Jia-Hu Ouyang

Three-dimensional graphene oxide/polypyrrole composite electrodes fabricated by one-step electrodeposition for high performance supercapacitors

Three-dimensional (3D) graphene oxide/polypyrrole (GO/PPy) composite electrodes have been fabricated via one-step electrochemical co-deposition in an aqueous solution containing pyrrole monomers, GO and LiClO4. The concentration of GO in the solution plays an important role in controlling the morphologies of the as-deposited GO/PPy composites, and a relatively low concentration of 0.1 mg mL−1 is favorable for the formation of a 3D interconnected structure. The unique 3D interconnected structure ensures fast diffusion of electrolyte ions through the electrode. As a result, the GO/PPy composite electrode with a mass loading of 0.26 mg cm−2 exhibits the highest specific capacitance of 481.1 F g−1, while the electrode with a larger mass loading of 1.02 mg cm−2 delivers the best area capacitance of 387.6 mF cm−2, at a current density of 0.2 mA cm−2. Moreover, the GO/PPy composite electrodes exhibit good rate capability with capacitance retentions over 80% when the current density load increases from 0.2 to 10 mA cm−2. Both the aqueous and solid-state supercapacitors based on GO/PPy composite electrodes show excellent capacitive properties with good cycling stability, indicating their suitability for applications in energy storage and management.

Low-pressure plasma-sprayed ZrO2-CaF2 composite coating for high temperature tribological applications

Abstract The microstructure and tribological behavior of low-pressure plasma-sprayed (LPPS) ZrO2–CaF2 composite coatings were studied. Optimum spray parameters were obtained to produce a less porous and strongly adherent ZrO2–CaF2 composite through carefully selecting the powder feed rate, primary gas pressure and spraying distance. The as-sprayed composite coating exhibited a typical lamellar structure of ZrO2 and CaF2 constituents, with a lot of microcracks in the splats. The resolidified interfacial structure featured by fine columnar grains were observed at the boundaries of ZrO2 lamellae and were considered to have formed due to the local temperature and compositional variations during the solidification process of the molten splats. Small amounts of discontinuous oxides distributed at the interface region between the coating and substrate were demonstrated to be a mixture of the complicated oxidized products of iron, chromium, nickel and calcium, ZrO2(Y2O3) particles, and independent Al2O3 and SiO2 particles located within the rough surface of the substrate. The ZrO2–CaF2 composite surface exhibited a distinct improvement in wear resistance and frictional characteristics in comparison to Y2O3-stabilized ZrO2 (YPSZ) coating at elevated temperatures. At 600 and 700°C, the composite exhibited a lower friction and wear than at room temperature, 400 and 800°C. CaF2, acting as a solid lubricant at 600°C, effectively reduces friction and wear. Different tribological behaviors were observed on the worn surfaces, with different microstructural features after the 600°C wear test. In the individual ZrO2 splats, microcracking and microfracture dropping led to material removal. However, in CaF2 splats smooth CaF2 surface films containing fine ZrO2 hard particles was formed to reduce the friction and wear. Brittle fracture and delamination of ZrO2–CaF2 composite were demonstrated to be the dominant wear mechanisms at room temperature and 400°C. Plastic deformation, the continuous formation of CaF2 transfer films, adhesive wear and viscous flow appeared as the dominant wear mechanisms at the higher temperature used in this investigation.

Laser cladding of yttria partially stabilized ZrO2 (YPSZ) ceramic coatings on aluminum alloys

Abstract A feasibility study of laser clad 7 wt.% yttria partially stabilized zirconia ceramic coatings undoped or doped with 2.5 wt.% TiO 2 to produce single tracks and partially overlapped tracks on the surfaces of AlSi9Cu3, AlZn10Si8Mg and AlSi10Mg alloys was carried out by using a 6 kW RS5000 continuous wave CO 2 laser together with continuous powder-feeding system. The dependencies of the macro and microquality of ZrO 2 ceramic coatings on the properties of the coating powder and laser parameters, and microstructure (such as morphology, phase transformation, etc.) were studied. The addition of 2.5 wt.% TiO 2 into the YPSZ powder is beneficial for the controllable cracking in the ceramic layer on AlSi9Cu3 alloy. The ceramic layers on three kinds of aluminum alloys exhibit planar ZrO 2 crystals with different sizes at the lower, columnar grains in the intermediate, and equiaxed grains at the upper region of the cross-sections. Small amounts of molten aluminum alloy are observed to segregate at the boundary regions of columnar or equiaxed grains in the ZrO 2 layer because of the poor affinity. The ceramic layers consist mainly of non-transformable t’ phase and small amount of the retained c phase. Under conditions of optimum laser parameters and coating powder properties, the bonding to aluminum alloys is satisfactory with small dilution. Molten aluminum alloy is also observed to flow into the cracked ZrO 2 layer at the bottom to seal these cracks. The hardness of the ZrO 2 ceramic layer was 1415–1575 HV0.1, which is mainly attributed to the absence of porosity and fine grains in the ceramic layers.

The friction and wear characteristics of low-pressure plasma-sprayed ZrO2-BaCrO4 composite coating at elevated temperatures

Abstract The friction and wear characteristics of low-pressure plasma-sprayed (LPPS) ZrO 2 -BaCrO 4 (ZB) coating at elevated temperatures have been studied using a high-temperature reciprocating wear tester. The ZB coating exhibits a less porous, alternative lamellar structure and a better splat spreading as contrasted to the porous yttria partially stabilized zirconia (YPSZ) coating. The hardness of the ZB coating is 276–435 HV, which is much lower than that of the YPSZ coating (650–727 HV). The ZB coating shows distinct improvements in wear resistance and frictional characteristics at elevated temperatures, as contrasted to the YPSZ coating. At room temperature, the friction coefficient of the ZB coating against sintered Al 2 O 3 is quite high and exhibits an increase in trend with increasing load. At above 300 °C, the ZB coating exhibits low friction and mild wear. But for the YPSZ coating, the situation is reversed with increasing temperature. Brittle fracture and delamination featured by large wear sheets are considered as the dominated wear mechanisms of the ZB coating at low temperatures. However, plastic deformation, formation and transfer of BaCrO 4 lubrication films appears as the main wear mechanisms at elevated temperatures. BaCrO 4 , acting as an effective solid lubricant at above 300 °C, reduces the friction and wear of the coating.

Effects of different additives on microstructure and high-temperature tribological properties of plasma-sprayed Cr2O3 ceramic coatings

Abstract The microstructure and high-temperature tribological properties of low-pressure plasma-sprayed Cr 2 O 3 ceramic coatings undoped or doped with different additives, such as CaF 2 , Ag 2 O and ZrO 2 were studied. Under the identical plasma-spray condition, the composite coatings doped with additives exhibited a larger thickness and distinctly lower hardness than pure Cr 2 O 3 coating. Within the as-sprayed composites, ZrO 2 and Cr 2 O 3 constituents were observed as fine lamellar structures and Ag 2 O as isolated particles with different gray levels in the back-scattered electron (BSE) mode, while CaF 2 appeared as a darker region with a great volume fraction. Some resolidified structure of ZrO 2 crystals featured by fine equiaxed or dendritic grains were clearly observed in the ZrO 2 lamellae on the cross-section or the polished surface. Friction coefficients of the composite coatings doped with additives were quite high at room temperature and gradually decreased with increasing of temperature. The situation was reversed for the pure Cr 2 O 3 coating. Among the three kinds of Cr 2 O 3 -based coatings, Cr 2 O 3 –CaF 2 –Ag 2 O (CFA) composite coating exhibited the lowest friction and wear characteristics at above 500°C and reached a minimum value of friction coefficient at 800°C. The Ag 2 O additive was beneficial to improve the adherence and wettability of the CaF 2 transfer film to the counterface, while the ZrO 2 additive was detrimental to the formation of a complete CaF 2 transfer film because of crushing and microfracture debris of ZrO 2 crystals on the worn surface at elevated temperature.

Laser cladding of ZrO2-(Ni alloy) composite coating

The microstructure of laser-clad 60 vol.% ZrO2 (partially stabilized with 2 mol% Y2O3) plus 40 vol.% Ni alloy composite coating on steel 1045 was investigated by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, energy-dispersive X-ray analysis and microhardness tests. The composite coating consists of a pure ZrO2 clad layer in the outer region and a bonding zone of Ni alloy adjacent to the substrate. The pure ceramic layer exhibits fine equiaxed ZrO2 grains in the outer zone and columnar ZrO2 dendrites in the inner zone, growing from the ceramic layer-bonding zone interface. This ceramic layer is composed of metastable t′-ZrO2 phase and a very small amount of m-ZrO2 phase and displays a microhardness of 1700 HV0.2. The high heating and cooling rate caused by laser cladding restrains the t → m phase transformation in the ZrO2 ceramic layer. Interdiffusion of alloy elements takes place in the bonding zone, in which the coexistence of ZrO2 particles, Ni-based solid solution and (Fe,Cr)23C6 particles in the interdendritic regions was found.

Microstructure and tribological characteristics of ZrO2–Y2O3 ceramic coatings deposited by laser-assisted plasma hybrid spraying

Abstract ZrO2–Y2O3 ceramic coatings were deposited on AISI 304 stainless steel by both a low-pressure plasma spraying (LPPS) and a laser-assisted plasma hybrid spraying (LPHS). Microstructure and tribological characteristics of ZrO2–Y2O3 coatings were studied using an optical microscope, a scanning electron microscope, and an SRV high-temperature friction and wear tester. The LPHS coatings exhibit distinctly reduced porosity, uniform microstructure, high hardness and highly adhesive bonding, although more microcracks and even vertical macrocracks seem to be caused in the LPHS coatings. The ZrO2 lamellae in the LPHS coatings before and after 800°C wear test consist mainly of the metastable tetragonal (t′) phase of ZrO2 together with small amount of c phase. The t′ phase is very stable when it is exposed to the wear test at elevated temperatures up to 800°C for 1 h. The friction and wear of the LPHS coatings shows a strong dependence on temperature, changing from a low to a high wear regime with the increase of temperature. At low temperatures, friction and wear of the LPHS coatings is improved by laser irradiation because of the reduced connected pores and high hardness in contrary to the LPPS coating. However, at elevated temperatures, the friction and wear of the LPHS coatings is not reduced by laser irradiation. At room temperature, mild scratching and plastic deformation of the LPHS coatings are the main failure mechanism. However, surface fatigue, microcrack propagation, and localized spallation featured by intersplat fracture, crumbling and pulling-out of ZrO2 splats become more dominated at elevated temperatures.

Characterization of laser clad yttria partially-stabilized ZrO2 ceramic layers on steel 16MnCr5

Abstract One-step laser cladding technique was used to melt 7 wt.% yttria partially stabilized zirconia (YPSZ) powder particles to produce single tracks of ZrO 2 ceramic layers on steel 16MnCr5. The ceramic layers were analyzed using optical microscope, scanning electron microscope (SEM), X-ray diffractometer (XRD) and microhardness testing techniques for the study of microstructure, topography, cracking formation and hardness. This laser cladding process involved a continuous feeding of ZrO 2 powder into the melt pool as the metal substrate was moved under laser beam at various traverse speeds. The optimum parameters for laser cladding were: 1.6–2.0 kW laser power, 300–500 mm/min traverse speed, 3 mm beam diameter and 5–7 g/min feed rate. Under these conditions, the clad ceramic layers consisted mainly of non-transformable t ′ phase and the retained c phase. Cross-sections of the clad layers showed three distinct microstructural regions which consisted of planar crystals at the bottom, columnar grains in the intermediate, and equiaxed grains at the upper. Two kinds of different combination modes were observed to be straight interface combination and diffusion fusion connection. In the latter combination, Fe and Cr elements diffused preferably along the grain boundaries into the ceramic layer.

Unlubricated friction and wear behavior of low-pressure plasma-sprayed ZrO2 coating at elevated temperatures

Abstract The unlubricated friction and wear behavior of low pressure plasma-sprayed (LPPS) ZrO 2 coating were studied by using a standard SRV wear test system with reciprocating motion against sintered Al 2 O 3 ceramic sphere from room temperature to 800°C. Microstructural observations showed that ZrO 2 splats appeared as fine lamellae structures together with small amounts of Al 2 O 3 and SiO 2 inclusions with different gray levels in the back-scattered electron (BSE) mode. The friction and wear of ZrO 2 coating showed a strong dependence on temperature, changing from a low to a high wear regime with the increase of temperature. ZrO 2 coating exhibited low friction and wear at room temperature and 200°C. However, when temperature was increased to above 400°C, friction and wear of the coating increased rapidly and reached a maximum at 600°C together with significant noise and distinct vibration caused by severe surface fracture. Intersplat fracture and surface fatigue of ZrO 2 splats were demonstrated to be the dominated wear mechanism at room temperature. Delamination caused by subsurface crack propagation and inclusion cracking was the dominated wear mechanism at 600°C. Above 700°C, recrystallization and abrasive wear in the form of removal of fine ZrO 2 grains became the dominated wear mechanism together with local plastic flow and viscous deformation.

Order–disorder transition and thermal conductivity of (Yb x Nd1− x )2Zr2O7 solid solutions

X-ray diffraction, transmission electron microscopy and a laser-flash method were used to investigate the order–disorder transition and thermal conductivity of (Yb x Nd1− x )2Zr2O7 (0 ≤ x ≤ 1.00) solid solutions. The structures were found to be pyrochlore-type for 0 ≤ x ≤ 0.25, defect fluorite for 0.45 ≤ x ≤ 1.00 and a mixture of these at 0.30 ≤ x ≤ 0.40. The thermal conductivities of (Yb x Nd1− x )2Zr2O7 first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbNdZr2O7 has the lowest thermal conductivity due to the reduced cation mean free path at the compositional combination of equal molar Yb3+ and Nd3+ cations.