Shengkai Gong

Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe

Heat conversion gets a power boost Thermoelectric materials convert waste heat into electricity, but often achieve high conversion efficiencies only at high temperatures. Zhao et al. tackle this problem by introducing small amounts of sodium to the thermoelectric SnSe (see the Perspective by Behnia). This boosts the power factor, allowing the material to generate more energy while maintaining good conversion efficiency. The effect holds across a wide temperature range, which is attractive for developing new applications. Science, this issue p. 141; see also p. 124 A thermoelectric derived by sodium doping of tin selenide has a high power factor and conversion efficiency over a wide temperature range. [Also see Perspective by Behnia] Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally friendly means of energy savings and power generation. The thermoelectric efficiency is determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZTdev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.

Effect of Ni excess on phase transformation temperatures of NiMnGa alloys

A systematic substitution of Ni for Mn, Ga, or both Mn and Ga in the non-stoichiometric NiMnGa alloys is performed. The relationship among the composition, structure and martensitic transformation temperatures was studied in detail for the Ni excessive NiMnGa alloys. The martensitic transformation temperatures almost linearly increase with increasing Ni content in all the three series from lower than 0 °C up to 300 °C. The increases in rate of the martensitic transformation temperatures are different for the three cases. It is large for Ga substituted by Ni, slow for Mn and intermediate for both Mn and Ga. The size factor and electronic concentrations are thought to influence the martensitic transformation temperature in the NiMnGa alloys. The determined relationship will be significant for designing a suitable NiMnGa alloy with a required martensitic transformation temperature for application at a specific temperature.

Enhanced Thermoelectric Properties in the Counter-Doped SnTe System with Strained Endotaxial SrTe

We report enhanced thermoelectric performance in SnTe, where significantly improved electrical transport properties and reduced thermal conductivity were achieved simultaneously. The former was obtained from a larger hole Seebeck coefficient through Fermi level tuning by optimizing the carrier concentration with Ga, In, Bi, and Sb dopants, resulting in a power factor of 21 μW cm(-1) K(-2) and ZT of 0.9 at 823 K in Sn(0.97)Bi(0.03)Te. To reduce the lattice thermal conductivity without deteriorating the hole carrier mobility in Sn(0.97)Bi(0.03)Te, SrTe was chosen as the second phase to create strained endotaxial nanostructures as phonon scattering centers. As a result, the lattice thermal conductivity decreases strongly from ∼2.0 Wm(-1) K(-1) for Sn(0.97)Bi(0.03)Te to ∼1.2 Wm(-1) K(-1) as the SrTe content is increased from 0 to 5.0% at room temperature and from ∼1.1 to ∼0.70 Wm(-1) K(-1) at 823 K. For the Sn(0.97)Bi(0.03)Te-3% SrTe sample, this leads to a ZT of 1.2 at 823 K and a high average ZT (for SnTe) of 0.7 in the temperature range of 300-823 K, suggesting that SnTe is a robust candidate for medium-temperature thermoelectric applications.

Twinning and De-twinning via Glide and Climb of Twinning Dislocations along Serrated Coherent Twin Boundaries in Hexagonal-close-packed Metals

The (1¯012) twin boundaries experimentally observed in hexagonal-close-packed metals are often serrated rather than fully coherent. These serrated coherent twin boundaries (SCTBs) consist of sequential (1¯012) coherent twin boundaries and parallel basal–prismatic planes serrations (BPPS). We demonstrated that the formation of BPPS is geometrically and energetically preferred in the SCTBs, and an SCTB thus migrates by glide and climb of twinning dislocations, combined with atomic shuffling. Particularly, the climb mechanism, combined with the density and the height of BPPSs in the SCTBs, could be crucial in controlling twinning and de-twinning, and twinning-associated hardening.

Synergistically optimized electrical and thermal transport properties of SnTe via alloying high-solubility MnTe

Lead chalcogenides are the most efficient thermoelectric materials. In comparison, SnTe, a lead-free analogue of PbTe, exhibits inferior thermoelectric performance due to low Seebeck coefficient and high thermal conductivity. In this report, we show that we can synergistically optimize the electrical and thermal transport properties of SnTe via alloying Mn. We report that the introduction of Mn (0–50%) induces multiple effects on the band structure and microstructure of SnTe: for the former, it can tune the Fermi level and promote the convergence of the two valence bands, concurrently enhancing the Seebeck coefficient; for the latter, it can profoundly modify the microstructure into an all-scale hierarchical architecture (including nanoscale precipitates/MnTe laminates, stacking faults, layered structure, atomic-scale point defects, etc.) to scatter phonons with a broad range of mean free paths, strongly reducing the lattice thermal conductivity. Meanwhile, most significantly, the Mn alloying enlarges the energy gap of the conduction band (C band) and the light valence band (L band), thereby suppressing the bipolar thermal conductivity by increasing the band gap. The integration of these effects yields a high ZT of 1.3 at 900 K for 17% Mn alloyed SnTe.

Development of gradient thermal barrier coatings and their hot-fatigue behavior

Abstract In this paper, a new type of gradient thermal barrier coating was produced by the co-deposition of a tablet of Al–Al 2 O 3 –YSZ (Yttria Stabilized Zirconia, ZrO 2 +8 wt.% Y 2 O 3 ), or Al 2 O 3 –YSZ and YSZ onto a NiCoCrAlY bond coat by EB-PVD. The analysis of SEM with EDS showed that the composition and microstructure changed continuously across the thickness of the gradient coating. Due to interdiffusion of elements between the bond coat and the gradient zone, a thin layer of a γ′ phase (Ni 3 Al) formed at the surface of the bond coat. The coatings were subjected to a series of tests and characterization studies, including thermal cycling, isothermal oxidation, hot-corrosion and microanalyses. Results showed that the substrate temperature during deposition had great effect on the properties of the coatings. The higher the temperature of the substrate, the lower the microhardness of the gradient coating, which can be considered as the formation of microporosity in the two-phase ZrO 2 +Al 2 O 3 region in the gradient coating, and its epitaxy into the YSZ topcoat. The thermal cyclic tests of specimens were performed by exposure to air at 1323 K for 0.5 h, and then cooled to room temperature within 5 min by forced air cooling, with a lifetime of more than 500 h. In addition, compared with a conventional two-layered coating, this gradient coating exhibited a better resistance to hot-corrosion than a conventional two-layered coating.

Investigation of the failure mechanism of thermal barrier coatings prepared by electron beam physical vapor deposition

Abstract Two-layer structure thermal barrier coatings (TBCs) (NiCoCrAlY [bond coat]+[6–8 wt.%] Y2O3-stabilized ZrO2 [YSZ top coat]) were deposited by electron beam physical vapor deposition (EB-PVD) on a Ni-base superalloy. Pre-treatments were carried out in a vacuum to improve the oxidation resistance of the bond coat, and the thermal cyclic life of the TBC system was investigated through thermal cyclic tests. It was found that the pre-treatments in the vacuum at 1273 K increased the thermal cyclic life of the TBCs by improving the oxidation resistance of the bond coat. The results of thermal cyclic tests indicated that the TBCs prepared by EB-PVD were degraded mainly at the interface between the bond coat and ceramic coat, due to the spallation of the YSZ top coat from the bond coat. According to the results of microstructure observation and composition analysis, the mechanism of failure was proposed as follows: micro-cracks are first initiated in the YSZ top coat along columnar boundaries, and then propagate through the whole top coat. The cracks formed at the thermal growth oxide (TGO) layer, due to the growing micro-cracks, are considered to cause abnormal thermally-grown oxides of the bond coat beneath the cracks, and consequently, the build-up stresses due to the volume increase, which are significant in weakening the interface combination of the top coat and bond coat. During the following cooling process, compressive stresses are introduced, which tend to separate the YSZ top coat from the bond coat, and finally cause the occurrence of the failure of TBC system.

A study of aluminide coatings on TiAl alloys by the pack cementation method

Abstract The lower case halide-activated pack cementation method was utilized to deposit aluminide coatings on TiAl alloys. Emphasis was placed on the effect of alloying elements on the aluminizing behavior of TiAl alloy. The addition of a small amount of Nb or Cr in the TiAl improved significantly the aluminizing kinetics of TiAl alloys by increasing the solid-state diffusion of Al through the formation of stable TiAl 3 layer. The TiAl 3 layer formed on the TiAl alloyed with Nb or Cr had better toughness than the TiAl 3 formed on the non-alloyed TiAl. The reason for better toughness of the coating formed on TiAl was that partial TiAl 3 with tetragonal structure was changed to high symmetry cubic L1 2 structure since Nb or Cr was dissolved into TiAl 3 . The TiAl 3 layer formed on the TiAl alloyed with Nb or Cr had much better oxidation resistance than the TiAl 3 layer formed on the non-alloyed TiAl. It was attributed to change in the crystal structure of TiAl 3 from the brittle tetragonal DO 22 to the ductile cubic L1 2 by addition of small amount of Nb or Cr.

Influence of water vapor on the isothermal oxidation behavior of low pressure plasma sprayed NiCrAlY coating at high temperature

Abstract The oxidation of low pressure plasma sprayed (LPPS) NiCrAlY coatings on nickel base superalloy were studied at 1050 °C in flows of O 2 , and mixture of O 2 and 5% H 2 O under atmospheric pressure. NiCrAlY coating exhibits a very low oxidation rate at 1050 °C in pure O 2 and the oxidation kinetics accords with a parabolic law. The oxidation kinetics of NiCrAlY coating obeys almost a linear law after long exposure times in the presence of 5% water vapor. The oxide formed on the surface of LPPS NiCrAlY coating after oxidation at 1050 °C in pure O 2 consisted of Al 2 O 3 , whereas the oxide formed on the surface of LPPS NiCrAlY coating after oxidation at 1050 °C in a mixture of O 2 and 5% H 2 O is mainly composed of NiCr 2 O 4 , NiO, Cr 2 O 3 and Al 2 O 3 . It is suggested that the effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to the increase in Ni and Cr ions transport.

Effect of thermal exposure on the microstructure and properties of EB-PVD gradient thermal barrier coatings

Abstract The effects of thermal exposure on the microstructure, properties and failure of electron beam physical vapor deposited Al 2 O 3 –yttria stabilized zirconia (YSZ) gradient thermal barrier coatings (GTBCs) were studied. The GTBCs, with lifetimes of more than 500 h for 1-h cycles and 13 h for 0.25-h cycles at 1100 °C, exhibited a better thermal shock resistance than two-layered-TBCs consisting of a NiCoCrAlY bond coat and a YSZ topcoat. The GTBCs also showed a relatively low oxidation rate under cyclic exposure, due to the formation of a pre-deposited Al 2 O 3 film on the bond coat. The oxidation of the bond coat is dominated by the selective oxidation of aluminum. The values of hardness and modulus for the Al 2 O 3 –YSZ graded layer are in the range of 2.0–3 and 170–230 GPa, respectively, whereas after 500 h exposure a considerable reduction in the mechanical properties occurred in a rich Al 2 O 3 area of the graded layer. Micro-cracks initiated and propagated in the rich Al 2 O 3 area, and finally resulted in the spallation failure of the coating. The thermal conductivity of a GTBC was found to be 1.7 W/mK, which was marginally lower than that of the two-layered-TBC at 2.2 W/mK. However, the value of thermal conductivity increased up to 2.0 W/mK after 500 h exposure.