Optimizing the average power factor of p-type (Na,Ag) co-doped polycrystalline SnSe

Despite the achievable high thermoelectric properties in SnSe single crystals, the poor mechanical properties and the relatively high cost of synthesis restrict the large scale commercial application of SnSe. Herein, we reported that co-doping with Na and Ag effectively improves the thermoelectric properties of polycrystalline SnSe. Temperature-dependent carrier mobility indicates that the grain boundary scattering is the dominant scattering mechanism near room temperature, giving rise to low electrical conductivity for the polycrystalline SnSe in comparison with that of the single crystal. Co-doping with Na and Ag improves the electrical conductivity of polycrystalline SnSe with a maximum value of 90.1 S cm⁻¹ at 323 K in Na_0.005Ag_0.015Sn_0.98Se, and the electrical conductivity of the (Na, Ag) co-doped samples is higher than that of the single doped samples over the whole temperature range (300–773 K). Considering the relatively high Seebeck coefficient of 335 μV K⁻¹ at 673 K and the minimum thermal conductivity of 0.48 W m⁻¹ K⁻¹ at 773 K, Na_0.005Ag_0.015Sn_0.98Se is observed to have the highest PF and ZT among the series of samples, with values of 0.50 mW cm⁻¹ K⁻² and 0.81 at 773 K, respectively. Its average PF and ZT are 0.43 mW cm⁻¹ K⁻² and 0.37, which is 92% and 68% higher than that of Na_0.02Sn_0.98Se, 40% and 43% higher than that of Ag_0.02Sn_0.98Se, and 304% and 277% higher than that of the previously reported SnSe, respectively.

(Na, Ag) co-doping combines the advantages of Ag and Na single doping in terms of the electronic properties.     

The ultralow thermal conductivity and tunable thermoelectric properties of surfactant-free SnSe nanocrystals

Most studies to date on SnSe thermal transport are focused on single crystals and polycrystalline pellets that are obtained using high-temperature processing conditions and sophisticated instruments. The effects of using sub-10 nm-size SnSe nanocrystals on the thermal transport and thermoelectric properties have not been studied to the best of our knowledge. Here, we report the synthesis of sub-10 nm colloidal surfactant-free SnSe NCs at a relatively low temperature (80 °C) and investigate their thermoelectric properties. Pristine SnSe NCs exhibit p-type transport but have a modest power factor of 12.5 μW m⁻¹ K⁻² and ultralow thermal conductivity of 0.1 W m⁻¹ K⁻¹ at 473 K. Interestingly, the one-step post-synthesis treatment of NC film with methylammonium iodide can switch the p-type transport of the pristine film to n-type. The power factor improved significantly to 20.3 μW m⁻¹ K⁻², and the n-type NCs show record ultralow thermal conductivity of 0.14 W m⁻¹ K⁻¹ at 473 K. These surfactant-free SnSe NCs were then used to fabricate flexible devices that show superior performance to rigid devices. After 20 bending cycles, the flexible device shows a 34% loss in the power factor at room temperature (295 K). Overall, this work demonstrates p- and n-type transport in SnSe NCs via the use of simple one-step post-synthesis treatment, while retaining ultralow thermal conductivity.

This work demonstrates tunable transport in surfactant free SnSe nanocrystals that retain ultralow nature of thermal conductivity.     

Surface modification and thermal performance of a graphene oxide/novolac epoxy composite

Functionalized graphene oxide (GO) was successfully modified by grafting 1,3,5-triglycidylisocyanurate (TGIC) onto the surface of GO. The modified GO was then added to a novolac epoxy composite at various volume fractions to improve the interfacial compatibility between the filler and matrix. Samples of the modified GO/novolac epoxy composite were fabricated through the hot-pressing method. Microstructural analysis revealed that the modified GO dispersed well in the matrix and formed thermal conductive pathways across the matrix. The thermal degradation temperature of 50% weight loss of the modified GO/novolac epoxy composite was 166 °C higher than that of the novolac epoxy. The data for loss factor tan δ demonstrated that when the composite contained 36.8 wt% of modified GO, the glass transition temperature of the modified GO/novolac epoxy composite was 222 °C, which is 90 °C higher than that of the novolac epoxy. The thermal conductivity of the modified GO/novolac epoxy composite improved from 0.044 W m⁻¹ K⁻¹ to 1.091 W m⁻¹ K⁻¹. Results indicated that the incorporation of surface-modified GO into the novolac epoxy positively affects the thermal conductivity and various properties of the modified GO/novolac epoxy composite.

Functionalized graphene oxide (GO) was successfully modified by grafting 1,3,5-triglycidylisocyanurate (TGIC) onto the surface of GO.     

Manufacturing silica aerogel and cryogel through ambient pressure and freeze drying

Achieving a mesoporous structure in superinsulation materials is pivotal for guaranteeing a harmonious relationship between low thermal conductivity, high porosity, and low density. Herein, we report silica-based cryogel and aerogel materials by implementing freeze-drying and ambient-pressure-drying processes respectively. The obtained freeze-dried cryogels yield thermal conductivity of 23 mW m⁻¹ K⁻¹, with specific surface area of 369.4 m² g⁻¹, and porosity of 96.7%, whereas ambient-pressure-dried aerogels exhibit thermal conductivity of 23.6 mW m⁻¹ K⁻¹, specific surface area of 473.8 m² g⁻¹, and porosity of 97.4%. In addition, the fiber-reinforced nanocomposites obtained via freeze-drying feature a low thermal conductivity (28.0 mW m⁻¹ K⁻¹) and high mechanical properties (∼620 kPa maximum compressive stress and Young''s modulus of 715 kPa), coupled with advanced flame-retardant capabilities, while the composite materials from the ambient pressure drying process have thermal conductivity of 28.8 mW m⁻¹ K⁻¹, ∼200 kPa maximum compressive stress and Young''s modulus of 612 kPa respectively. The aforementioned results highlight the capabilities of both drying processes for the development of thermal insulation materials for energy-efficient applications.

Ambient pressure and freeze drying techniques enable silica aerogel and cryogel insulation composites.     

Electrical and thermal properties of silver nanowire fabricated on a flexible substrate by two-beam laser direct writing for designing a thermometer

Accurate knowledge of electrical conductivity and thermal conductivity temperature dependence plays a crucial role in the design of a thermometer. Here, by using a two-beam laser direct writing system, an individual silver nanowire (AgNW) with well-defined dimensions is fabricated on a polyethylene terephthalate (PET) substrate. The temperature dependence of the resistivity of the fabricated AgNW is measured ranging from 10 to 300 K, and fitted with the Bloch–Grüneisen formula. The residual resistivity ((1.62 ± 0.20) × 10⁻⁷ Ω m) of the AgNW is larger than that of the bulk material (1 × 10⁻¹¹ Ω m). The electron–phonon coupling constant of the AgNW is (1.08 ± 0.13) × 10⁻⁷ Ω m, which is larger than that of the bulk silver (5.24 × 10⁻⁸ Ω m). Moreover, the Debye temperature of the AgNW is 199.30 K and is lower than that of the bulk silver (235 K). The Lorenz number of the fabricated AgNW is found to decrease as the temperature increases. Besides, the Lorenz number (2.66 × 10⁻⁷ W Ω K⁻²) is larger than the Sommerfeld value (2.44 × 10⁻⁸ W Ω K⁻²) at room temperature. The measurement results allow one to design a thermometer in the temperature range 40–300 K. The flexibility of the AgNW is also excellent, and the resistance increase of the AgNW is only 2.58% when the AgNW bending about 1000 times with a bending radius of 1 mm.

Investigation of temperature dependence of electrical resistivity, thermal conductivity and Lorenz number of silver nanowire, and design of a thermometer.     

Hot pressing-induced alignment of hexagonal boron nitride in SEBS elastomer for superior thermally conductive composites

Styrene–ethylene–butylene–styrene (SEBS) composite films containing well-dispersed and highly aligned hexagonal boron nitride (hBN) platelets were achieved by a ball milling process followed by hot-pressing treatment. An ultrahigh in-plane thermal conductivity of 45 W m⁻¹ K⁻¹ was achievable in the SEBS composite film with 95 wt% hBN. The corresponding out-of-plane thermal conductivity was also as high as 4.4 W m⁻¹ K⁻¹. The hBN/SEBS composite film was further used to cool a CPU connected to a computer, resulting in a decrease by about 4 °C in the stable temperature. Percolation thresholds over 40 wt% and 60 wt% in the hBN/SEBS composites were obtained in the in-plane and out-of-plane directions, respectively. This phenomenon has rarely been reported in polymer composites. Molecular dynamics simulations were also conducted to support this percolation threshold. The linear coefficients of the thermal expansion value of the hBN/SEBS composite with 95 wt% hBN was as low as 16 ppm K⁻¹. This was a significant decrease compared to that of pure SEBS (149 ppm K⁻¹). The proposed strategy provides valuable advice about the heat-transfer mechanism in polymer composites containing oriented two-dimensional materials.

Orientational hBN/SEBS composite films embued with superior thermal conductivity and improved dimensional stability were prepared by hot-pressing treatment.     

Effect of networked hybridized nanoparticle reinforcement on the thermal conductivity and mechanical properties of natural rubber composites

Thermal conductivity of natural rubber (NR) was enhanced by incorporating novel conductive hybrid nanofillers, namely polyaniline grafted carbon black (PANI/CB) nanoparticles and carbon black nanoparticles linked with carbon microfiber (CF/CB) composites. The PANI/CB hybrid fillers were synthesized using an in situ method, where aniline monomers were initially adsorbed onto carbon black spherical domains and, afterwards, it was polymerized in the presence of an oxidizer. Final rubber composites were prepared through melt mixing, where PANI/CB and CF/CB filler loading was kept at 40 parts per hundred of rubber (phr). The thermal conductivity values of the rubber composites with CF/CB (20 : 20) and PANI/CB (20 : 20) yield were 0.45 W m⁻¹ K⁻¹ and 0.31 W m⁻¹ K⁻¹, respectively and the thermal conductivity improved significantly compared to the control (0.25 W m⁻¹ K⁻¹) sample. The higher thermal conductivity values of CF/CB and PANI/CB incorporated composites suggest that the generated networked structure of CF and PANI nanofibers with CB nanoparticles has immensely contributed to enhancing the heat dissipation compared to that of the neat CB rubber composite. Scanning electron micrographs (SEM) confirmed the attachment of the synthesized PANI onto the spherical CB nanoparticles and interconnected morphology of CF/CB and PANI/CB hybrid fillers. The synthesized PANI/CB hybrid filler was further characterized using Fourier-transform infrared (FTIR) spectroscopy to evaluate the chemical properties. Furthermore, thermogravimetric analysis revealed the higher thermal stability of CF/CB (20 : 20) and PANI/CB (20 : 20) composites compared to the control. Moreover, the addition of CF/CB (20 : 20) and PANI/CB (20 : 20) improved the mechanical properties such as ultimate tensile strength, modulus at break, resilience and abrasion resistance significantly and well above the minimum required standard mechanical parameters in the tyre industry. These reinforced composites show great potential to be used as heat dissipating rubber composites in the tyre industry.

Thermal conductivity of natural rubber was enhanced by incorporating novel conductive hybrid nanofillers, namely polyaniline grafted carbon black nanoparticles and carbon black nanoparticles linked with carbon microfiber composites.     

Electrochemical performance of activated carbon fiber with hydrogen bond-induced high sulfur/nitrogen doping

The sulfur/nitrogen co-doped activated carbon fiber (S/N-ACF) is prepared by the thermal treatment of thiourea-bonded hydroxyl-rich carbon fiber, which can bond the decomposition products of thiourea through hydrogen bond interaction to avoid the significant loss of sulfur and nitrogen sources during the thermal treatment process. The sulfur/nitrogen co-doped carbon fiber (S/N-CF) is prepared by the thermal treatment of thiourea-adsorbed carbon fiber. The doping degree of the carbon fiber is improved by reasonable strategy. S/N-ACF shows a higher amount of S/N doping (4.56 at% N and 3.16 at% S) than S/N-CF (1.25 at% N and 0.61 at% S). S/N-ACF with high S/N doping level involves highly active sites to improve the capacitive performance, and high delocalization electron to improve the conductivity and rate capability when compared with the normal S/N co-doped carbon fiber (S/N-CF). Accordingly, the specific capacitance increases from 1196 mF cm⁻² for S/N-CF to 2704 mF cm⁻² for S/N-ACF at 1 mA cm⁻². The all-solid-state flexible S/N-ACF supercapacitor achieves 184.7 μW h cm⁻² at 350 μW cm⁻². The results suggest that S/N-ACF has potential application as a CF-based supercapacitor electrode material.

Sulfur/nitrogen co-doped activated carbon fiber is prepared by thermal treatment of thiourea-bonded hydroxyl-rich carbon fiber, which achieves high doping level and electrochemical performance.     

The effect of 3D carbon nanoadditives on lithium hydroxide monohydrate based composite materials for highly efficient low temperature thermochemical heat storage

Lithium hydroxide monohydrate based thermochemical heat storage materials were modified with in situ formed 3D-nickel-carbon nanotubes (Ni-CNTs). The nanoscale (5–15 nm) LiOH·H₂O particles were well dispersed in the composite formed with Ni-CNTs. These composite materials exhibited improved heat storage capacity, thermal conductivity, and hydration rate owing to hydrogen bonding between H₂O and hydrophilic groups on the surface of Ni-CNTs, as concluded from combined results of in situ DRIFT spectroscopy and heat storage performance test. The introduction of 3D-carbon nanomaterials leads to a considerable decrease in the activation energy for the thermochemical reaction process. This phenomenon is probably due to Ni-CNTs providing an efficient hydrophilic reaction interface and exhibiting a surface effect on the hydration reaction. Among the thermochemical materials, Ni-CNTs–LiOH·H₂O-1 showed the lowest activation energy (23.3 kJ mol⁻¹), the highest thermal conductivity (3.78 W m⁻¹ K⁻¹) and the highest heat storage density (3935 kJ kg⁻¹), which is 5.9 times higher than that of pure lithium hydroxide after the same hydration time. The heat storage density and the thermal conductivity of Ni-CNTs–LiOH·H₂O are much higher than 1D MWCNTs and 2D graphene oxide modified LiOH·H₂O. The selection of 3D carbon nanoadditives that formed part of the chemical heat storage materials is a very efficient way to enhance comprehensive performance of heat storage activity components.

3D carbon modified LiOH·H₂O particles were well dispersed into nanoparticles (5–15 nm) and tested using in situ DRIFT spectroscopy.     

Simple in situ synthesis of SiC nanofibers on graphite felt as a scaffold for improving performance of paraffin-based composite phase change materials

A paraffin phase change material absorbed in a composite material of silicon carbide nanofibers and graphite fibers was prepared by vacuum impregnation to solve problems of current organic phase change materials, such as the low thermal response rate, low thermal conductivity, and high vulnerability to leakage. A composite material containing a phase change material as the scaffold combined with silicon carbide nanofibers is prepared by a simple vapor–solid reaction using industrially produced porous graphite felt as the raw material. The thermal conductivity of the composite phase change material is increased to 1.29 W m⁻¹ K⁻¹ by thermal strengthening of the supporting scaffold. Compared with the thermal conductivity of pure paraffin of 0.25 W m⁻¹ K⁻¹, not only is the thermal response rate improved, but the composite phase change material also exhibits chemical stability, shape stability, and stable thermal properties. The phase change enthalpies of the melting and freezing processes are 180.5 and 176.4 J g⁻¹, respectively. In addition, the use of low-cost graphite felt and the simple synthetic process of the composite phase change material facilitates large-scale production. Moreover, the composite is extremely flexible in terms of its shape design and has good energy storage properties in terms of photo-thermal conversion. These features promise to accelerate the effective application of the prepared composite phase change material in solar and buildings energy storage with great potential for application practices.

The composite phase change material has excellent thermal properties, good photo-thermal conversion efficiency and flexible design in size, which produces a type of material for applications in solar and buildings energy storage.     

Carbon-dot doped,transfer-free,low-temperature,high mobility graphene using microwave plasma CVD

Microwave plasma chemical vapor deposition is a well-known method for low-temperature, large-area direct graphene growth on any insulating substrate without any catalysts. However, the quality has not been significantly better than other graphene synthesis methods such as thermal chemical vapor deposition, thermal decomposition of SiC, etc. Moreover, the higher carrier mobility in directly grown graphene is much desired for industrial applications. Here, we report chemical doping of graphene (grown on silicon using microwave plasma chemical vapor deposition) with carbon dots to increase the mobility to a range of 363–398 cm² V⁻¹ s⁻¹ (1 × 1 cm van der Pauw devices were fabricated) stable for more than 30 days under normal atmospheric conditions, which is sufficiently high for a catalyst-free, low-temperature, directly grown graphene. The sheet resistance of the graphene was 430 Ω □⁻¹ post-doping. The novelty of this work is in the use of carbon dots for the metal-free doping of graphene. To understand the doping mechanism, the carbon dots were mixed with various solvents and spin coated on graphene with simultaneous exposure to a laser. The significant information observed was that the electron or hole transfer to graphene depends upon the functional group attached to the carbon dot surface. Carbon dots were synthesized using the simple hydrothermal method and characterized with transmission electron microscopy revealing carbon dots in the range of 5–10 nm diameter. Doped graphene samples were further analyzed using Raman microscopy and Hall effect measurements for their electronic properties. This work can open an opportunity for growing graphene directly on silicon substrates with improved mobility using microwave plasma CVD for various electronic applications.

Microwave plasma chemical vapor deposition is a well-known method for low-temperature, large-area direct graphene growth on any insulating substrate without any catalysts.     

Preparation of boron nitride nanosheets via polyethyleneimine assisted sand milling: towards thermal conductivity and insulation applications

Hexagonal boron nitride (h-BN) is often used as a filler in polymer composites due to its good thermal conductivity and insulation properties. However, the compatibility between h-BN and the matrix limits its application areas. To overcome this issue, a combination of mechanical liquid phase exfoliation and chemical interfacial modification was adopted in this work. Polyethyleneimine (PEI) was used as the exfoliation reagent to prepare PEI-functionalized h-BN nanosheets, denoted as PEI@BNNS. Thermoplastic polyurethane (TPU) composites with different contents of h-BN and PEI@BNNS which were recorded as h-BN/TPU and PEI@BNNS/TPU were successfully prepared through a hot-pressing process, respectively. The results show that PEI@BNNS/TPU composites have better in-plane thermal conductivity while maintaining insulation, and with the content of 5 wt% PEI@BNNS, the in-plane thermal conductivity of the PEI@BNNS/TPU composite is up to 0.61 W m⁻¹ K⁻¹, which is three times that of pure TPU (0.22 W m⁻¹ K⁻¹).

The PEI-grafted boron nitride nanosheets were successfully prepared via sand-milling process, which were doped into thermoplastic polyurethane matrix for better in-plane thermal conductivity while maintaining insulation properties.     

Epoxy composite with high thermal conductivity by constructing 3D-oriented carbon fiber and BN network structure

As electronic devices tend to be integrated and high-powered, thermal conductivity is regarded as the crucial parameter of electronic components, which has become the main factor that limits the operating speed and service lifetime of electronic devices. However, constructing continuous thermal conductive paths for low content particle fillers and reducing interface thermal resistance between fillers and matrix are still two challenging issues for the preparation of thermally conductive composites. In this study, 3D-oriented carbon fiber (CF) thermal network structures filled with boron nitride flakes (BN) as thermal conductive bridges were successfully constructed. The epoxy composite was fabricated by thermal conductive material with a 3D oriented structure by the vacuum liquid impregnation method. This special 3D-oriented structure modified by BN (BN/CF) could efficiently broaden the heat conduction pathway and connected adjacent fibers, which leads to the reduction of thermal resistance. The thermal conductivity of the boron nitride/carbon fiber/epoxy resin composite (BN/CF/EP) with 5 vol% 10 mm CF and 40 vol% BN reaches up to 3.1 W m⁻¹ K⁻¹, and its conductivity is only 2.5 × 10⁻⁴ S cm⁻¹. This facile and high-efficient method could provide some useful advice for the thermal management material in the microelectronic field and aerospace industry.

As electronic devices tend to be integrated and high-powered, thermal conductivity is regarded as the crucial parameter of electronic components, which is the main factor that limits the operating speed and service lifetime of electronic devices.     

Catalytic graphitization assisted synthesis of Fe3C/Fe/graphitic carbon with advanced pseudocapacitance

We report an environmentally friendly strategy for the synthesis of Fe₃C/Fe/graphitic carbon based on hydrothermal carbonization and graphitization of carbon spheres with potassium ferrate (K₂FeO₄) at 800 °C. The obtained sample consisting of Fe₃C/Fe nanoparticles and graphitic carbon (FC-1-8) delivered an enhanced pseudocapacitance of 428.0 F g⁻¹ at a current density of 1 A g⁻¹. After removal of the Fe₃C/Fe electroactive materials, the graphitic carbon (FC-1-8-HCl) possessed a large specific surface area (SSA) up to 2813.6 m² g⁻¹ with a capacity of 243.3 F g⁻¹ at 1 A g⁻¹, far outweighing the other amorphous carbon electrodes of FC-0-8 (carbon spheres annealed at 800 °C without the treatment of K₂FeO₄). The graphitic material with a porous structure could offer more electroactive sites and improved conductivity of the sample. This method provided guidelines for the synthesis of superior performance supercapacitors with synchronous graphitic carbon and electroactive species.

We report an environmentally friendly strategy for the synthesis of Fe₃C/Fe/graphitic carbon based on hydrothermal carbonization and graphitization of carbon spheres with potassium ferrate (K₂FeO₄) at 800 °C.     

Synergistic effect of reduced graphene oxide/carbon nanotube hybrid papers on cross-plane thermal and mechanical properties

Graphene paper has attracted great attention as a heat dissipation material due to its excellent thermal conductivity and mechanical properties. However, the thermal conductivity of graphene paper in the normal direction is relatively poor. In this work, the cross-plane thermal conductivities (K_⊥) and mechanical properties of the reduced graphene oxide/carbon nanotube papers with different CNT loadings were studied systematically. It was found that the K_⊥ decreased from 0.0393 W m⁻¹ K⁻¹ for 0 wt% paper to 0.0250 W m⁻¹ K⁻¹ for 3 wt% paper, and then increased to 0.1199 W m⁻¹ K⁻¹ for 20 wt% paper. The papers demonstrated a maximum elastic modulus of 6.1 GPa with 10 wt% CNT loading. The CNTs acted as scaffolds to restrain the graphene sheets from corrugating and to reinforce the mechanical properties of the hybrid papers. The more CNTs that filled the gaps between graphene sheets, the greater the number of channels of the transmission of phonons and the looser the structure in the cross-plane direction. Further mechanism analysis revealed the synergistic effects of CNT loadings and graphene sheets on enhancing the thermal and mechanical performance of the papers.

The top-view SEM images for (a) rGO, (b) rGO/CNT-3%, (c) rGO/CNTs-20% and the corresponding schematic diagram of photon transmission with different spacer CNTs loadings (a-i, b-ii, c-iii).     

Thermal conductivity of hexagonal BC2P – a first-principles study

In this work, we report a high thermal conductivity (k) of 162 W m⁻¹ K⁻¹ and 52 W m⁻¹ K⁻¹ at room temperature, along the directions perpendicular and parallel to the c-axis, respectively, of bulk hexagonal BC₂P (h-BC₂P), using first-principles calculations. We systematically investigate elastic constants, phonon group velocities, phonon linewidths and mode thermal conductivity contributions of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Interestingly, optical phonons are found to make a large contribution of 30.1% to the overall k along a direction perpendicular to the c-axis at 300 K. BC₂P is also found to exhibit high thermal conductivity at nanometer length scales. At 300 K, a high k value of ∼47 W m⁻¹ K⁻¹ is computed for h-BC₂P at a nanometer length scale of 50 nm, providing avenues for achieving efficient nanoscale heat transfer.

In this work, we report a high thermal conductivity (k) of 162 W m⁻¹ K⁻¹ and 52 W m⁻¹ K⁻¹ at room temperature, along the directions perpendicular and parallel to the c-axis, respectively, of bulk hexagonal BC₂P (h-BC₂P), using first-principles calculations.     

Ultralow lattice thermal conductivity and high thermoelectric performance of monolayer KCuTe: a first principles study

Monolayer KCuTe is a new-type of two-dimensional (2D) semiconductor material with high carrier mobility and large power energy conversion efficiencies, suggesting its potential application in thermoelectric (TE) and photoelectric fields. Based on the density functional theory (DFT) and semiclassical Boltzmann transport equation, the electronic and phonon transport properties of monolayer KCuTe are systematically studied. Our results show that it possesses an ultralow lattice thermal conductivity value of nearly ∼0.13 W m⁻¹ K⁻¹ at 300 K, mainly attributed to its small phonon group velocity, large Grüneisen parameters, and strong phonon–phonon scattering. Furthermore, the intralayer opposite phonon vibrations greatly restrict the heat transport. Monolayer KCuTe shows an ideal direct band gap of ∼1.21 eV, and a high twofold degeneracy appearing at the Γ point gives a high Seebeck coefficient of ∼2070 μV K⁻¹, leading to high TE performance. Using the transport coefficients together with constant electron relaxation time, the figure of merit (ZT) can reach 2.71 at 700 K for the p-type doping, which is comparable to the well-known TE material SnSe (2.6 ± 0.3 at 935 K). Our theoretical studies may provide perspectives to TE applications of monolayer KCuTe and stimulate further experimental synthesis.

The excellent thermoelectric performance of monolayer KCuTe is discovered by first-principles study for the first time.     

High ion adsorption densities of site-selective nitrogen doped carbon sheets prepared from natural lignin

Carbon fibers and sheets were prepared from jet-milled natural chitin and cellulose samples, and from natural lignin sample using ice-templating technique, respectively. Nitrogen doping treatments using melamine were also performed for the carbon fibers and sheets. Electric double layer capacitor (EDLC) electrode properties of the prepared carbon fibers and sheets including the nitrogen doped samples were investigated with aqueous (sulfuric acid) and organic (tetraethylammonium tetrafluoroborate in propylene carbonate) electrolytes. It was found that the nitrogen doped lignin carbon sheets having very small specific surface area of 66 m² g⁻¹ show very high EDLC capacitances of 227 F g⁻¹ and 80 F g⁻¹ determined by charge–discharge measurements at current density of 50 mA g⁻¹ for aqueous and organic electrolytes, respectively. X-ray photoelectron spectroscopy (XPS) measurements revealed that nitrogen atoms of the nitrogen doped lignin carbon sheets exist dominantly in pyridinic sites unlike other chitin and cellulose carbon fibers. We discussed that this site-selective nitrogen doping gives exceptionally high ion adsorption density per unit surface area of the nitrogen doped lignin carbon sheets.

Carbon fibers and sheets were prepared from jet-milled natural chitin and cellulose samples, and from natural lignin sample using ice-templating technique, respectively.     

Enhanced thermal conductivity of epoxy composites filled with tetrapod-shaped ZnO

Epoxy composites with ZnO powders characterized by different structures as inclusion are prepared and their thermal properties are studied. The experimental results demonstrate that the epoxy resins filled by tetrapod-shaped ZnO (T-ZnO) whiskers have the superior thermal transport property in comparison to ZnO micron particles (ZnO MPs). The thermal conductivity of ZnO/epoxy and T-ZnO/epoxy composites in different mass fraction (10, 20, 30, 40, 50 wt%) are respectively investigated and the suitable models are compared to explain the enhancement effect of thermal conductivity. The thermal conductivity of T-ZnO/epoxy composites with 50 wt% filler reaches 4.38 W m⁻¹ K⁻¹, approximately 1816% enhancement as compared to neat epoxy. In contrast, the same mass fraction of ZnO MPs are incorporated into epoxy matrix showed less improvement on thermal conduction properties. This is because T-ZnO whiskers act as a thermal conductance bridge in the epoxy matrix. In addition, the other thermal properties of T-ZnO/epoxy composites are also improved. Furthermore, the T-ZnO/epoxy composite also presents a much reduced coefficient of thermal expansion (∼28.1 ppm K⁻¹) and increased glass transition temperature (215.7 °C). This strategy meets the requirement for the rapid development of advanced electronic packaging.

Epoxy composites with ZnO powders characterized by different structures as inclusion are prepared and their thermal properties are studied.     

The excellent TE performance of photoelectric material CdSe along with a study of Zn(Cd)Se and Zn(Cd)Te based on first-principles

Zn(Cd)Se and Zn(Cd)Te are well known for their excellent photoelectric performance, however, their thermoelectric (TE) properties are usually ignored. By taking advantage of first-principles calculations, the Boltzmann transport equation and semiclassical analysis, we executed a series of thermal and electronic transport investigations on these materials. Our results show that CdSe has the lowest anisotropic thermal conductivity, κ_L, of the four materials, at 4.70 W m⁻¹ K⁻¹ (c axis) and 3.85 W m⁻¹ K⁻¹ (a axis) at a temperature of 300 K. Inspired by the very low lattice conductivity, other thermoelectric parameters were calculated in the following research. At a temperature of 1200 K we obtained a pretty large power factor, S²σ, of 4.39 × 10⁻³ W m⁻¹ K⁻², and based it on the fact that the corresponding figure of merit ZT can reach 1.8 and 1.6 along the a axis and c axis, respectively. We revealed the neglected thermoelectric potential of CdSe by means of systematic studies and demonstrated that it is a promising material with both excellent photoelectric performance and thermoelectric performance.

We reveal the neglected thermoelectric potential of CdSe by means of systematic studies and demonstrate that it is a promising material with excellent photoelectric and thermoelectric performance..