Spontaneously formed porous and composite materials

In recent years, a number of routes to porous materials have been developed which do not involve the use of pre-formed templates or structure-directing agents. These routes are usually spontaneous, meaning they are thermodynamically downhill. Kinetic control, deriving from slow diffusion of certain species in the solid state, allows metastable porous morphologies rather than dense materials to be obtained. While the porous structures so formed are random, the average architectural features can be well-defined, and the porosity is usually highly interconnected. The routes are applicable to a broad range of functional inorganic materials. Consequently, the porous architectures have uses in energy transduction and storage, chemical sensing, catalysis, and photoelectrochemistry. This is in addition to more straightforward uses deriving from the pore structure, such as in filtration, as a structural material, or as a cell-growth scaffold. In this feature article, some of the methods for the creation of porous materials are described, including shape-conserving routes that lead to hierarchical macro/mesoporous architectures. In some of the preparations, the resulting mesopores are aligned locally with certain crystallographic directions. The coupling between morphology and crystallography provides a macroscopic handle on nanoscale structure. Extension of these routes to create biphasic composite materials are also described

Effective nn-type Doping of Mg3_3Sb2_2 with Group-3 Elements

The recent discovery of high thermoelectric performance in Mg$_3$Sb$_2$ has been critically enabled by the success in $n$-type doping of this material, which is achieved under Mg-rich growth conditions, typically with chalcogens (Se, Te) as extrinsic dopants. Using first-principles defect calculations, we previously predicted that higher electron concentrations ($\sim10^{20}$ cm$^{-3}$) can be achieved in Mg$_3$Sb$_2$ by doping with La instead of Se or Te. Subsequent experiments showed that free electron concentration in La-doped Mg$_3$Sb$_{2-x}$Bi$_x$ indeed exceeds those in the Te-doped material. Herein, we further investigate $n$-type doping of Mg$_3$Sb$_2$ and predict that, in addition to La, other group-3 elements (Sc, Y) are also effective as $n$-type dopants; Y is as good as La while Sc slightly less. Overall, we find that doping with any group-3 elements should lead to higher free electron concentrations than doping with chalcogens

Zintl Chemistry for Designing High Efficiency Thermoelectric Materials

Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb_(14)MnSb_(11), clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors

Phonon engineering through crystal chemistry

Mitigation of the global energy crisis requires tailoring the thermal conductivity of materials. Low thermal conductivity is critical in a broad range of energy conversion technologies, including thermoelectrics and thermal barrier coatings. Here, we review the chemical trends and explore the origins of low thermal conductivity in crystalline materials. A unifying feature in the latest materials is the incorporation of structural complexity to decrease the phonon velocity and increase scattering. With this understanding, strategies for combining these mechanisms can be formulated for designing new materials with exceptionally low thermal conductivity

Composition and the thermoelectric performance of β-Zn_4Sb_3

β-Zn_4Sb_3 is a promising thermoelectric material due to the abundance of zinc and antimony and reports of high efficiency in bulk samples. This work establishes the high temperature properties of β-Zn_4Sb_3 across the phase stability window. By controlling the stoichiometry, the Hall carrier concentration can be tuned from 6–9 × 10^(19) cm^(−3) without requiring extrinsic dopants. The trend in Seebeck coefficient on carrier concentration is rationalized with a single, parabolic band model. Extremely low lattice thermal conductivity (0.4–0.6 W m^(−1) K^(−1)) coupled with a moderate effective mass (1.2 m_e) and mobility leads to a large figure of merit (zT of 0.8 by 550 K). The single parabolic band model is used to obtain the carrier concentration dependence of the figure of merit and an optimum carrier concentration near 5 × 10^(19) cm^(−3) is predicted

Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb_2

The layered Zintl compound SrZnSb_2 is investigated using transmission electron microscopy (TEM) to understand the low lattice thermal conductivity. The material displays out-of-phase boundaries with a spacing from 100 down to 2 nm. Density functional theory calculations confirm that the TEM-derived defect structure is energetically reasonable. The impact of these defects on phonon scattering is analyzed within the Debye–Callaway model, which reveals a significant reduction in the acoustic phonon mean free path. This enhancement in phonon scattering leads to an ~30% reduction in lattice thermal conductivity at 300 K

Entropic Stabilization and Retrograde Solubility in Zn4Sb3

Zn4Sb3 is shown to be entropically stabilized versus decomposition to Zn and ZnSb though the effects of configurational disorder and phonon free energy. Single phase stability is predicted for a range of compositions and temperatures. Retrograde solubility of Zn is predicted on the two-phase boundary region between Zn4Sb3 and Zn. The complex temperature dependent solubility can be used to explain the variety of nanoparticle formation observed in the system: formation of ZnSb on the Sb rich side, Zn on the far Zn rich side and nano-void formation due to Zn precipitates being reabsorbed at lower temperatures.Comment: 5 pages, 5 figure

Ca_3AlSb_3: an inexpensive, non-toxic thermoelectric material for waste heat recovery

Thermoelectric materials directly convert thermal energy into electrical energy, offering a promising solid-state solution for waste heat recovery. For thermoelectric devices to make a significant impact on energy and the environment the major impediments are the efficiency, availability and toxicity of current thermoelectric materials. Typically, efficient thermoelectric materials contain heavy elements such as lead and tellurium that are toxic and not earth abundant. Many materials with unusual structures containing abundant and benign elements are known, but remain unexplored for thermoelectric applications. In this paper we demonstrate, with the discovery of high thermoelectric efficiency in Ca_3AlSb_3, the use of elementary solid-state chemistry and physics to guide the search and optimization of such materials