Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

/pdf/review-liquid-phase-sintering-19fo5ee4w3.pdf

Review: liquid phase sintering

The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently

We report on the controlled synthesis of single-crystal platelets of alpha- and beta-Co(OH)2 via homogeneous precipitation using hexamethylenetetramine as a hydrolysis agent. The alpha- and beta-Co(OH)2 hexagonal platelets of several micrometers in width and about 15 nm in thickness were reproducibly yielded in rather dilute CoCl2 solutions in the presence and absence of NaCl at 90 degrees C, respectively. The phase and size control of the products were achieved by varying both CoCl2 and NaCl concentrations. Polarized optical microscope observations revealed clear liquid crystallinity of colloidal suspensions of these high aspect-ratio platelets. The as-prepared alpha-Co(OH)2 containing interlayer chloride ions was intercalated with various inorganic or organic anions, keeping its high crystallinity and hexagonal platelike morphology.

Selective and Controlled Synthesis of α- and β-Cobalt Hydroxides in Highly Developed Hexagonal Platelets

TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants

Research progress on polymer–inorganic thermoelectric nanocomposite materials

Plate-like particles of Ca3Co4O9 have been prepared in which Bi and Na were partially substituted for Ca to improve the thermoelectric performance. Successfully fabricated dense and highly textured ceramics have been obtained by combining the templated grain growth technique (TGG) with hot-pressing (HP). Ca3Co4O9 single crystals, which have alternating layers of Co–O and Ca–Co–O in the direction of the c-axis, show high electrical conductivity along the layer compared with that across the layer. Hence, a highly oriented TGG + HP specimen showed high electrical conductivity compared with the specimen sintered under uniaxial pressure (UP + PLS). The improved electrical conductivity with high Seebeck coefficient of the highly oriented specimen (TGG + HP) gave a high thermoelectric power factor of 5.9 × 10−4 W m−1 K−2 at 1073 K. The figures-of-merit at 773 K and 1073 K were calculated to be 8.54 × 10−5 K−1
(ZT
= 0.066 at 773 K) and 1.69 × 10−4 K−1
(ZT
= 0.18 at 1073 K), respectively. These values are quite high among Ca–Co–O polycrystalline systems reported so far.

Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing

Textured bulk ceramics have been prepared of various cobaltites including the common CdI2-type CoO2 layer (i.e., [Ca2(Co0.65Cu0.35)2O4]0.624[CoO2]: CCO-4-layer, [Bi2M2−xO4]p[CoO2]
				(M2
				= Sr2: BSCO, M2
				= Ca1Sr1: B(SC)CO, M2
				= Ca2: BCCO)) by using β-Co(OH)2
				(CdI2-type) platelets as reactive templates, and their thermoelectric (TE) properties have been examined. All the specimens were found to be highly c-axis aligned ceramics. Especially for CCO-4-layer, B(SC)CO and BCCO, the present study is the first report on the fabrication of textured ceramics. Our textured ceramic specimens exhibited larger values of electrical conductivity (σ) than the ceramics prepared by a conventional solid-state reaction (SSR specimens): e.g., σab
				= 0.46 × 104 S m−1 for the textured specimen and σSSR
				=
				∼0.26 × 104 S m−1 at 1060 K for the SSR specimen for BCCO. This study indicated that our preparation method using β-Co(OH)2 templates is a versatile and effective technique to fabricate cobaltites, including CoO2 layers, with a high degree of orientation and improved TE properties.

Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH) 2 platelets as reactive templates

Muon spin rotation-relaxation $(\ensuremath{\mu}\mathrm{SR})$ spectroscopy has been used to investigate the magnetic properties of polycrystalline ${\mathrm{Ca}}_{3\ensuremath{-}x}{\mathrm{M}}_{x}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ $(xl~0.5,M=\mathrm{Sr},\mathrm{Y},$ and Bi) and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$ samples in the temperature range between 2.5 and 300 K. It was found that ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ exhibits a magnetic transition at around ${T}_{c}=100\mathrm{K};$ at lower temperatures, two types of relaxation were observed using a weak transverse field $\ensuremath{\mu}\mathrm{SR}$ technique with $H=104\mathrm{Oe}:$ one with a fast relaxation rate ${\ensuremath{\lambda}}_{F}\ensuremath{\sim}10\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$ and the other with a slow ${\ensuremath{\lambda}}_{S}\ensuremath{\sim}0.1\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}.$ Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements suggest the existence of an incommensurate spin-density-wave (SDW) state below ${T}_{c}$ (i.e., ${T}_{c}{=T}_{\mathrm{SDW}}),$ although a ferrimagnetic $M\ensuremath{-}H$ loop was observed by a dc susceptibility measurement below 19 K. The substitution of Y or Bi for Ca increased ${T}_{\mathrm{SDW}},$ while the substitution of Sr for Ca did not affect ${T}_{\mathrm{SDW}}.$ This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$ showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2},$ we suggest that Co ions in the rocksalt-type layers of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ are likely to play a significant role in inducing the SDW transition around 100 K.

Magnetism of layered cobalt oxides investigated by muon spin rotation and relaxation

A positive muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ experiment on $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{]}_{0.62}[{\mathrm{CoO}}_{2}]$ (i.e., ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9},$ a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\ensuremath{\sim}100\mathrm{K}$ and 400--600 K; the former is a transition from a paramagnetic state to an incommensurate (IC) spin-density wave (SDW) state. The anisotropic behavior of zero-field ${\ensuremath{\mu}}^{+}\mathrm{SR}$ spectra at 5 K suggests that the IC-SDW propagates in the $a\ensuremath{-}b$ plane, with oscillating moments directed along the c axis; also the IC-SDW is found to exist not in the $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]$ subsystem but in the $[{\mathrm{CoO}}_{2}]$ subsystem. In addition, it is found that the long-range IC-SDW order completes below $\ensuremath{\sim}30\mathrm{K},$ whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions above 400 K. These two magnetic transitions detected by ${\ensuremath{\mu}}^{+}\mathrm{SR}$ are found to correlate closely with the transport properties of $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{]}_{0.62}[{\mathrm{CoO}}_{2}].$

Hidden magnetic transitions in the thermoelectric layered cobaltite [ Ca 2 CoO 3 ] 0.62 [ CoO 2 ]

Using muon spin spectroscopy we have found that, for both ${\mathrm{N}\mathrm{a}}_{x}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ ($0.6\ensuremath{\le}x\ensuremath{\le}0.9$) and three- and four-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the ${\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ planes. Here we summarize those results and report an almost dome-shaped relation between the transition temperature into the low-$T$ magnetic state and the composition $x$ for ${\mathrm{N}\mathrm{a}}_{x}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ and/or the high-temperature asymptotic limit of thermopower in the more complex three- and four-layer cobaltites. This behavior is explained using the Hubbard model on a two-dimensional triangular lattice in the ${\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ plane.

Hiroshi Itahara

Papers

Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing

Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH) 2 platelets as reactive templates

Magnetism of layered cobalt oxides investigated by muon spin rotation and relaxation

Hidden magnetic transitions in the thermoelectric layered cobaltite [ Ca 2 CoO 3 ] 0.62 [ CoO 2 ]

Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites.