Covalently bonded silicon carbide (SiC) ceramics exhibit preeminent properties, yet their application scope is still challenged because the low self-diffusivity makes densification difficult. Herein, we use different sintering aids (TiO 2 , TiO 2 /Y 2 O 3 , and ZrO 2 /Y 2 O 3 ) to liquid-phase sinter SiC ceramics. The formation mechanism as well as the microstructure of the intergranular phase (IGP) and the interfacial relation between IGP and matrix grains on the densification and mechanical properties of the material were studied. The results showed that the formed IGP greatly differed in morphology, crystalline nature, and interface structure with the matrix grains. Particularly, fully dense SiC ceramic with a continuous network structure crystallized IGP was fabricated using ZrO 2 /Y 2 O 3 . Benefiting from the proper bonding strength between the IGP and matrix grains, crack deflection took place at the interface between the two, which could enhance the flexural strength and fracture toughness of SiC ceramic up to 725 MPa and 7.27 MPa·m 0.5 , respectively.
LiNiO 2 (LNO) is one of the most promising cathode materials for lithium-ion batteries. Tungsten element in enhancing the stability of LNO has been researched extensively. However, the understanding of the specific doping process and existing form of W are still not perfect. This study proposes a lithium-induced grain boundary phase W doping mechanism. The results demonstrate that the introduced W atoms first react with the lithium source to generate a Li–W–O phase at the grain boundary of primary particles. With the increase of lithium ratio, W atoms gradually diffuse from the grain boundary phase to the interior layered structure to achieve W doping. The feasibility of grain boundary phase doping is verified by first principles calculation. Furthermore, it is found that the Li 2 WO 4 grain boundary phase is an excellent lithium ion conductor, which can protect the cathode surface and improve the rate performance. The doped W can alleviate the harmful H2↔H3 phase transition, thereby inhibiting the generation of microcracks, and improving the electrochemical performance. Consequently, the 0.3 wt% W-doped sample provides a significant improved capacity retention of 88.5 % compared with the pristine LNO (80.7 %) after 100 cycles at 2.8–4.3 V under 1C.
Ziming Zhang,Chen Ming,Qingfeng Song,Lei Wang,Hanbing Chen,Jincheng Liao,Chao Wang,Lidong Chen,Shengqiang Bai
Mg 3 (Sb,Bi) 2 -based Zintl compounds are rising stars with superior features of high thermoelectric (TE) performance, nontoxicity and low cost. However, the poor thermodynamic stability hinders its practical application. Here, we propose a design strategy of modulating the local chemical composition with grain boundary (GB) phase guided by computational phase equilibrium diagrams. By in-situ constructing ZrSb 1- x second phase in the GB regions of Mg 3 (Sb,Bi) 2 -based compounds, the migration of Mg is blocked and the Mg deletion is depressed. Benefit from these, the driving force of Mg diffusion from matrix to GB and Mg migration along GBs is weakened, resulting in an improved thermal stability without degradation of electrical transport properties. The fabricated 8-pair TE module using GB modulated n-type Mg 3.1 Sb 1.5 Bi 0.49 Te 0.01 and p-type Ge 0.89 Cu 0.06 Sb 0.08 Te presents an efficiency of 8.9 % under the temperature difference of 450 K and shows excellent durability in the thermal cycling test. This study provides an effective strategy to improve thermal stability by constructing GB phase, and proves the feasibility of the practical application for Mg 3 Sb 2 -based TE modules.
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