Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

Study on Gelcasting of Fused Silica Glass Using Glutinous Rice Flour as Binder

Gelcasting is a colloidal processing method for fabricating high-strength and complex shape ceramic green bodies. However, industry has been reluctant to use the gelcasting technique because the most commonly used gel, acrylamide (AM), is a neurotoxin. Here, we report an attempt at the gelcasting of fused silica glass using a natural and nontoxic gel, glutinous rice flour (GRF) as binder. The GRF-based aqueous system was found to behave excellently in the gelcasting process. Flexural strength of fused silica green bodies solidified with only 3 wt% GRF is up to 11.87 MPa. Bulk density and flexural strength of fused silica glass sintered at 1275 °C are 1.75 g/cm³ and 47.02 MPa, respectively.     

Cold compaction molding and sintering of polystyrene

It is the objective of this paper to demonstrate the applicability of cold compaction molding followed by a sintering treatment to the processing of polystyrene powders. The influence of pressure, compaction speed, and peak pressure dwell time on the green (as compacted) density and the green tensile strength, as well as the effect of sintering on the tensile strength and dimensional change, were evaluated. The resulting data indicate that room temperature compaction alone is insufficient to provide adequate tensile strength for the compacts. Sintering the green compacts at temperatures of 150 to 173°C markedly improves the tensile strength while simultaneously causing a thickness change in the compacts. This thickness change results from gas evolution, pore shrinkage, and viscoelastic recovery of the residual stresses induced by pressure. For compacts of 0.225 in. thickness, an optimum sintering treatment of 173°C for 30 mins is recommended to provide a tensile strength of 4,000 psi and a thickness change of less than + 7 percent. Coining (repressing) the green compacts does not appreciably affect the sintered strength. However, a finer particle size improves the sintered properties. A review of the literature on the flow behavior of polystyrene suggests that a non-Newtonian viscous flow mechanism is followed by a Newtonian one as sintering progresses.     

Correlation between the morphology of cobalt oxalate precursors and the microstructure of metal cobalt powders and compacts

Metal cobalt powders of well-controlled size and morphology were synthesized by thermal decomposition under hydrogen of precipitated cobalt oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strength of the metal compacts was measured.A precipitation from ammonium oxalate and oxalic acid gives rise to the formation of β-CoC₂O₄·2H₂O particles of parallelepipedic and acicular morphology, respectively. An increase in the length to diameter ratio of the precursor particles favours an entanglement of the elementary grains during the thermal decomposition. Therefore, irregular and rough metal particles have been obtained. This specific morphology favours a mechanical interlocking of the particles during the compaction, leading to high values of green density and green strength of the metal compacts.     

A replacement of traditional insulation refractory brick by a waste-derived lightweight refractory castable

Lightweight insulation refractories are essential for high-temperature performance to reduce energy consumption. This study investigates a new insulation material, that is, solid waste rice husk ash (RHA) derived lightweight refractory castable, replacing traditional insulation refractory brick. The RHA is generated after the burning of rice husk as biomass fuel. The RHA is used as an aggregate and alkali-extracted silica sol from RHA as a binder to fabricate the insulation castable. The nanosilica containing (~30 wt%) sol is employed to synthesize the refractory castable by varying the sol amount (2.5-12.5 wt% silica from sol). The castable specimens are cast by a vibro-caster and fired at 900-1200°C in a muffle furnace. The physic-mechanical and thermal conductivity (κ) of the castable is investigated. At 1100°C with 10 wt% dry sol retaining sample shows an excellent apparent porosity (~65%), low bulk density (~ 0.8 g/cm³), and κ (0.136 W/m k) with sustainable compressive strength (6 MPa). The acquired results are a good match with the literature (other wastes-derived insulation materials) and industrial (silica insulation brick) obtained data. These promising outcomes may inspire the refractory industries for using RHA as an aggregate and RHA extracted sol as a binder for making insulation castable.     

Reactive Hydrothermal Liquid‐Phase Densification (rHLPD) of Ceramics – A Study of the BaTiO3[TiO2] Composite System

A densification process called reactive hydrothermal liquid‐phase densification (rHLPD), based on principles of hydrothermal reaction, infiltration, reactive crystallization, and liquid‐phase sintering, is presented. rHLPD can be used to form monolithic ceramic components at low temperatures. The densification of barium titanate–titania composite monoliths was studied to demonstrate proof of concept for this densification model. Permeable, green titania (anatase) compacts were infiltrated with aqueous barium hydroxide solutions and reacted under hydrothermal conditions in the temperature range 90°C–240°C. The effects of reaction time and temperature on the conversion of titania (anatase) into barium titanate were studied. Utilizing a 72 h reaction at 240°C between l.0 M Ba(OH)₂, an anatase (TiO₂) powder compact, and a corresponding Ba/Ti ratio of 1.5, it was possible to crystallize a composite 95 wt% (88 mol%) BaTiO₃ and 5 wt% (12 mol%) TiO₂. The composite had a relative density of ~90% with a compressive strength of 172 ± 21 MPa and a flexural strength of 49 ± 4 MPa.     

Fabrication and characteristics of porous Ni and NiO(Li) cathodes for MCFC

In-situ Ni andex-situ NiO(Li) cathodes were fabricated by cold pressing using Ni powder and Li₂CO₃ (in case ofexsitu cathode) with paraffin wax as a binder. The effects of compaction pressure, amount of binder, and sintering temperature on porosity, the average size of pore, Li^* cation fraction, and the fracture strength were investigated in this study. The optimum compaction pressure was 100 kg/cm² to prevent the brittleness and stress concentration caused by high compaction pressure. Also, 10 wt% of binder yielded best results and the strength decreased dramatically and revealed nonuniform pore and pore distribution beyond 10 wt%. To obtain reasonably high electric conductivity inexsitu cathode, the sintering temperature was chosen to be below 950°C to maintain the ration fraction of Li* is in the range of 0.02–0.06. In terms of the Ni-dissolution rate,exsitu cathode was more efficient thaninsitu cathode in the long-term operation except the initial 24hr period.     

Reliability studies of gelcast fused silica between organic and inorganic binder systems

Fused silica ceramics was prepared by using conventional organic binder, mathacrylamide‐N,N′‐methylenebisacrylamide (MAM‐MBAM) system by gelcasting process. Mechanical properties of green bodies were studied as a function of solid loading varying from 60 to 72 vol%. After evaluating the green body mechanical properties, the samples were densified at different sintering temperature from 1200 to 1450°C with definite intervals of 50°C and subjected to flexural strength analysis. Variation in flexural strength with sintering temperature was observed and correlated with the quantity of devitrification of fused silica during sintering. Quantification of devitrified cristobalite was carried out by using 20 wt% rutile (TiO₂) as an internal standard by X‐ray diffraction. It was found that, as the cristobalite content increased, flexural strength decreased. Reliability studies were carried out for the samples having maximum flexural strength with and without crystalline content. Reliability studies have shown that for this organic binder system the sample sintered at 1300°C is crystalline free and most reliable product. The mechanical properties and reliability of this product processed with organic binder are compared with inorganic binder system. Results indicate that the sample fabricated using inorganic binder system is exhibiting high Weibull modulus and thus better reliability.     

Defect Formation during Supercritical Extraction of Binder from Green Ceramic Components

Supercritical extraction in carbon dioxide has been used to remove binder consisting of 60 wt% poly(vinyl butyral) and 40 wt% dioctyl phthalate from multilayer ceramic capacitors containing barium titanate as the dielectric. The amount of binder removed increases as both the temperature and pressure of the supercritical fluid increases; a maximum of ∼50% of the binder can be removed using carbon dioxide in 3 h at 75°C and 40 MPa. During depressurization from the supercritical state, defects such as fracture and delamination were occasionally observed in the green body. The occurrence of defects is related to the rate of depressurization, the permeability of the body, and the green strength of the body. The origin of the defects is discussed, and ways to mitigate them are presented.     

Enhancing the physical and mechanical properties of pellet-shaped hydroxyapatite by controlling micron- and nano-sized powder ratios

Hydroxyapatite (HA) was fabricated in microns as its basic size. The particle size distribution was controlled by mixing micron- and nano-sized HA to obtain the optimum amount of mixture to improve its properties. HA powder with a size of 2.5 μm was mixed with that with a size of 200 nm, with a variety of concentrations of up to 20 wt%. A green body was fabricated using the uniaxial pressing method at a pressure of 200 MPa. The sintering process was conducted at a temperature of 1200 °C, heating rate of 3 °C/min, and holding time of 2 h in air. The physical characteristics of the HA sintered body were determined using X-ray diffraction, scanning electron microscopy, linear shrinkage, and density testing. The mechanical properties of the HA sintered body were tested using compressive strength testing. The test results indicated that the mechanical properties of the HA sintered body increased with the addition of nano-sized HA. The mechanism of the increasing strength occurred because nano-sized HA particles filled the gaps between the micron-sized particles. In this study, the highest mechanical properties were obtained by adding 20 wt% nano-sized HA. The compressive strength in the sample without added nano-sized HA was 132.2 MPa and increased significantly to 208.6 MPa with the addition of nano-sized HA of 20 wt%. No change in the phase in HA was observed within a sintering temperature of 1200 °C.     

Effect of Polyethylene Glycol (PEG) Powder on Compressibility and Microstructural Properties of Sintered α-Alumina

This study examines the effect of various contents of polyethylene glycol (PEG) powders on density, compressibility, and microstructural properties of sintered α-alumina samples. Moreover, the effect of compaction pressure on the green density of the compacts is studied by applying different pressures ranging from 400 to 550 MPa. Samples were prepared by mechanical blending of alumina and various amounts of PEG powders in a Turbula mixer. The binder contents vary from 1 wt.% to 4 wt.%. The as-prepared mixture was compacted in a universal machine at room temperature under the pressure of 6 MPa to produce disk-shaped samples in a pre-compaction step. Experimental results revealed that adding various amounts of PEG powders has a detrimental effect on the green density of alumina pellets and decreases the green density from 1.95 to 1.87 g/cm³. The results also show that sintered density of samples increased by increasing the compaction pressure to pressures higher than 400 MPa. It is observed that a sudden increase in green density has been observed between 450 and 550 MPa.     

Low-pressure injection molding of magnesium aluminate nanoparticles: Rheological behavior and flexural properties of sintered component

This research aims to investigate the density and flexural strength of nanostructured spinel parts fabricated using the low-pressure injection molding (LPIM) method. For this purpose, firstly, the effect of the amount of binder was tested on the rheology behavior of the feedstocks containing spinel nanopowder for producing ceramic parts using the LPIM method. The rheometric analysis indicated that the feedstocks containing 80 wt% powder and 20 wt% binders showed shear-thinning fluid behavior and were chosen as the optimal low-viscosity feedstocks for the LPIM process. After binder removal from LPIMed part, secondly, the effect of sintering temperature was examined on the relative density and flexural strength of the spinel parts. The results indicated that by increasing sintering temperature from 1550 °C to 1700 °C, the size of pores was reduced and grain size was increased from 2 μm to 6 μm. Furthermore, the flexural strength of the parts sintered at 1700 °C was 10 MPa greater than that of the sample sintered at 1650 °C.     

Effect of compaction on the sintering of borosilicate glass/alumina composites

The effect of initial compaction on the sintering of borosilicate glass matrix composites reinforced with 25 vol.% alumina (Al₂O₃) particles has been studied using powder compacts that were uniaxially pressed at 74, 200 and 370 MPa. The sintering behaviour of the samples heated in the temperature range 850–1150 °C was investigated by density measurement, axial and radial shrinkage measurement and microstructural observation. The density of the sintered composites increased continuously with temperature for compacts pressed at 74 MPa, while for compacts pressed at 200 and 370 MPa it reached the maximum value at 1050 °C and at higher temperatures it decreased slightly due to swelling. The results showed anisotropic shrinkage behaviour for all the samples, which exhibited an axial shrinkage higher than the radial shrinkage, and the anisotropic character increased with the initial compaction pressure.     

Detailed microstructural and mechanical evaluation of yttria-stabilized zirconia-Ti composite compacts developed using spark plasma and electric discharge sintering processes

In this work, electric discharge sintering (EDS) and spark plasma sintering (SPS) methods are used to fabricate different yttria-stabilized zirconia–titanium (YSZ-Ti) composite compacts. The compacts were prepared using the ball-milled precursors and by varying the YSZ content of the YSZ-Ti composite (0, 1, 3, 5, 10 wt%). Then, the microstructures and mechanical properties of these composite compacts are analyzed in detail and compared. The results indicated that the YSZ contents of the SPS produced composite compacts are agglomerated and dispersed mostly at the Ti particle boundaries, according to the microstructural analyses. In the EDS composite compacts, the YSZ component is uniformly distributed throughout the Ti matrix. As a result, the mechanical properties such as the hardness (from 88 HV to 611 HV), yield strength (366 MPa–922 MPa), and tensile strength (from 743 MPa to 1944 MPa) of the EDS composite compacts are improved significantly. In comparison, as the YSZ content of Ti increased, the mechanical properties of the SPS prepared composite compacts are also enhanced but not as much as those of the EDS processed composite compacts (hardness (from 219 HV to 382 HV), yield strength (321 MPa–502 MPa), and tensile strength (from 692 MPa to 1076 MPa). To summarize, a composite comprising a Ti matrix reinforced with YSZ was efficiently fabricated by EDS rather than by SPS using Ti powder with surface-embedded YSZ. The excellent mechanical properties of the Ti-YSZ composites fabricated by EDS can be ascribed to the uniform and homogeneous distribution of YSZ in the Ti matrix phase. EDS is thus a promising powder metallurgy technique for the production of low-cost isotropic Ti matrix composites reinforced with ceramic particles.     

Influence of Viscous Deformation at the Contact Point of Primary Particles on Compaction of Alkoxide-Derived Fine SiO2 Granules under Ultrahigh Isostatic Pressure

Viscous deformation and the adhesion force at the contact point between amorphous silica particles under ultrahigh isostatic pressure (up to 1 GPa) are important in the densification of powder compacts. The amount of viscous deformation and the strength of adhesion force have been changed in the present study by altering the calcination temperature and particle diameter, and the new values have been determined successfully using a diametral compression test. The diameter of spherical and monosized alkoxide-derived silica powders has been controlled within the range of 10–400 nm. Close-packed granules of these powders have been produced by spray drying. Because of viscous deformation, as-spray-dried ultrafine silica powders without calcination could be consolidated into highly dense compacts (>74% of theoretical density) by applying ultrahigh isostatic pressure (1 GPa). Relatively high temperature in the calcined particles (400°C) causes viscous deformation at the contact point to disappear almost completely and clearly increases the adhesion force, because of neck growth that has resulted from viscous sintering. At temperatures >200°C, the green density of the calcined powders decreases to 65% of theoretical density, even under 1 Gpa pressure. The relationship between green density and viscous deformation in silica particles at the point of contact has been analyzed quantitatively by the Hertz and Rumpf model. On the other hand, if relatively low isostatic pressure ( P c < 100 MPa) is applied, the green density and intergranular pore volume depend on the strength of the spray-dried granules. The relationship between granule strength and neck growth at the contact point with calcination has been estimated quantitatively.     

A facile route for the preparation of silica foams using rice husk ash

This work aims to synthesize silica foam with around 75 vol. % open porosity without using any additive or pore forming agents, in order to prevent the generation of greenhouse gases during pore formation in the silica matrix. Waste rice husk ash (RHA) derived silica is used as a silica source, which is extracted through the alkali extraction synthesis route. Several physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), differential thermal-thermogravimetric (DTA-TGA) and FTIR analysis have been done for RHA extracted silica. Silica foam specimens are fabricated through control compaction pressure and at low foaming temperature. Samples which are fired at 550°C for 30 minutes exhibit both a adequate apparent porosity (AP; ~75.82%) and significant compressive strength (~1.54 MPa). It can also be observed that the porosity and strength values are changed with the variation in compaction pressure and foaming temperature.     

Transparent nanocomposites with enhanced performances from poly(propylene carbonate) and silica

In this study, silica nanoparticles with a refractive index matching PPC and a diameter smaller than the visible light wavelength were chosen to prepare enhanced PPC/silica nanocomposites by a simple melt compounding method. The result exhibited all the nanocomposites possessed excellent transparency (about 90%), even in the nanocomposite with a silica content of 10 wt%. For PPC/silica nanocomposites, the percolation threshold was determined to be 7.5 wt% based on the dynamic rheological behavior and percolation theory. Moreover, the overall performance of the PPC-based nanocomposite with a silica content of 7.5% is the best. The optimal nanocomposite showed a Young's modulus of 3792 MPa, a yield strength of 46.5 MPa, a storage modulus of 3812 MPa and a highest temperature at maximum weight-loss rate (T_max, 309°C). These characteristics are very important for potential commodity applications of PPC.     

Effect of Relative Humidity on the Viscoelastic and Mechanical Properties of Spray-Dried Powder Compacts

Spray-dried powder compacts exhibit viscoelastic properties such as stress relaxation, creep, and delayed elastic strain. This behavior is attributed to the organic binder, which forms bridges between the particles in spray-dried granules, thereby affecting their deformation characteristics. The viscosity and distribution of the binder within the powder compact can affect its mechanical and viscoelastic properties. In this study, the powder was conditioned at different ambient relative humidity (RH) levels, to vary the binder viscosity. Load deformation, stress relaxation, fracture strength, and fracture toughness behavior of ferrite powder compacts were studied as a function of ambient RH both before and after compaction. The loading rate was found to significantly affect the time-dependent response, and the relaxation times decreased at high humidity levels during compaction. It is proposed that increasing the humidity level during compaction increases the number of particle–particle contacts. This simple mechanism of binder redistribution led to slower relaxation times, increases in fracture strength, and elastic modulus of the green bodies, without significantly altering the fracture toughness when powders were compacted at high humidity to a given density.     

Large-scale synthesis of AlN nanofibers by direct nitridation of aluminum

AlN nanopowders and nanofibers were synthesized by direct nitridation of Al and rice bran mixture compacts in a tube furnace up to 1300 °C in a nitrogen flow without addition of extra catalyst. The effect of the compaction pressure applied onto the green bodies on the morphology of the final AlN products was investigated. A green body compacting pressure in between 320 MPa and 480 MPa was found to be favorable for the synthesis of AlN fibers with aspect ratio up to 400, diameter in the range of 50–500 nm, and length up to tens of micrometers; for a lower pressure of 160 MPa and a higher pressure of 640 MPa, nano-sized AlN powders were the primary morphology in the final product. The AlN products were characterized by several techniques and the VLS growth mechanism was proposed as the main reason for the AlN fibers formation.     

Effects of monomer composition on the mechanical and machinability properties of gel-cast alumina green compacts

In the present work, machinable green gel-cast alumina compacts were prepared by using polyethyleneglycol (400) diacrylate (P4) and polyethyleneglycol (600) dimethacrylate (P6) together with the acrylamide (AAm) comonomer. The glass transition temperatures of copolymers decreased with the increasing of P4 or P6 amount in total copolymer. The green samples obtained in an aqueous system were mechanically analyzed by means of three-point bending. Flexural strength values increased, from 2 MPa to 25 MPa, as the weight ratio of P4 or P6 in total copolymer (AAm-P4 or AAm-P6) decreased from 90% to 3.5%, respectively. The green gel-cast samples prepared by using P4 or P6 were machined easily by using a lathe, drill and milling machine without damaging the samples, which have good surface finish. The binder removal was achieved at lower temperatures than those samples prepared by using only AAm.