in numerous industry fields such as aerospace, automotive, and biomedical devices. Ceramic-polymer piezoelectric composites combine the advantages of ceramics as well as the mechanical flexibility and weight of the polymers. This paper reviews various factors such as crystallinity, the orientation of filler particles, etc. that affect the piezoelectric performance. Generally, the piezoelectric performance can be measured using a variety of key parameters, such as piezoelectric charge coefficient (d33), piezoelectric voltage coefficient (g33), and dielectric constant (). The parameters are presented throughout the review to justify the enhancement of the piezoelectric performance of piezoceramic-polymer composites

Tunable microwave dielectric properties in SrO‐V2O5 system through compositional modulation

Adjustment on resonance frequency stability against the sintering temperature of Sr3V2O8 was realized by adjusting the Sr:V mole ratio. Effects of Sr:V ratio on sintering behavior and dielectric properties of Sr3V2O8 were studied. The sintering temperature was sucessfully reduced to 950°C from 1150°C. With increasing vanadium content, both relative permittivity and quality factor decreased, while the temperature coefficient of resonance frequency shifted from positive to negative values. Especially, a near‐zero τf of −1.1 ppm/°C along with a low permittivity (εr) of 9.8 and a quality factor Q × f of 24 120 GHz was successfully achieved in Sr3‐yV2O8‐y ceramic (y = 0.6, sintered at 950°C). The wide compositional and processing adjustment window, favorable dielectric performances, and good chemical compatibility with silver render Sr3‐yV2O8‐y ceramics potential candidates in multilayer electronic devices

In Situ Printing and Functionalization of Hybrid Polymer-Ceramic Composites Using a Commercial 3D Printer and Dielectrophoresis—A Novel Conceptual Design

Three-dimensional printing-based additive manufacturing has emerged as a new frontier in materials science, with applications in the production of functionalized polymeric-based hybrid composites for various applications. In this work, a novel conceptual design was conceived in which an AC electric field was integrated into a commercial 3D printer (-based fused filament fabrication (FFF) working principle) to in situ manufacture hybrid composites having aligned ceramic filler particles. For this work, the thermoplastic poly lactic acid (PLA) was used as a polymer matrix while 10 vol% KNLN (K0.485Na0.485Li0.03NbO3) ceramic particles were chosen as a filler material. The degree of alignment of the ceramic powders depended upon print speed, printing temperature and distance between electrodes. At 210 °C and a 1 kV/mm applied electric field, printed samples showed nearly complete alignment of ceramic particles in the PLA matrix. This research shows that incorporating electric field sources into 3D printing processes would result in in situ ceramic particle alignment while preserving the other benefits of 3D printing

Experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites

A comprehensive experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites (EP/CNF) is presented in this study. The machinability indicators including cutting force and surface roughness have been investigated. Tensile properties, morphology of tensile fracture surfaces, glass transition temperature, machined chip morphology, and machined surface morphology were also characterised. To investigate the effect of both workpiece material properties and operating conditions on the machinability of EP/CNF, three controlled quantitative factors were selected at different levels, namely CNF loading, cutting speed and feed per tooth (FPT). Micromilling experiments were performed on an ultra-precision desktop micro-machine tool using titanium‑carbon-nitride (TiCN) coated micro-end mills. Among all compositions with CNF concentration ranging from 0.3 to 1 wt%, EP/1 wt% CNF exhibited the best machinability among other nanocomposites with its lowest cutting force of approximately 0.5 N and surface roughness of 0.18 μm. Size effect appeared at FPT below minimum uncut chip thickness (MUCT) indicated by the strong deterioration of surface quality owing to the dominant ploughing effect

A Review on Nanocomposites. Part 2: Micromachining

Micromachining of nanocomposites is deemed to be a complicated process due to the anisotropic, heterogeneous structure and advanced mechanical properties of these materials associated with the size effects in micromachining. It leads to poorer machinability in terms of high cutting force, low surface quality, and high rate of tool wear. A comprehensive review on mechanical properties of nanocomposites aiming to pointout their effects on micro-machinability has been addressed in part 1. In part 2, the subsequent micro-machining processes are critically discussed based on relevant studies from both experimental and modeling approaches. The main findings and limitations of these micro-machining methods in processing nanocomposites have been highlighted together with future prospects

Synthesis of LiBGeO4 using compositional design and its dielectric behaviors at RF and microwave frequencies

Borates are promising candidates as dielectric substrate materials in low temperature cofired ceramics technology (LTCC) due to their relative low sintering temperatures and relative permittivities compared to their counterparts. However, synthesizing borates having single-phase is still challenging because of the volatility and hydrophilicity of boron resources. In this work, a compositional design was utilized to synthesize single-phase LiBGeO 4 ceramics over a broad temperature range from 600 to 840 °C. Radio-frequency dielectric behaviours featured a strong temperature dependence, especially at high temperatures (>400 °C), which is related to the thermally activated polarizations. LiBGeO 4 ceramic sintered at 820 °C has optimum microwave dielectric properties with the relative permittivity (ε r) of 6.28, a quality factor (Q × f) of 21,620 GHz, and a temperature coefficient of resonance frequency (τ f) of -88.7 ppm/ °C. LiBGeO 4 also showed chemical inertness when cofired with silver (Ag), provided an evidence for its utilization in LTCC technology. Overall, this work provides a strategy for facile synthesis of phase pure borates, via the proposed two-step process to obtain stable boron resources

A Review on Nanocomposites. Part 1: Mechanical Properties

Micromachining of nanocomposites is deemed to be a complicated process due to the anisotropic, heterogeneous structure, and advanced mechanical properties of these materials associated with the size effects in micromachining. It leads to poorer machinability in terms of high cutting force, low surface quality, and high rate of tool wear. In part 1 of this two-part review paper, a comprehensive review on mechanical properties of various nanocomposites will be presented while the second part of the paper will focus on the micro-machinability of these nanocomposite materials

Low-temperature sintering, dielectric performance, and far-IR reflectivity spectrum of a lightweight NaCaVO4 with good chemical compatibility with silver

As one of the candidates of low-temperature cofired ceramics (LTCC), the sintering behavior, dielectric properties, and silver chemical compatibility of NaCaVO4 were studied. The bulk ceramic samples were densified well at a low sintering temperature of 720 oC with an optimal bulk density of 2.87 g/cm3. Excellent microwave dielectric properties with a low relative permittivity εr ~ 9.9, a high quality factor Q × f ~ 31,600 GHz, and a temperature coefficient of resonance frequency τf ~ -66.5 ppm/°C were achieved at 11.5 GHz. The far infrared reflectivity spectra revealed the intrinsic dielectric properties (relative permittivity of 8.34 and dielectric loss of 2.7×10-4) are comparable to the measured values. The absence of chemical reaction between NaCaVO4 and silver at firing temperature, evidenced by X-ray diffraction and energy dispersive spectroscopy indicated the potential prospect for LTCC application. Furthermore, the very low density of NaCaVO4 opens up an avenue for its potential applications in light-weight devices