Fabrication of 3D structures via direct ink writing of kaolin/graphene oxide composite suspensions at ambient temperature

Kaolin/graphene oxide composite has been widely utilized in aero-space and architectural engineering applications due to its excellent mechanical property. Direct ink writing (DIW) is a freeform rapid prototyping technology that could be used to accurately fabricate the resulting size with complex shapes. In this study, we reported the DIW of kaolin/graphene oxide (GO) composite suspensions (KGCS) to assemble 3D structures at ambient temperature for the first time. The effects of GO on the chemical constitution and microstructure of kaolin suspensions were investigated. Rheology was characterized to ensure printability of KGCS. The addition of GO in kaolin suspensions quickened a flocculation structure, which dramatically changed their rheology properties. The DIW of 3D structures from the optimal KGCS sample maintained their initial shape without spreading. The flexural and compressive strengths of the dried optimal KGCS samples were obviously enhanced due to the improvement and reduction of the micro-defects compared from cured kaolin matrix.     

Drilling machinability of engineering ceramics under low-frequency axial vibration processing by sintering/brazing composite diamond trepanning bit

Thin-wall diamond trepanning bits are extensively used for processing hard and brittle materials such as engineering ceramics. However, it is difficult to achieve high-efficiency processing of engineering ceramics at a constant feed speed, because of high dynamic compressive strength, high hardness, and low density of engineering ceramics. In this study, a novel composite diamond bit combining sintering and brazing has been designed, along with the low-frequency axial vibration technology, to realize the continuous hole processing of engineering ceramics. Drilling experiments have been conducted on Al₂O₃ and SiC engineering ceramics with a constant feed speed. The variation of axial force, micromorphology of hole wall surface drilled, as well as the method of removing nesting during the drilling process were analyzed. According to the results, the novel composite diamond bit fabricated by combining sintering and brazing, can achieve the continuous hole processing of engineering ceramics at a constant feed speed, including Al₂O₃ and SiC. Compared to the conventional drilling (CD), the low-frequency axial vibration drilling (LFVD) can significantly reduce the axial force, and produce fewer plastic scratches on the hole wall surface drilled. In particular, the automatic blanking ratio approaches to 100% by LFVD, and only about 73.58% by CD. It can be concluded that LFVD technology can be used to realize continuous hole processing of engineering ceramics. The research results achieved in this study show that the drilling machinability of engineering ceramics by LFVD and novel composite diamond bit is good. Accordingly, this study provides a useful reference for continuous processing or batch production of engineering ceramics at a constant feed speed.     

Room temperature R-curve and stable crack growth behaviour of ZrB2–SiC ceramic composites

ABSTRACT

R-curve and controlled stable crack growth behaviour of ZrB₂–17vol.-%SiC and ZrB₂–45vol.-%SiC ceramic composites was studied on V-notched samples using four-point bending at room temperature. The rising K_1R behaviour was determined as a function of the crack extension Δa with a crack bridging mechanism being dominant in such behaviour. Significant differences in crack growth rates were found within the same composition of ceramics simply as the crack length varied during crack growth processes. These differences are indicative of the significant influence of microstructural parameters of the ceramics on crack propagation. The peculiarities of stress intensity factor K₁ and the crack growth-specific behaviour in ZrB₂–SiC particulate ceramic composites are discussed.     

Feasibility study to control fiber distribution for enhancement of composite properties via three-dimensional printing

The distribution of fibers in the composite (which takes into account both their locations and orientations) is one of the important factors that affect the mechanical properties of FRCs. However, this parameter depends on various factors during composite fabrication, and controlling the distribution of fibers in the produced material represents a significant challenge. In this study, the applicability of three-dimensional (3D) printing technique for controlling fiber distributions was evaluated. The fibers fabricated using a 3D printer were placed inside a mold to produce cementitious composites. Three-point bending tests were conducted and the results of the experiment were discussed.     

High strain rate compressive response of the Cf/SiC composite

Carbon fiber reinforced ceramic owns the properties of lightweight, high fracture toughness, excellent shock resistance, and thus overcomes ceramic's brittleness. The researches on the advanced structure of astronautics, marine have exclusively evaluated the quasi-static mechanical response of carbon fiber reinforced ceramics, while few investigations are available in the open literature regarding elastodynamics. This paper reports the dynamic compressive responses of a carbon fiber reinforced silicon carbide (C_f/SiC) composite (CFCMC) tested by the material test system 801 machine (MTS) and the split Hopkinson pressure bar (SHPB). These tests were to determine the rate dependent compression response and high strain rate failure mechanism of the C_f/SiC composite in in-plane and out-plane directions. The in-plane compressive strain rates are from 0.001 to 2200?s^?1, and that of the out-plane direction are from 0.001 to 2400?s^?1. The compressive stress-strain curves show the C_f/SiC composite has a property of strain rate sensitivity in both directions while under high strain rate loadings. Its compressive stiffness, compressive stress, and corresponding strain are also strain rate sensitive. The compressive damage morphologies after high strain rate impacting show different failure modes for each loading direction. This study provides knowledge about elastodynamics of fiber-reinforced ceramics and extends their design criterion with a reliable evaluation while applying in the scenario of loading high strain rate.     

Alginate–chitosan composite hydrogel film with macrovoids in the inner layer for biomedical applications

Making of a layered composite using two biopolymer gels with regularly aligned voids in the inner layer is described in this article. Calcium alginate constituted the inner layer, within which voids of 500 μm diameter were embedded in monolayer or in multiple layers using a fluidic device for bubbling. The chitosan without any additional crosslinker was used to form the outer layer. The layered structure enabled compartmentalization of drug hold-up, and differential release rates. These aspects were reviewed using bovine serum albumin and vitamin B₁₂ as model solutes. The presence of voids at the inner layer of alginate increased the uptake, raising the level of absorptivity to more than 4000%. The composite film could hold two solutes at a time. The one, held inside the alginate layer started releasing only after 1 h of dipping in the release media. The adhesive strength between layers and the response of the composite film to compressive deformation are studied here. The effect of single or multiple layers of voids in the inner layer is reviewed. The slowing of degradation rate due to chitosan-encapsulation is experimentally determined. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47599.     

A Monte Carlo study of spring-exchange magnets containing ultra-high coercive phases

The aim of the present work is to study magnetization processes in the spring-exchange magnetic composites containing magnetically soft, hard and ultra-high coercive phases, experimentally found in Tb-Fe-B-Nb group of alloys. In order to modeling hysteresis loops a special Monte Carlo procedure,suitable for irregular geometry of the composites, was applied. The obtained results indicate that sufficiently strong coupling between soft and hard magnetic phase leads to collective, single-phase-like movement of such phases, which in turn, causes increase of the coercivity and internal magnetic energy.     

Preparation of Al2O3-mullite thermal insulation materials with AlF3 and SiC as aids by microwave sintering

Al₂O₃-mullite composites were prepared under the synergy effect of AlF₃ and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF₃ content. The microstructure showed that the lower content (≤1 wt%) of AlF₃ led to the formation of granular mullite and the higher content (≥3 wt%) of AlF₃ led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF₃ was 86%. The composite with 3 wt% AlF₃ additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.     

Size-dependent magnetoelectric response of (Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3)-(Ni0.8Zn0.2)Fe2O4 particulate composites

In this paper, we investigated the effect of magnetic grain size on magnetoelectric responses of particulate magnetoelectric 0.7(Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃)-0.3(Ni_0.8Zn_0.2)Fe₂O₄ (BNKT-NZFO) composites. The coexistence of two chemically separated phases was confirmed using x-ray diffraction analysis. The composites had homogeneous microstructure with controlled grain size. The magnetoelectric response of the BNKT-NZFO composites sensitively depended on the grain size of the NZFO phase and the magnetoelectric voltage coefficients presented a marked enhancement of 33% in the engineered grain size range. This result indicated that tailoring the magnetic grain size physically will provide a powerful mean of enhancing magnetoelectric coupling in a two-phase particulate composite, with large potential application in area of magnetic field sensor.