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In this study, 37 kinds of high-entropy pyrochlore/fluorite ceramics with equiatomic multicomponent compositions were successfully synthesized and the process of phase transformation was also investigated. Eventually, the experimental results show that 33 among the high-entropy compounds exhibit a single pyrochlore/fluorite structure with all cations randomly and uniformly distributed at A and B sites. Meanwhile, the ionic radii, valence, and size disorder may be the main driving forces for the formation of single-phase high-entropy pyrochlore/fluorite oxides. Furtherly, the average radius ratio(rA/rB) is the main factor of the pyrochlore/fluorite phase transformation. And when the average radius ratio(rA/rB) extends from 1.46 to 1.78, the high-entropy pyrochlore structure is stable. When the average radius ratio rA/rB is less than 1.46, it transforms into the defective fluorite structure. This study not only further expands the high-entropy pyrochlore family and but also is expected to guide the design of high-entropy pyrochlore through composition customization.

Dense SiC ceramics with a high alignment of graphene nanoplatelets (GNPs) were fabricated via a liquid-phase hot pressing (HP) process. The effects of GNPs on the phase composition, microstructure evolutions, mechanical and thermal properties of SiC ceramics were analyzed in detail. As the GNPs content increased from 0 to 3 wt %, the density and hardness of the SiC/GNPs composites decreased monotonically, whereas the flexural strength, fracture toughness and thermal conductivity increased initially and then decreased. The decrease in density and hardness is due to the low density and low hardness of GNPs themselves. The increase in thermal conductivity is mainly due to electron thermal transmission and the widening effect of phonon thermal transmission channel by the textured GNPs. The main strengthening and toughening mechanisms include fine grain strengthening, dislocation strengthening, crack bridging, crack deflection and crack termination. The SiC/GNPs composites sintered under uniaxial pressure condition (HP) display anisotropic thermal and mechanical properties. The anisotropic thermal conductivities of the SiC/GNPs composites are likely due to in plane GNPs parallel to the direction of heat flow, while through plane GNPs are perpendicular to the direction of heat flow. The anisotropy in mechanical properties of the SiC/GNPs composites are due to the through plane GNPs preferentially oriented to intersect the cracks, while the in plane GNPs preferentially oriented to parallel the cracks.

Porous SiOC ceramic was successfully prepared by pyrolysis of dimethylsilicone oil, silane coupling agent and melamine foam. The microwave absorbing properties of porous SiOC were studied for the first time. At the matching layer thickness of 3.0 mm, the paraffin-based composite with porous SiOC displays a minimum reflection coefficient (RC) of −39.13 dB (11.76 GHz) and an effective absorption bandwidth (EAB) of 4.64 GHz which are much larger than that of paraffin-based composite with ordinary SiOC. It is found that the porous structure of SiOC is crucial to achieve its high microwave absorption performance by improving both the polarization loss and conduction loss. The enhanced polarization loss is originated from the dipole polarization and interfacial polarization, while the improvement of conduction loss is attributed to the carbon skeleton of porous SiOC. These results indicate that porous SiOC ceramic is a promising candidate for high-performance ceramic-based microwave absorbing materials.

Ti3AlC2 ceramics were successfully joined with TiO2 and Nb2O5 respectively as interlayers via solid-state diffusion bonding at 1300 °C. The joints bonded with TiO2 and Nb2O5 exhibit distinct microstructures. Two continuous thin Al2O3 oxidation layers with a thickness around 1 μm were formed in the joints bonded with TiO2. Between the oxidation layers there exists a dense oxycarbide (TiC1-xOx) layer. For the joint bonded with Nb2O5, a dense bonding layer with Nb2AlC and Nb4AlC3 grains surrounded by thin Al2O3 oxidation layers at the grain boundaries were obtained. The shear strength of the final joints shows clear dependences on both the thickness and microstructure of the joints. Smaller joint thickness and the microstructure with complex phases favour for higher shear strength. Those result implies that bonding with oxides is a practical and efficient method for joining Ti3AlC2.

Oxygen 18 was used as a tracer to quantify anion diffusion in thin-film UO2 using secondary ion mass spectroscopy to measure one-dimensional depth profiles. Both thermal and heavy ion bombardment (1.8 MeV Kr+) treatments were employed over a temperature range from 295 K to 623 K. Textured and single-crystal thin-film samples were grown using reactive-gas magnetron sputtering at ambient temperature. Both microstructures resulted in similar thermal activation energies, Ea = 0.2 eV. This activation energy is significantly lower than the known value for intrinsic anion vacancy-self diffusion in stoichiometric UO2.00 (Ea = 2.5 eV). We attribute this to an interstitialcy mechanism in our hyper-stoichiometric films. The activation energy for irradiated textured films was approximately half that of thermal diffusion, consistent with the chemical rate theory. The opposite was true for the single-crystal microstructure (irradiated Ea = 0.36 eV). This may be due to radiation-induced changes in the microstructure. The mixing parameter was quantified on the anion sublattice as well, ξ = 2.1 ± 0.2 A5eV−1.

Cr coatings deposited on zircaloy-4 substrates have been oxidized in high temperature steam environment from 1000 °C to 1200 °C in this study. It has been reported by many authors that Cr coatings have exhibited excellent oxidation resistance in steam environment because of the formation of a dense and adherent Cr2O3 layer on the surface. However, we have found that in the stage when the Cr coating has completely been oxidized and approached the coating-substrate interface, it is reduced by the Zr substrate and mostly transformed into a metallic Cr layer. The mechanisms of oxidation in this stage is therefore more interesting than generally assumed and we have proposed a new model based on the diffusion and redox reaction between Zr and Cr layers in high temperature steam environment.

Macroscopic parallel aligned non-woven carbon fibers were incorporated into Al2O3 composites in this study to evaluate the contribution of multiple reflections to the total electric magnetic interference (EMI) shielding. Results indicate that parallel aligned non-woven carbon fiber layers contribute significantly to the total EMI shielding effectiveness (SET) of Al2O3 composites by largely enhancing the EMI absorption, and seven parallel aligned thin non-woven carbon fiber layers finally make the almost microwave-transparent Al2O3 an excellent EMI shielding material with an EMI SET as high as 29–32 dB in the X-band frequency range. The volume fraction of carbon fibers in Al2O3 composites with seven carbon fiber layers is calculated to be only 0.5%, and therefore the EMI SE enhancement efficiency by parallel aligned large non-woven carbon fiber layers is much higher than other highly conducting nano fillers. It validates the significance of multiple reflections in achieving high EMI shielding properties in ceramic composites and provides an instructive approach to design efficient EMI shielding ceramic composites.

FeCrAl alloys have exhibited excellent oxidation resistance in high temperature steam environment. However, over the past few years, thin film FeCrAl on zircaloy substrate can only tolerate up to 1000 °C steam because of the fast diffusion between zirconium and iron. In this study, we introduce Mo as a buffer layer and significantly reduced the inter-diffusion of zirconium and iron. The 7-micron thick FeCrAl coatings have shown excellent oxidation resistance up to 1200 °C, which is the highest temperature of thin FeCrAl coatings reported in literature. A systematic study of the oxidation behavior of FeCrAl coatings at 1000, 1100, 1200 °C is presented in this paper.

TiO2 was selected as effective sintering aid for pressureless sintering of Ti3AlC2 ceramics in this study. The addition of only 5 wt% TiO2 largely promotes the densification and nearly dense Ti3AlC2 ceramic was obtained by pressureless sintering at 1500 °C. Significant strengthening and toughening effects were observed with the addition of TiO2. High Vickers hardness, flexural strength and fracture toughness of 3.22 GPa, 298 MPa and 6.2 MPa m−1/2, respectively, were achieved in specimen pressureless sintered with 10 wt% TiO2. Additionally, the addition of 5 wt% TiO2 had no deleterious effect on the excellent oxidation resistance of Ti3AlC2 ceramic under 1200 °C water vapor atmosphere, while addition of 10 wt% TiO2 accelerates the oxidation rate by two orders of degree.

Graphene nanoplatelets (GNPs) were successfully incorporated into silicon carbide (SiC) ceramic matrix in a self-aligned pattern and the obtained materials displayed extremely high value of shielding effectiveness (SE) over 40 dB by adding only 3 wt.% GNPs, which was the highest SE value in all SiC-based composites reported in literature up to now. It was found that the texture distribution of GNPs was crucial to achieve the high electromagnetic interference shielding performance of SiC/GNPs composites, which can contribute to the significant improvement of both absorption and reflection. The improved absorption originated from the formation of network of mini capacitors comprised of self-aligned GNPs and multiple reflections. The improvement of reflection was attributed to the high electrical conductivity of the composite due to the introduction of GNPs. These results indicate that SiC/GNPs composites can be used as high-performance ceramic-based EMI shielding materials.

A ZrO2/FeCrAl bilayer coating was prepared on Zr-4 alloy. Plasma electrolytic oxidation (PEO) was used to synthesize the ZrO2 buffer layer and magnetron sputtering was used to deposit the FeCrAl outer layer. The oxidation resistance of the ZrO2/FeCrAl coated Zr-4 alloy was investigated by using 1000 °C steam oxidation tests and was compared with the mono-layer FeCrAl coated and uncoted Zr-4 alloys. Morphologies, phase compositions and elemental distributions of the ZrO2/FeCrAl and FeCrAl coated Zr-4 alloys before and after the steam oxidation tests were characterized by SEM, XRD and EDX mapping and line scans. Results indicated that the introduction of the ZrO2 buffer layer effectively eliminated the high temperature inter-diffusion between FeCrAl and Zr-4 alloy, and consequently improved the adherence and integrity of the FeCrAl layer after the 1000 °C steam oxidation. As a result, the parabolic rate constant of the ZrO2/FeCrAl coated Zr-4 alloy was decreased by about 20 times compared with that of the uncoated and FeCrAl coated Zr-4 alloys in 1000 °C steam environment.

Cr coatings deposited on zircaloy substrates generally exhibit (110) and (200) preferred orientations, which in turn affects the microstructure and oxidation performance of coatings. In this study, the preferential orientation of Cr coatings has been regulated in order to enhance the oxidation resistance of Cr coatings. For the pristine coatings, the deposition rate and surface morphologies depend strongly on the bias voltage and Ar pressure. While the evolutions of the texture coefficients are affected significantly by temperature, bias voltage and Ar pressure, especially for the (110) texture. The oxidation resistance of coatings with different texture has been investigated according to the energy-dispersive X-ray spectrometry. After oxidized in steam at 800 ℃ for 1 h, a transition layer and dispersed particles with higher oxygen content have been discovered. In addition, coatings deposited at pressure higher than 1.0 Pa exhibit through-film cracks and the zirconium substrates have been partially oxidized.

Iron-chromium-aluminum (FeCrAl) coatings deposited on Zircaloy 2 (Zy2) and yttria-stabilized zirconia (YSZ) by magnetron sputtering have been tested with respect to oxidation weight gain in high-temperature steam. In addition, autoclave testing of FeCrAl-coated Zy2 coupons under pressure-temperature-dissolved oxygen coolant conditions representative of a boiling water reactor (BWR) environment has been performed. Four different FeCrAl compositions have been tested in 700 °C steam; compositions that promote alumina formation inhibited oxidation of the underlying Zy2. Parabolic growth kinetics of alumina on FeCrAl-coated Zy2 is quantified via elemental depth profiling. Autoclave testing under normal BWR operating conditions (288 °C, 9.5 MPa with normal water chemistry) up to 20 days demonstrates observable weight gain over uncoated Zy2 simultaneously exposed to the same environment. However, no FeCrAl film degradation was observed. The 900 °C eutectic in binary Fe–Zr is addressed with the FeCrAl-YSZ system.

Cr coatings have been deposited on the outer surface of zircaloy-4 tube with well control of thickness variations along the tube axis. A series of samples have been oxidized in high temperature steam environment at 1200 °C for different durations. Cr2O3 layers are formed on the surface of the Cr coatings after high temperature oxidation, however, we have found that the thickness of the Cr2O3 layers increases first, and then decreases. The reason is because the Cr2O3 is reduced by the zirconium substrate when the thickness of Cr2O3 reaches its maximum. More detailed study revealed that the entire process can be divided into three stages: initial oxidation stage (I); reduction stage (II); and the oxidation of the sub-coating zirconium at a lower rate than the uncoated Zry4 (III). In addition, precipitation of secondary phases both within the coatings and the zirconium substrate has been studied in detail which plays an important role during the oxidation process. Therefore, the oxidization behavior of the Cr-Zr system is more complex than generally assumed. This study has provided sufficient experimental evidences for the redox reaction between the Cr coating and Zry4 substrate. Diffusion channels of oxygen ions during the reduction reaction have also been studied systematically.

High-entropy (Zr,Ti,Hf,Nb,Ta)C ceramic with single-phase rock salt structure was prepared by spark plasma sintering and their oxidation behavior was investigated at 1200 °C water vapor atmosphere. Results indicate that high-entropy carbides exhibit high oxidation resistance with a small mass gain per unit area of 6.6 × 10−2 kg/m2 after oxidation for 5 h. The parabolic rate constant kp of (Zr,Ti,Hf,Nb,Ta)C ceramic is almost two order of magnitude lower than that of ZrC ceramic. The high oxidation resistance is generally ascribed to the large lattice distortion and the formation of dense oxide scale which significantly suppressed the metal elements and H2O diffusion.

High-energy synchrotron X-ray diffraction was utilized to study the mechanical response of the f.c.c $\delta$-hydride phase, the intermetallic precipitation with hexagonal C14 lave phase and the $\alpha$-Zr phase in the Zircaloy-4 materials with a hydride rim/blister structure near one surface of the material during in-situ uniaxial tension experiment at 200 \si{\degreeCelsius}. The f.c.c $\delta$ was the only hydride phase observed in the rim/blister structure. The conventional Rietveld refinement was applied to measure the macro-strain equivalent response of the three phases. Two regions were delineated in the applied load versus lattice strain measurement: a linear elastic strain region and region that exhibited load partitioning. Load partitioning was quantified by von Mises analysis. The three phases were observed to have similar elastic modulus at 200 \si{\degreeCelsius}.