Preparation and anisotropic properties of textured structural ceramics: A review

Ceramics are usually composed of randomly oriented grains and intergranular phases, so their properties are the statistical average along each direction and show isotropy corresponding to the uniform microstructures. Some methods have been developed to achieve directional grain arrangement and preferred orientation growth during ceramic preparation, and then textured ceramics with anisotropic properties are obtained. Texture microstructures give particular properties to ceramics along specific directions, which can effectively expand their application fields. In this review, typical texturing techniques suitable for ceramic materials, such as hot working, magnetic alignment, and templated grain growth(TGG), are discussed. Several typical textured structural ceramics including α-Al2O3 and related nacre bioinspired ceramics, Si3N4 and SiAlON, h-BN, MB2 matrix ultra-high temperature ceramics, MAX phases and their anisotropic properties are presented.

Microstructural evolution of h-BN matrix composite ceramics with La-Al-Si-O glass phase during hot-pressed sintering

BN/La-Al-Si-O composite ceramics were fabricated by hot-pressed sintering using hexagonal boron nitride(h-BN),lanthanum oxide(La_(2)O_(3)),aluminia(Al_(2)O_(3)),and amorphous silica(SiO_(2))as the raw materials.The effects of sintering temperature on microstructural evolution,bulk density,apparent porosity,and mechanical properties of the h-BN composite ceramics were investigated.The results indicated that La-Al-Si-O liquid phase was formed during sintering process,which provided an environment for the growth of h-BN grains.With increasing sintering temperature,the cristobalite phase precipitation and h-BN grain growth occurred at the same time,which had a significant influence on the densification and mechanical properties of h-BN composite ceramics.The best mechanical properties of BN/La-Al-Si-O composite ceramics were obtained under the sintering temperature of 1700℃.The elastic modulus,flexural strength,and fracture toughness were 80.5 GPa,266.4 MPa,and 3.25 MPa·m^(1/2),respectively.

First-principles study of the anisotropic thermal expansion and thermal transport properties in h-BN

The thermal expansion coefficient(TEC)and thermal conductivity(k)of thermal fillers are key factors for designing thermal management and thermal protection composite materials.Due to its unique advantages,hexagonal boron nitride(h-BN)is one of the most commonly used thermal fillers.However,its TEC and k values are still unclear due to the inconsistency of characterization techniques and sample preparations.In this work,these disputes were addressed using the quasi-harmonic approximation(QHA)method and phonon Boltzmann transport equation(BTE)theory based on the density functional theory(DFT),respectively.The accuracy of our calculated TEC and k values was confirmed by previously reported experimental results,and the underlying physical principles were analyzed from the phonon behaviors.Our TEC results show that the h-BN has small in-plane negative value and large cross-plane positive value,which are-2.4×10^(-6) and 36.4×10^(-6) K^(-1) at 300 K,respectively.And the anisotropic TEC is mainly determined by the anisotropic isothermal bulk modulus and the low-frequency out-of-plane longitudinal phonon modes.We found that the convergence of cutoff radius and q-grid size have significant effect on the accuracy of k of h-BN.Our results show that the in-plane k is much higher than the cross-plane k,and the values at 300 K are 286.6 and 2.7 W m^(-1) K^(-1),respectively.The anisotropic phonon group velocity arising from the vibration behaviors of acoustic phonon modes should be primarily responsible for the anisotropic k.Our calculated TEC and k values will provide important references for the design of h-BN composite materials.

Texture and anisotropy of hot-pressed h-BN matrix composite ceramics with in situ formed YAG

Textured hexagonal boron nitride(h-BN)matrix composite ceramics were prepared by hotpressing using different contents of 3Y_(2)O_(3)–5Al_(2)O_(3)(molar ratio of 3:5)as the sintering additive.During hot-pressing,the liquid Y_(3)Al_(5)O_(12)(YAG)phase showing good wettability to h-BN grains was in situ formed through the reaction between Y_(2)O_(3) and Al_(2)O_(3),and a coherent relationship between h-BN and YAG was observed with[010]_(h-BN)//[111]_(YAG) and(002)_(h-BN)//(321)_(YAG).In the YAG liquid phase environment formed during hot-pressing,plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction,forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction,which give these composite ceramics anisotropy in their mechanical and thermal properties.The highest texture degree was found in the specimen with 30 wt%YAG,which also possesses the highest anisotropy degree in thermal conductivity.The aggregation of YAG phase was observed in the specimen with 40 wt%YAG,which resulted in the buckling of h-BN plates and significantly reduced the texture degree.

Microstructural evolution and mechanical properties of in situ nano Ta_(4)HfC_(5) reinforced SiBCN composite ceramics

The in situ nano Ta_(4)HfC_(5) reinforced SiBCN-Ta_(4)HfC_(5) composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering.The microstructural evolution and mechanical properties of the resulting SiBCN-Ta_(4)HfC_(5) were studied.After the first-step milling of 30 h,the raw materials of TaC and HfC underwent crushing,cold sintering,and short-range interdiffusion to finally obtain the high pure nano Ta_(4)HfC_(5).A hybrid structure of amorphous SiBCN and nano Ta_(4)HfC_(5) was obtained by adopting a second-step ball-milling.After reactive hot-pressing sintering,amorphous SiBCN has crystallized to 3C-SiC,6H-SiC,and turbostratic BN(C)phases and Ta_(4)HfC_(5) retained the form of the nanostructure.With the in situ generations of 2.5 wt% Ta_(4)HfC_(5),Ta_(4)HfC_(5) is preferentially distributed within the turbostratic BN(C);however,as Ta_(4)HfC_(5) content further raised to 10 wt%,it mainly distributed in the grain-boundary of BN(C) and SiC.The introduction of Ta_(4)HfC_(5) nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics,reaching to 344.1 MPa and 4.52 MPa·m^(1/2),respectively.This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix,which is beneficial to the real applications of SiBCN ceramics.