TEM Investigations on Layered Ternary Ceramics

Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics.These unique properties are strongly related to their layered crystal structures and microstructures. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy （STEM） and transmission electron microscopy （TEM）, selected area electron diffraction （SAED）, convergent beam electron diffraction （CBED） represents a powerful method to link microstructures of materials to macroscopic properties, allowing layered ternary ceramics to be investigated in an unprecedented detail. Vicrostructural information obtained using TEM is useful in understanding the formation mechanism, layered stacking characteristics, and defect structures for layered ternary ceramics down to atomic-scale level; and thus provides insight into understanding the ＂Processing-Structure-Property＂ relationship of layered ternary ceramics. Transmission electron microscopic characterizations of layered ternary ceramics in Ti-Si-C, Ti-Al-C, Cr-Al-C, Zr-Al-C, Ta-Al-C and Ti-Al-N systems are reviewed.

Current Status in Layered Ternary Carbide Ti_3SiC_2,a Review

This article provides a review of current research activities that concentrate on Ti3SiC2. We begin with an overview of the crystal and electronic structures, which are the basis to understand this material. Followings are the synthetic strategies that have been exploited to achieve, and the formation mechanism of Ti3SiC2. Then we devote much attentions to the mechanical properties and oxidation/hot corrosion behaviors of Ti3SiC2 as well as some advances achieved recently. At the end of this paper, we elaborate on some new discoveries in the Ti3SiC2 system, and also give a brief discussion focused on the ＂microstructure -property＂ relationship.

Fabrication of ceramic layer on an Al-Si alloy by MAO process

The MAO (Micro-Arc Oxidation) process is applied to a eutectic Al-Si alloy(Al-12.0 percent Si-l.0 percent Cu-0.9 percent Mg (mass fraction)). The oxide ceramic layer wasfabricated with about 220 mum thickness and 3000 Hv micro-hardness. By XRD (X-ray diffractometry)and DSC (differential scanning calorimetry) analyses, the oxide layer consists of amorphous Al_2O_3,which is distinct from the results reported by the other researchers. The SEM photographs of suchlayer show that the layer is fixed tightly on the substrate alloy. So this alloy can he used in thehigh temperature and friction environment alter it is treated with such process.

High-Temperature Electric Properties and X-ray Photoemission Spectra of Layered Co-Based Oxides Bi_(2-x)La_xSr_2Co_2O_(8-δ)

Layered Co-based ceramics with a nominal composition Bi2-xLaxSr2Co2O8-δ (x=0.0, 0.4, 0.8, short by BLC-222) were prepared using conventional solid state reaction method. X-ray photoemission spectroscopy (XPS) was used to investigate their electronic structures. The cobalt ions are highly mixed valences of Co3+ and Co4+. The fraction of Co4+ almost keeps unchanged with the increase of x. The O-1s photoemission spectra show that there are lattice oxygen and chemical absorbed oxygen in all the samples. The substitution of Bi3+ by La3+ results in a change from metallic-like behavior to semiconductor behavior. This abnormal phenomenon means that La3+ plays a key role in effecting the electrical transport property of BLC-222. The O-Co covalence bond is strengthened by the increase of La3+, which results in the decrease of conductivity.

Preparation and corrosion resistance of the micro-arc oxidation coating on Al_2O_3f/ZL109 composites

A ceramic layer was prepared on the surface of Al2O3f/ZL109 composites by means of micro-arc oxidation （MAO） technique. The surface morphology and phase constituent of the ceramic layer were analyzed using scanning electron microscope and X-ray diffraction. The polarization curves of the composites before and after MAO treatment were measured and analyzed. The results showed that after Al2O3f/ZL109 composites were treated using MAO technique in silicate solution, the ceramic layer formed, and it was composed of Al, Si, and mullite phase. Al and Si came from Al alloy matrix of the composites, and the mullite phase formed in process of MAO. Al2O3 fiber in the composites affects the electric conductivity of the composites, the MAO reaction is promoted, and the cera- mic layer forming on the composite material side is slightly thicker than that on the Al alloy side. After Al2O3f/ZL109 composites were treated using MAO technique, the corro- sion resistance of the composites is significantly improved.

Investigation on plasma-sprayed ZrO_2 thermal barrier coating on nickel alloy substrate

The thermal barrier coatings with NiCrAlY alloy bonding layer, NiCrAlY Y 2O 3 stabilized ZrO 2 transition layer and Y 2O 3 stabilized ZrO 2 ceramic layer are prepared on nickel alloy substrates using the plasma spray technique. The relationship among the composition, structure and property of the coatings are investiga ted by means of optical microscope, scanning electronic microscope and the experiments of thermal shock resistance cycling and high temperature oxidation resistance. The results show that the structure design of introdu cing a transition layer between Ni alloy substrate and ZrO 2 ceramic coating guarantees the high quality and properties of the coatings; ZrO 2 coatings doped with a little SiO 2 possesses better thermal shock resistance and more excellent hot corrosion resistance as compared with ZrO 2 coating materials without SiO 2 ;the improvement in performance of ZrO 2 coating doped with SiO 2 is due to forming more dense coating structure by self closing effects of the flaws and pores in the ZrO 2 coatings.

High-performance (K,Na)NbO_(3)-based multilayer piezoelectric ceramic actuators with nickel inner electrodes

Multilayer ceramic actuator(MLCA)has been widely employed in actuators due to the large cumulative displacement under the low driving voltage.In this work,the MLCA devices consisting of a lead-free MnCO_(3-)and CuO-doped 0.96(K_(0.48)Na_(0.52))(Nb_(0.96)Ta_(0.04))O_(3)-0.04CaZrO_(3) piezoelectric ceramics and a base nickel(Ni)metal inner electrode were well co-fired by the two-step sintering process in a reducing atmosphere.The ceramic layer/electrode interface is well-integrated and clearly continuous without distinct interdiffusion and chemical reaction,which is beneficial to the electrical reliability of the MLCA.As a result,the MLCA laminated with nine active ceramic layers obtains an ultrahigh piezoelectric coefficient d_(33) of 3157 pC/N,about 9 times than bulk ceramics.The 0.5 mm-thick MLCA composed of a series of~50μm-thick ceramic layers and~3μm-thick Ni electrodes reaches a high 1.8μm displacement under the low applied voltage of 200 V(the same displacement requires a voltage as high as 3700 V for~1 mm-thick bulk ceramics).The excellent electrical performance and low-cost base electrode reveal that the(K,Na)NbO_(3)(KNN)-based MLCAs are promising lead-free candidate for actuator application.

Microstructure and properties of nano-laminated Y_(3)Si_(2)C_(2) ceramics fabricated via in situ reaction by spark plasma sintering

A nano-laminated Y_(3)Si_(2)C_(2) ceramic material was successfully synthesized via an in situ reaction between YH_(2)and SiC using spark plasma sintering technology.A MAX phase-like ternary layered structure of Y_(3)Si_(2)C_(2) was observed at the atomic-scale by high resolution transmission electron microscopy.The lattice parameters calculated from both X-ray diffraction and selected area electron diffraction patterns are in good agreement with the reported theoretical results.The nano-laminated fracture of kink boundaries,delamination,and slipping were observed at the tip of the Vickers indents.The elastic modulus and Vickers hardness of Y_(3)Si_(2)C_(2) ceramics(with 5.5 wt%Y_(2)O_(3)) sintered at 1500℃were 156 and 6.4 GPa,respectively.The corresponding values of thermal and electrical conductivity were 13.7 W·m^(-1)·K^(-1) and 6.3×10^(5)S·m^(-1),respectively.

Observation of low thermal expansion behavior and weak thermal anisotropy in M_(3)A_(2)C phases

M_(3)A_(2)X phases,named 321 phases,are an atypical series of MAX phases featuring in the MA-triangular-prism bilayers,with the A=As/P,exhibiting excellent elastic properties.This work systematically studies the thermal expansion properties of 321 phases.We found their average linear thermal expansion coefficients(TECs),α_(L)=5-6μK^(–1),are the lowest among the reported values of MAX phases.The lowest average TEC was found in Nb_(3)As_(2)C(αa=4.46(4)μK^(–1),αc=5.09(4)μK–1,αL=5.09(4)μK–1).The average TEC and anisotropy factor(αc/αa)of Nb_(3)As_(2)C and Nb_(3)P_(2)C were lower than the ones of the corresponding 211 phases.The best isotropy performance was found in Nb_(3)P_(2)C (αc/αa=1.11).Moreover,our first-principles calculations demonstrate that the weaker chemical bonding between Nb-As/P than Nb-C induces thermal expansion in M_(3)A_(2)X phases.Furthermore,a relatively weaker anharmonic effect in 321 phases than in the 211 phases was revealed by the as-calculated average Grüneisen parameters,which account for the lower TECs in 321 phases.The low TECs and enhanced thermal isotropy make 321 phases outstanding among MAX phases,which could be sound candidates for varying-temperature structural-functional components.

Cross-polarization suppression in C-shaped microstrip patch antenna employing anisotropic dielectrics

An anisotropic dielectric realized by layered ceramic structures was adopted to design a low cross-polarization C-shaped patch antenna.The anisotropic dielectric performs as a substrate and can cause additional cross-polarized fields which are able to cancel the cross-polarized fields generated by the C-shaped patch itself,and then reduce the cross-polarization level.Compared to the C-shaped patch antenna with an isotropic substrate,the cross-polarization of the proposed antenna is suppressed by more than 15 dB with a little gain enhancement at 2.4 GHz.The anisotropic dielectric has a little impact on the direction of the C-shaped patch antenna.The gain of the proposed C-shaped patch antenna is 6.8 dB with a cross-polarization of
28 dB.

Phase transformation of ZrO2 doped with CeO2

Thermal barrier coatings（TBCs） protection is widely used to prolong the lifetime of turbine components.The outermost layer of TBCs is ceramic layer, whose function is heat insulation, and the main composition of the ceramic layer is ZrO2. In this study, the micro-Zr02 and the nano-ZrO2 doped with 10 wt% CeO2 as well as microZrO2 and nano-ZrO2 were prepared by air plasma spraying（APS） to study the advantages of the addition of rare earth element. The effect of CeO2 on the phase transformation of ZrO2 was studied. The results show that there are few cracks in micro-and nano-ZrO2 doped with 10 wt% CeO2,and rare earth oxides can affect the phase transformation significantly. The morphologies, hardness and elastic modulus of the four ceramic layers were also discussed.

In situ selective laser gas nitriding for composite TiN/Ti-6Al-4V fabrication via laser powder bed fusion

Laser-assisted gas nitriding of selective Ti-6Al-4V surfaces has been achieved during laser powder bed fusion fabrication by exchanging the argon build gas environment with nitrogen.Systematic variation of processing parameters allowed microdendritic Ti N surface coatings to be formed having thicknesses ranging from a few tens of microns to several hundred microns,with TiN dendrite microstructure volume fractions ranging from 0.6 to 0.75;and corresponding Vickers microindentation hardness values ranging from^7.5 GPa–9.5 GPa.Embedded TiN hard layers ranging from 50μm to 150μm thick were also fabricated in the laser-beam additively manufactured Ti-6Al-4V alloy producing prototype,hybrid,planar composites having alternating,ductile Ti-6Al-4V layers with a hardness of^4.5 GPa and a stiff,TiN layer with a hardness of^8.5 GPa.The results demonstrate prospects for fabricating novel,additively manufactured components having selective,hard,wear and corrosion resistant coatings along with periodic,planar or complex metal matrix composite regimes exhibiting superior toughness and related mechanical properties.