Effects of electrical conductivity on the formation and annihilation of positronium in porous materials

Extremely Low Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Performance in α-Cu2Se through the Incorporation of Porous Carbon

In this work, a series of Cu2Se/x wt % porous carbon (PC) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) composite materials were synthesized by ball milling and spark plasma sintering (SPS). The highly ordered porous carbon was synthesized by a hydrothermal method using mesoporous silica (SBA-15) as the template. X-ray diffraction results show that the incorporation of porous carbon induces a phase transition of Cu2Se from the β phase to the α phase. Meanwhile, the addition of porous carbon reduces the carrier concentration from 2.7 × 1021 to 2.45 × 1020 cm-3 by 1 order of magnitude. The decrease of the carrier concentration leads to the reduction of electrical conductivity and the increase of the Seebeck coefficient, which results in the enhancement of the power factor. On the other hand, the incorporation of porous carbon into Cu2Se increases the porosity of the composites and also introduces more interfaces between the two materials, which is evidenced by positron annihilation lifetime measurements. Both pores and interfaces greatly enhance phonon scattering, leading to extremely low lattice thermal conductivity. In addition, the decrease of electrical conductivity also causes a sufficient reduction in electronic thermal conductivity. Due to the above synergistic effects, the thermoelectric performance of the Cu2Se/PC composite is significantly enhanced with a maximum ZT value of 0.92 at 403 K in the Cu2Se/1 wt % PC composite, which is close to that of the Bi2Te3-based materials. Our work shows that α-Cu2Se has great potential for near-room-temperature thermoelectric materials.

Investigation of ortho-positronium annihilation for porous materials with different geometries and topologies

In this work, we present the results of the ortho-positronium (o-Ps) annihilation lifetimes and nitrogen adsorption measurements for different porous materials and an approach for describing the annihilation of o-Ps in a pore, which results in a surface-volume formula (SVF) for calculating the pore-related o-Ps lifetime. This proposed formula gives the relationship between the o-Ps annihilation rate and the effective pore radius, bulk composition, and pore structure, including pore geometry and topology. The pore-related o-Ps lifetimes of different materials calculated by the SVF are consistent with experimental results for both micro- and mesopores (and macropores) with different geometries and topologies. The SVF is convenient for calculations of pore dimensions for many cases of metal organic frameworks and zeolites. This approach enables us to fully explain the temperature dependence of the o-Ps annihilation lifetime over a wide temperature range, 20-700 K.

Significant Enhancement in the Thermoelectric Performance of Aluminum-Doped ZnO Tuned by Pore Structure

In this paper, 2 atom % Al-doped ZnO (AZO) was prepared by the co-precipitation method together with sparking plasma sintering (SPS) treatment. The as-synthesized AZO powders show the morphology of hollow hexagonal towers, which result in a high porosity of 50.6% in the bulk sample consolidated by SPS sintering at 400 °C, and the porosity decreases gradually with increasing sintering temperature up to 1000 °C. Positron annihilation measurements reveal that even after sintering at 1000 °C, there are still a considerable number of small pores. A high electrical conductivity of 3 × 10⁵ S m^-1 is achieved at room temperature for the AZO sample sintered at 1000 °C, while the absolute values of Seebeck coefficient keep at relatively high values between 59 and 144 μV K^-1 in the measurement temperature range of 27-500 °C, leading to a high power factor of 3.4 × 10^-3 W m^-1 K^-2. On the other hand, the pores in AZO act as strong phonon scattering centers, and an extremely low thermal conductivity of 1.5 W m^-1 K^-1 measured at room temperature is obtained for AZO sintered at 400 °C. Due to the residual pores in the 1000 °C-sintered sample, the thermal conductivity is still relatively low. As a result, a maximum ZT of 0.275 measured at 500 °C is obtained in this sample, which is the highest ZT reported for ZnO around this temperature.

Porous liquid zeolites: hydrogen bonding-stabilized H-ZSM-5 in branched ionic liquids

Porous liquids, as a newly emerging type of porous material, have great potential in gas separation and storage. However, the examples and synthetic strategies reported so far likely represent only the tip of the iceberg due to the great difficulty and challenge in engineering permanent porosity in liquid matrices. Here, by taking advantage of the hydrogen bonding interaction between the alkane chains of branched ionic liquids and the Brønsted sites in H-form zeolites, as well as the mechanical bond of the long alkyl chain of the cation penetrated into the zeolite channel at the interface, the H-form zeolites can be uniformly stabilized in branched ionic liquids to form porous liquid zeolites, which not only significantly improve their gas sorption performance, but also change the gas sorption-desorption behavior because of the preserved permanent porosity. Furthermore, such a facile synthetic strategy can be extended to fabricate other types of H-form zeolite-based porous liquids by taking advantage of the tunability of the counter-anion (e.g., NTf2-, BF4-, EtSO4-, etc.) in branched ionic liquids, thus opening up new opportunities for porous liquids for specific applications in energy and environment.

Porous Liquid: A Stable ZIF-8 Colloid in Ionic Liquid with Permanent Porosity

We reported an example of metal-organic framework (MOF)-based porous liquid by dispersing ZIF-8 ({Zn(mim)₂}, mim = 2-methylimidazole) nanocrystallites in ionic liquid (IL) of [Bpy][NTf₂] ( N-butyl pyridinium bis(trifluoromethyl sulfonyl)imide). Two essential challenges, stable colloid formation and porosity retention, have been overcome to prepare MOF-based porous liquid. Preventing ZIF-8 nanocrystals from aggregation before dispersing is vital to form a stable ZIF-8 colloid in IL via enhancing the interaction between ZIF-8 and IL. The resultant ZIF-8-[Bpy][NTf₂] colloid is able to be stable over months without precipitating. [Bpy][NTf₂] with larger ion sizes cannot occupy pores in ZIF-8, leaving the ZIF-8 cage empty for enabling access by guest molecules. The porosity of this porous liquid system was verified by positron (e⁺) annihilation lifetime spectroscopy and I₂ adsorption in ZIF-8 in the colloid. MOF-based porous liquids could provide a new material platform for liquid-bed-based gas separations.

Positron annihilation characteristics, water uptake and proton conductivity of composite Nafion membranes

The connection between microstructure evolution, in terms of o-Ps lifetime, and proton conductivity in Nafion membranes with different water uptakes.