Whisker Network Research Articles

Whisker Bond: From Findings to Concept in Refractories

Ceramic bonds are conventionally formed during the<sup> </sup>burning of<sup> </sup>refractory bricks and by using pre-fabricated blocks or monolithic lining, which is characterized as the coalescence of the particles by liquid sintering. However, the whisker bond was discovered with the outstanding performance of unburnt periclase–spinel–Al bricks while substituting magnesia–chrome bricks in the chromium-free campaign of refractory lining of RH degassers. Thanks to the prominent effect of the whisker bond, such a refractory material demonstrates ultrahigh hot strength, high resistance to slag penetration, and thermomechanical stress. Investigations reveal the initial melting of metal Al at the melting point of 660°C, aluminum liquid rims around the cavities formed before ~800°C, gaseous AlN yielded and distributed throughout<sup> </sup>the matrix with increasing temperature, gaseous Mg reduced from ~1000°C, and MgAlON whiskers eventually formed in the matrix. Microstructure observations show a dense interwoven whiskers bonded matrix in most residual parts of the used periclase–spinel–Al bricks. The whisker network in the matrix is made up of<sup> </sup>straight columns<sup> </sup>of 1–5 µm in diameter and 20–40 µm in length, which is different from birdnesting, nano-size curly whiskers observed in the past. The findings suggest a whisker-bond concept in terms of the bond mode of the whisker network and the process of the vapor-solid forming mechanism.

In situ mullite whisker network formation for high strength and lightweight ceramic proppants

In this study, a facile in situ sintering method has been proposed for fabricating high strength lightweight mullite whisker network reinforced proppants. The effect of TiO2 on the microstructure, properties and fracture behaviours of the mullite whisker network was researched in detail. The addition of TiO2 promoted the formation of a modified mullite whisker network structure, which effectively improved the flexural strength. The sample containing 4 wt.% TiO2 exhibited an excellent flexural strength of 215.25MPa and low bulk density of 1.53 g/cm3 at 1400 ?C. The TiO2-added mullite whiskers mostly exhibited rod-like crystals with about 0.5- 0.8 ?m in diameter and 10-15 ?m in length. The as-prepared ceramic proppant showed excellent properties, such as low breakage ratio (2.19%under 52MPa), low density (1.36 g/cm3) and low acid solubility (2.28 wt.%).

Microstructure control of porous Si3N4 whisker scaffolds by in-situ carbothermal reduction and nitridation reaction

Si3N4 whisker scaffolds have great potentials owing to the superior properties of Si3N4 and their unique porous structures. Obviously, the properties of the scaffolds are closely related to the morphology and distribution of phases, and the bonding between whiskers. This study proves that the above-mentioned microstructural features can be tailored to a great extent by the in-situ growth of secondary Si3N4 products via carbothermal reduction and nitridation (CRN) of SiO2 inside a β-Si3N4 whisker network. Specifically, the yield and morphology (grain size, aspect ratio, etc.) of the CRN product are found to be strongly dependent on the composition and amount of the reactants (SiO2 and C/SiO2 molar ratio) and reaction temperature. When reacted at 1300 ℃, a distinctive microstructure is formed in which nano sized and CRN-derived Si3N4 whiskers are uniformly distributed in the original micron sized whisker network. The whisker scaffolds reveal improved flexural strength if compared with that without CRN-derived Si3N4. This finding indicates the formation of a strong bonding between the β-Si3N4 whiskers by the growth of the CRN product. The measured low dielectric constant (2.6 ∼ 3.1) and low dielectric loss (<1.1 ×10−2) of the scaffolds makes them suitable for wave transmission application.

Three bond modes of basic refractories used for Ruhrstahl Heraeus degassing process

AbstractMagnesia–spinel brick and unburnt periclase–spinel–Al brick are being employed as a substitution of traditional magnesia–chrome brick in the chromium‐free campaign of lining materials in Ruhrstahl Heraeus (RH) degasser. These three materials are investigated, in terms of physical properties, corrosion resistance and flexibility by wedge splitting test. Tracking their physical alterations and chemical reactions through burning or heating, three bond modes are discovered. Magnesia–chrome brick is subject to a series of phase transformation with rising temperature to yield a liquid envelop around chromite‐ore particles, to further form porous rim while liquid is gradually absorbed by surrounding magnesia and eventually to precipitate secondary chromite spinel lied between magnesia particles by thoroughly dissociating chrome ore. The precipitated chromite spinel functions as the featured bond that enhances hot strength and corrosion resistance to slag, and additionally liquid coexistence improves the flexibility. The direct bond mode of magnesia particles in magnesia–spinel brick endures slag penetration by immanent character of MgO. Spinel incorporation in magnesia effectively improves thermal shock resistance. Due to minor negative value of permanent linear change after reheating, further sintering (densifying) in using at high temperature would bring a risk of loosening and open joints of magnesia–spinel lining. While used in RH degasser, unburnt periclase–spinel–Al bricks undergo a miraculous process of metallic Al melting, gaseous AlN and AlON formation, MgAlON whiskers germination combined with gaseous Mg reduced, and micron‐size whisker network bond domination in their matrix. Such a whisker‐network bond renders the material a successful eco‐friendly alternative to magnesia–chrome refractory.

Structure evolution and ablative mechanism of C/C-ZrC-ZrxCuy composites with low porosity fabricated by pressure assisted reactive melt infiltration

C/C-ZrC-ZrxCuy composites were fabricated by pressure assisted reactive melt infiltration (PARMI). The infiltrated matrix contained three different ZrC-ZrxCuy regions due to the gradient distribution of carbon and Cu during PARMI, and the dimensions of ZrC particles decreased sequentially from matrix boundary to inside. During 30 s ablation, heat-absorbing effect of Cu-phase slowed down the increasement of surface temperature. After 60 s ablation, porous ZrO2 skeleton evolved to a protective integrated cover, including expanded large-scale infiltrator and network of curly whiskers. Thus, ablation rates of composites were reduced to − 1.7 ± 0.1 µm/s and 1.7 ± 0.1 mg/s, separately.

Near‐net‐size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

AbstractPorous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade‐off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near‐net‐size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α‐Al2O3 powders as the starting materials and using MoO3 to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid‐state reaction during reaction sintering. The in situ formed three‐dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near‐net‐shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.

The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient, and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si3N4 porous ceramics (Si3N4-PC) have attracted attention due to their excellent intrinsic properties of Si3N4 and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si3N4-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si3N4-PC while it has been neglected in recent years. In this study, RB and SRB Si3N4-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si3N4-PC in wave-transparent materials. The results show that RB Si3N4-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si3N4-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σF and γwof. As a result, the RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σF of 109.94 MPa and 119.56 MPa as well as maximum γwof of 990.74 J m-2 and 1167.88 J m-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10-3 and 22.90 × 10-3, respectively. It is concluded that RB Si3N4-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.

Fabrication and characterization of porous mullite ceramics derived from fluoride-assisted Metakaolin-Al(OH)3 annealing for filtration applications

In this work, polycrystalline mullite whiskers are synthesized by fluoride-assisted method from metakaolin and several aluminum-containing compounds such as γ-Al(OH)3, AlF3·3H2O, and α-Al2O3 (corundum). The mullite formation and crystallization are assessed both in ex situ and in situ synchrotron X-ray diffraction experiments under synthesis conditions. Polycrystalline mullite starts to form from metakaolin, Al(OH)3, and AlF3·3H2O reactants at 680 °C, whereas mullite does not form even at 1000 °C when corundum is used. Porous mullite ceracmics are fabricated at sintering temperatures between 1000 and 1700 °C and tested for water permeance. Scanning Electron Microscopy (SEM) and synchrotron X-ray tomography (μCT) reveal that ceramics are comprised of pore channels with an interlocked network of mullite whiskers. With competitive porosity (up to 63 %), compressive strength (up to 20 MPa), and pure water flux (up to 579 L/m2·h at 1 bar), fabricated mullite ceramics are promising candidates for water filtration and purification.

Dual-functional application of Al2O3–B2O3 composite bubble in Al2O3–SiC–C castables as antioxidant and mechanical reinforcement agents

In the present work, the influence of low-cost self-synthesized Al2O3–B2O3 composite bubble on the oxidation resistance and mechanical properties of Al2O3–SiC–C (ASC) castables were investigated. The pre-fabricated composite bubble were characterized in detail. The properties of the ASC castables were measured, including their oxidation indexes, ambient and high-temperature mechanical properties, and microstructural evaluations. The results showed that the introduction of moderate contents of Al2O3–B2O3 composite bubble effectively improved the oxidation resistance and mechanical properties of the ASC castables by volumetric expansion, improved powder filling performance, and the in-situ formation of a high-strength whisker network in the high-temperature flowing liquid phase. These results demonstrated the dual-functional application of Al2O3–B2O3 composite bubble in ASC castables as antioxidants and mechanical reinforcements simultaneously.

Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites

The hybrid network of Si3N4 whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si3N4 into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si3N4 (Cf-RBSN) composites have been discussed in detail. The Cf reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % Cf-RBSN composite exhibit a minimum reflection loss (RLmin) of −36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g−1 for 5.9 mm thickness, while 20 vol % Cf-RBSN composites resulted in RLmin of −22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si3N4 whiskers and Cf.

Preparation and characterization of mullite microspheres based porous ceramics with enhancement of in-situ mullite whiskers

Abstract High-strength insulating ceramic materials were prepared using lightweight mullite microspheres with dense surfaces and high internal porosity as the main raw material and silica sol as a binder. The effects of AlF 3 ·3H 2 O content on the in situ formation and growth of mullite whiskers were analyzed by X-ray diffraction and scanning electron microscopy. The obtained results showed that mullite whiskers were formed in large quantities at 1200 °C using AlF 3 ·3H 2 O and V 2 O 5 as additives; their optimal growth was observed at 4 wt% AlF 3 ·3H 2 O and 1 wt% V 2 O 5 . The apparent porosity of the produced specimens was 39%; the MOR and CCS of the specimens were 31 and 152 MPa, respectively; the HMOR at 1300 °C was 11.32 MPa; and the thermal conductivity at 900 °C was 0.783 W m −1 K −1 . The staggered whisker network structure formed between mullite microspheres not only improved the mechanical properties of the material, but also refined its pore size, reduced the thermal conductivity, and enhanced the thermal insulation properties.

Fabrication and morphology control of high strength lightweight mullite whisker network

This work presents a simple approach to produce controlled mullite whisker network structure. Mullite whisker networks were facilely prepared by direct sintering of coal fly ash, bauxite and kaolin in BaCO3-potash feldspar-pyrolusite-talcum catalytic system at 1350 °C. Controlled mullite whiskers were obtained with about 200 nm in diameter and 3 μm in length by controlling the rate of Al2O3 diffusion through feldspar addition in catalytic system. Morphology evolution process and growth characteristics of mullite whisker networks were further investigated using scanning electronic microscope (SEM) and their possible fabrication mechanism were also investigated. The optimum mullite whisker network structural material exhibits some attractive properties such as high strength (199.89 MPa), low density (1.51 g cm−3) and low acid solubility (2.63 wt%). These results demonstrate an effective method to prepare large-scale mullite whisker network with potential applications in producing ceramic proppants.

Crystallization, mechanical, thermal and rheological properties of polypropylene composites reinforced by magnesium oxysulfate whisker

Polypropylene (PP) composites containing magnesium oxysulfate whisker (MOSw) or lauric acid (LA) modified MOSw (LAMOSw) were prepared via melt mixing in a torque rheometer. The heterogeneous nucleating effect of LAMOSw was clearly observed in polarized light microscopy (PLM) pictures with the presence of an abundance of small spherulites. MOSw exhibited no nucleation effect and formed a few spherulites with large size. Compared with PP/MOSw composites, PP/LAMOSw exhibited better impact strength, tensile strength and nominal strain at break, ascribing to three possible reasons: (i) more β-crystal PP formed, (ii) better dispersity of LAMOSw in PP matrix and (iii) the plasticizing effect of LA. The results of dynamic mechanical thermal analysis (DMTA) indicated that brittleness of the PP matrix at low temperature was improved by the addition of LAMOSw, while the interfacial interactions between MOSw and PP matrix were actually weakened by LA, as evidenced by the higher tanδ values over the entire range of test temperatures. In terms of the rheological properties of the composites, both the η* and G′ at low frequencies increase with the addition of MOSw or LAMOSw, indicating that the PP matrix was transformed from liquid-like to solid-like. However, a network of whiskers did not form because no plateau was found in the G′ at low frequencies. With low filler content, LAMOSw produced a stronger solid-like behavior than MOSw mainly due to the better dispersion of the LAMOSw in PP matrix. However, for highly-filled composites, the η* of PP/LAMOSw at low frequencies was smaller than that of PP/MOSw composite, since the particle-particle contact effect played a major role.

Mullite whisker network reinforced ceramic with high strength and lightweight

High strength lightweight mullite whisker network reinforced ceramic materials was successfully prepared by firing a bauxite - kaolin - coal fly ash mixture with additions of varying mixtures of feldspar - talcum - BaCO3 - pyrolusite. The mullite whisker network exhibits a unique architecture in which thin mullite crystals layers with anisotropic properties and well controlled crystal size were interweaved with one another. The effects of pyrolusite content on morphologies and properties of resultant ceramic materials were investigated. The phase compositions and microstructures of several samples were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The flexural strength and acid solubility of ceramic materials as functions of sintering temperature were systematically investigated by measuring the flexural strength and acid solubility at different sintering temperature, respectively. The resulting mullite whisker network structure ceramic materials showed optimum performance at sintering temperature around 1390 °C when the content of pyrolusite equals to 6 wt%, such as high strength (190.10 MPa), low density (1.48 g cm−3) and low acid solubility (2.55 wt%). The approach opens new opportunities for the sintered ceramic as a proppant material since the ceramic system displays low acid solubility and good flexural strength.

Spark plasma sintering of fine-grained alumina ceramics reinforced with alumina whiskers

Densification of alumina whisker-reinforced alumina ceramics by spark plasma sintering (SPS) has been investigated with the aim of obtaining a fine-grained microstructure and also studying the effect of whisker addition on the room-temperature mechanical properties. It was found that whisker addition retards slightly the sinterability of alumina by whisker hindering of particle rearrangement. Besides, the internal stress on the alumina matrix particles reduced due to the presence of a whisker network structure of strong rigid boundaries. Nevertheless near fully-dense and fine-grained alumina ceramics with alumina whisker content between 3wt% and 10wt% could be obtained under appropriate SPS conditions. The hardness of alumina ceramics with 3wt% was comparable to that of pure alumina ceramics (∼26GPa) whereas its fracture toughness (5.6MPam1/2) was higher (4.2MPam1/2). Crack bridging by well-dispersed whiskers and whiskers pull-out were identified as the main toughening mechanisms.

Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers

Nanocomposites based on poly(styrene-co-hexylacrylate) copolymer and cellulose whiskers as the nanosize filler were prepared by in situ miniemulsion polymerization and their melt rheological behaviours were investigated under dynamic shear conditions. The effects of γ-methacryloxypropyl triethoxysilane (MPS) content along with the whisker loading were explored. In the absence of whiskers, a transition from a liquid- to a solid-like behaviour was observed when the polymer was synthesized in the presence of MPS. When cellulose nanofiller was added, the storage modulus G′ and the dynamic viscosities η of the nanocomposites increased monotonically with whisker content and the resulting materials displayed a solid-like behaviour. Above 2 wt.%. loading, a percolated interconnected whisker–whisker network is built up, producing a jump in the storage modulus and strong shear-thinning behaviour of the viscosity. However, as the nanocomposites were prepared in the presence of 3% of MPS, no enhancement nor in the storage modulus nor in the viscosity was observed up to 5 wt.%. of whisker loading. Such a phenomenon was ascribed to inhibition of build-up of the whisker network. The non-linear viscoelastic behaviour of the nanocomposites was also investigated and analysed in terms of the breakdown of different networks, namely the filler–filler and the polymer–filler networks.

Cellulose Whisker/Epoxy Resin Nanocomposites

New nanocomposites composed of cellulose nanofibers or "whiskers" and an epoxy resin were prepared. Cellulose whiskers with aspect ratios of approximately 10 and approximately 84 were isolated from cotton and sea animals called tunicates, respectively. Suspensions of these whiskers in dimethylformamide were combined with an oligomeric difunctional diglycidyl ether of bisphenol A with an epoxide equivalent weight of 185-192 and a diethyl toluenediamine-based curing agent. Thin films were produced by casting these mixtures and subsequent curing. The whisker content was systematically varied between 4 and 24% v/v. Electron microscopy studies suggest that the whiskers are evenly dispersed within the epoxy matrix. Dynamic mechanical thermoanalysis revealed that the glass transition temperature (T(g)) of the materials was not significantly influenced by the incorporation of the cellulose filler. Between room temperature and 150 degrees C, i.e., below T(g), the tensile storage moduli (E') of the nanocomposites increased modestly, for example from 1.6 GPa for the neat polymer to 4.9 and 3.6 GPa for nanocomposites comprising 16% v/v tunicate or cotton whiskers. The relative reinforcement was more significant at 185 degrees C (i.e., above T(g)), where E' was increased from approximately 16 MPa (neat polymer) to approximately 1.6 GPa (tunicate) or approximately 215 MPa (cotton). The mechanical properties of the new materials are well-described by the percolation model and are the result of the formation of a percolating whisker network in which stress transfer is facilitated by strong interactions between the whiskers.

Anelastic Behavior Modeling of SiC Whisker‐Reinforced Al2O3

Anelastic recovery during creep deformation of a whisker‐reinforced Al2O3 matrix is examined in order to study the complex interactions of the reinforcement network with the matrix. In this paper, the finite‐element method is used to model the elastic and creep properties of a representative volume element during creep deformation. Two types of unit cells are investigated. The first is a three‐dimensional (3D) unit cell in which randomly oriented short fibers do not contact each other. The second case examined is a 2D cell within which fibers are aligned regularly so as to percolate through the unit cell. Simulation results from the 3D model verify the necessity of a percolating network for the anelastic recovery. Results from the 2D model show that anelastic recovery is attributed more to whisker bending, rather than to contact effects between fibers of the whisker network. Finally, FEM simulations of the maximum recoverable strain and characteristic relaxation time are found to be in good agreement with an analytical model recently developed by Wilkinson and Pompe, which is based on the application of Euler's beam theory in a 2D visco‐elastic unit cell.

Synthesis, electrical properties, and nanocomposites of poly(3,4-ethylenedioxythiophene) nanorods

Cellulose nanofibers or “whiskers” derived from tunicates were coated with PEDOT:PSS to produce high-aspect-ratio nanorods with high electrical conductivity. Films made from these PEDOT:PSS whiskers have surface resistivities as low as 97 Ω/□. The nanorods were found to disperse well in polar organic solvents such as dimethylformamide, which allowed the fabrication of nanocomposites with inert host polymers such as poly(methyl methacrylate) and polystyrene. These materials display surface resistivities as low as 760 Ω/□. Due to the high aspect ratio of the PEDOT:PSS-coated whiskers (64) and the formation of well-dispersed whisker networks within the polymer matrices, these materials display a percolation threshold as low as ∼2% v/v. Using a sol–gel process, electrically conducting gels and aerogels with a density of 6.4 mg cm−3 and a surface resistivity of ∼200 kΩ/□ were also produced.

In-Situ Formation and Coating of Cordierite Whiskers on Cordierite Based Honeycomb Support

In this work, cordierite whiskers were successfully coated on cordierite honeycomb support by in-situ method. Whisker coating was performed on walls of the pore channels of cordierite support from aqueous slurry. Sintering was carried out at 1300 °C for 4 hours. XRD measurements revealed a complete formation of cordierite phase, no traces of other phases were identified. A well adherence of coating on the support was identified from the microstructure. SEM analysis also revealed the formation of three-dimensional network of cordierite whiskers with open pores between them on the cordierite support.