Hydrolysis-assisted Solidification Research Articles

The Hydrolysis of AlN Powder - A Powerful Tool in Advanced Materials Engineering.

The tendency for aluminium nitride (AlN) powder to undergo hydrolysis, which can lead to a complete degradation of the material, is unique in metal nitrides. Although this form of hydrolysis has been known for a long time, it is generally considered as a nuisance, because it prevents the aqueous powder processing of AlN-based ceramics. However, careful investigations of the course of hydrolysis, the reaction kinetics and the evolution of aluminium hydroxides have uncovered exceptional possibilities for the exploitation of this naturally driven process in the area of advanced materials engineering. It can be employed as superior synthesis path for hierarchically-assembled, mesoporous alumina powders or coatings consisting of 2D nanocrystalline lamellas. The beneficial surface characteristics of the powder serve as an ideal template for further modifications useful in catalysis, while the powder's flowability enables facile preparation of high-performance hierarchically porous structures. The coatings, on the other hand, are suitable as templates for superhydrophobic surfaces or as adhesive coatings for cementing zirconia dental ceramics. Finally, the hydrolysis-assisted solidification (HAS) process has proved to be an important asset in the processing science and technology for fabrication of porous and dense ceramics and nanocomposites.

Novel composites of $$\upbeta $$ β -SiAlON and radome manufacturing technology developed at ARCI, Hyderabad, for hypervelocity vehicles

Keeping the importance of developing suitable radome (a word derived from radar $$+$$ dome) materials and near-net shape consolidation technique for manufacturing radomes suitable for hypersonic (>mach 5) radar-guided missiles in India, the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, has initiated an in-house R&D programme and successfully developed a complete process know-how for manufacturing defect-free prototype $$\upbeta $$ -SiAlON-based radome structures with all the desired properties. As a part of this R&D programme, total six separate sub-projects mentioned below were undertaken and executed: (i) identification of the best composition out of $$\upbeta $$ - $$\hbox {Si}_{6-z}\hbox {Al}_{z}\hbox {O}_{z}\hbox {N}_{8-z}$$ (0 $$\le z \le $$ 4.1) solid solution, which possesses a right combination of properties required for radome applications, (ii) designing of an AlN-free precursor mixture for consolidating $$\upbeta $$ - $$\hbox {Si}_{4}\hbox {Al}_{2}\hbox {O}_{2}\hbox {N}_{6}$$ ceramics by following aqueous colloidal processing routes, (iii) development of a process for passivating water-sensitive AlN powder against hydrolysis, (iv) development of aqueous gelcasting (GC) and hydrolysis-assisted solidification (HAS) powder processing routes for consolidating dense $$\upbeta $$ -SiAlON ceramics using highly solids loaded (>50 vol%) aqueous slurries, (v) development of an hydrolysis-induced aqueous gelcasting (GCHAS) process, a novel near-net-shape consolidation technique, to produce radomes with very high-production yields and (vi) development of an economic route for synthesizing the low-dielectric constant and high strength novel $$\upbeta $$ -SiAlON- $$\hbox {SiO}_{2}$$ ceramic composites. In this paper, (i) the basis for choosing $$\upbeta $$ -SiAlON-based ceramics for hypervelocity radome applications, and (ii) the various bottle-neck problems faced, while executing this entire R&D work and the way they were overcome have been critically analysed and discussed systematically, while citing all the relevant and important references.

Characterization of the Microstructure of an AlN-Mullite-Al2O3 Ceramic Layer on WCu Composite Alloy for Microelectronic Application

An AlN composite ceramic layer was designed and fabricated on WCu substrates by hydrolysis-assisted solidification and firing. First, the surface of WCu substrates were pre-coated with polycarbosilane/AlN ceramic layers by spinning; the layers were then fabricated by firing. The phase composition, microstructure, and element distribution of the ceramic layer and interfacial reaction layer were investigated by use of scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction. The results showed that the ceramic layers were composed of AlN, mullite, and Al2O3. There were many nanocrystalline rods on the surface of the ceramic layers. The Cr layer prevented the WCu substrate from reacting with water vapor during firing, and the Ni layer prevented diffusion of tungsten into the Cr layer. Study of the cross section of the ceramic layer fired on the Cr/Ni/WCu substrate revealed a perfect interfacial reaction layer.

Hydrolysis-Induced Aqueous Gelcasting: The Latest Concept for Net-Shape Consolidation of Ceramics—A Review

This is a review article covering literature reported on consolidation of ceramic components out of aqueous-based Al2O3, MgAl2O4 spinel, β-Si4Al2O2N6, β-SiAlON-SiO2 composite, and zirconia toughened alumina (ZTA) slurries following hydrolysis induced aqueous gelcasting (henceforth referred to as GCHAS) route. GCHAS is a combination of aqueous gelcasting (GC) and hydrolysis assisted solidification (HAS) routes. The advantages of GCHAS over other net-shape forming techniques in the fabrication of certain important ceramic structures such as thin wall radomes have been reviewed in this article. Interestingly, among the various colloidal-based net shape forming techniques available, GCHAS has been found to be a most promising one for consolidating complex-shaped ceramic structures with significantly high green strength and high product yield. Sintered properties of ceramics consolidated by GCHAS are compared and discussed with those of similar ceramics consolidated by aqueous slip casting (SC), GC, HAS, and a conventional dry-powder pressing (DP) of freeze dried granules in this article.

Nanoparticle assisted coagulation of aqueous alumina suspensions

Colloidal processing of ceramics offers a high potential to achieve homogeneous microstructures with improved material properties. In this study, a novel forming method is investigated, which was already applied successfully for the fabrication of ceramic matrix composites (CMC), but is also considered to be a suitable direct casting technique for the fabrication of advanced ceramics with tailored microstructure and properties. The so-called nanoparticle assisted coagulation method (NPAC) represents a modification of the hydrolysis-assisted solidification (HAS) technique. It promises green components with high green strength, uniform density as well as homogeneous and tailored microstructure. Electrostatically stabilized colloidal suspensions with high solid loadings were produced by dispersing various fractions of submicron alumina powder and aluminium hydroxide nano-powder in water without use of any organic binder. Rheology and coagulation kinetics of suspensions and green part properties were studied regarding to modifications of pH value, setting temperature, amount of setting agent, amount of nano-powder, solids loading and process parameters like ultrasound treatment. It could be revealed that the homogeneous core-shell arrangement of submicron and nanoparticles in the colloidal state can be transmitted to the green state, which improves the microstructure and green density of the green parts. For this, the NPAC method is seen as a promising technique for the fabrication of advanced ceramics with tailored microstructure and properties.

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

This paper reports on synthesis of MgAl2O4 spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45 vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5 wt‐%Al2O3 into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1 h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.

Porous Alumina Ceramics Prepared by the Hydrolysis-Assisted Solidification Method

We used the hydrolysis-assisted solidification (HAS) process to form the alumina green bodies from the suspensions. A total of 2.5 wt% of AlN powder, with respect to the solids content, was added to an aqueous alumina suspension containing 53.5 vol% of solids. During the hydrolysis of the AlN powder, the aluminum hydroxide reaction product precipitated on the surface of the alumina particles. The effect of the precipitated aluminum hydroxide on the density and the bending strength, after sintering the alumina samples prepared by the HAS process at temperatures up to 1300°C, was investigated. In comparison with conventionally formed samples, a similar strength was measured at a lower sintered density for the partially sintered alumina prepared by the HAS process. These improved properties could be ascribed to the formation of aluminum hydroxide in the form of boehmite lamellas on the surface of the particles during the HAS consolidation process. Boehmite lamellas topotactically transform with temperature into various transition aluminas; however, the change in the crystal structure did not affect the morphology, which remained the same up to 1100°C. These lamellas hindered the densification of the samples up to 1100°C, but at higher temperatures, the lamellas transformed into nanosized α-alumina particles, which enhanced the neck growth, resulting in a very high measured flexural strength at sintered densities up to 70%.

A novel colloidal processing route to alumina ceramics

The fabrication of complex-shaped alumina ceramics following a new near-net shape technique based on hydrolysis induced aqueous gelcasting (GCHAS) is reported in this paper. Aqueous suspension containing 50 vol.% solids loading was prepared by dispersing alumina in an aqueous solution of methacrylamide and methlynebisacrylamide (17 wt.% in 6:1) using polycarboxylic acid as dispersing agent. Consolidation was accomplished by adding a polymerization initiator, a catalyst and AlN powder (4 wt.%). For comparison purposes, alumina ceramics were also consolidated by aqueous gelcasting (GC) and by hydrolysis assisted solidification (HAS) from the same concentrated suspensions, and by conventional dry pressing (DP) from freeze dried granules prepared from the same suspensions by freeze granulation. Among the four shaping techniques used, GCHAS was found to be best for consolidating near-net shape alumina components like thin wall radomes with the highest green strength ever reported for alumina ceramics. Green samples were sintered for 2 h at 1600 °C and then characterized for microstructure and mechanical properties.

Influence of processing route on microstructure and mechanical properties of MgAl 2O 4 spinel

This paper reports on process dependant microstructural and mechanical properties of MgAl 2O 4 spinel (MAS) ceramics. Two MAS powders with different chemical compositions were synthesized by solid-state reaction of alumina and calcined caustic magnesia at 1400 °C for 1 h. The surface of the as obtained MAS powders was passivated against hydrolysis by coating it with H 3PO 4 and Al(H 2PO 4) 3 species dissolved in ethanol at 80 °C for 24 h. The as protected powders could then be dispersed in aqueous solutions of tetramethylammonium hydroxide (TMAH) and Duramax D-3005 as dispersing agents to obtain stable slurries with 45 vol.% solids loading. The stable aqueous MAS slurries were consolidated by slip casting (SC), gelcasting (GC), hydrolysis assisted solidification (HAS) and hydrolysis induced aqueous gelcasting (GCHAS) routes, fully dried and then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS ceramics were also prepared following a conventional dry-powder pressing (DP) and temperature induced gelation (TIG) routes. All the sintered MAS ceramics were thoroughly characterized for bulk density, apparent porosity, water absorption capacity, SEM microstructure, XRD phase, hardness, 3-point bend strength, and percentage of shrinkage to evaluate the suitability of the processing routes for fabricating defect free components with near-net shape. Among the various techniques employed, the GCHAS was found to be best for fabricating near-net shape MAS ceramics.

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia‐alumina composites, ZTA‐30 (70 wt% Al2O3+30 wt% ZrO2) and ZTA‐60 (40 wt% Al2O3+60 wt% ZrO2), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis‐assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze‐dried granules (FG). In the HAS process, 5 wt% of Al2O3 in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three‐point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

A new near-net shape forming process called “hydrolysis-induced aqueous gelcasting” (GCHAS) is reported in this paper for the consolidation of ZTA composites, ZTA-30 (70 wt.% Al 2O 3 + 30 wt.% ZrO 2) and ZTA-60 (40 wt.% Al 2O 3 + 60 wt.% ZrO 2). For comparison purposes, ceramics having the same chemical compositions were also consolidated by hydrolysis-assisted solidification (HAS). All the starting suspensions contained a solids loading of 50 vol.%. In the precursor powder mixtures, 1–5 wt.% of Al 2O 3 was replaced by equivalent amounts of AlN to enhance or promote or co-promote the consolidation of suspensions by HAS or by GCHAS, respectively. The suspensions for GCHAS were prepared by dispersing the ZTA powder precursor mixtures in a premix solution of 20 wt.% MAM (methacrylamide), MBAM (methylenebisacrylamide) and NVP ( n-vinylpyrrolidinone) in 3:1:3 ratio in de-ionized water. Ceramics consolidated via GCHAS exhibited superior mechanical properties after consolidation and after sintering for 1 h at 1600 °C in comparison to those consolidated by HAS.

Dense β-SiAlONs consolidated by a modified hydrolysis-assisted solidification route

Dense β-Si 4Al 2O 2N 6 materials were fabricated by a modified hydrolysis-assisted solidification (HAS) route from aqueous slurries containing 48–50 vol.% solids, in which 5–22 wt.% of the required α-Al 2O 3 was replaced by equivalent amounts of unprotected aluminium nitride (AlN) powder to promote consolidation via AlN hydrolysis. A fixed amount (9.37 wt.%) of AlN passivated against hydrolysis with a coating phosphate layer was also added to all the samples consolidated by the modified HAS method. The aqueous slurries were cast in non-porous moulds, allowed to set and dried before sintering at 1675 °C for 4 h. For comparison purposes, ceramics with the same predicted final composition (having 64.33% α-Si 3N 4, 23.36% α-Al 2O 3, 9.37% AlN and 7% Y 2O 3 as starting materials) were also consolidated by a conventional dry-powder pressing (CDPP). The β-Si 4Al 2O 2N 6 ceramics consolidated by the modified HAS route exhibited superior outstanding properties (bulk density, apparent porosity, water absorption capacity, hardness and fracture toughness) in comparison to the traditional dry-powder pressing route.

Aqueous Processing of AlN Powder

For the production of ceramics containing AlN as a major or minor constituent it is necessary to avoid hydrolysis. To do that, non-aqueous powder processing is required or waterresistant AlN powders must be used. Alternatively, as in the Hydrolysis Assisted Solidification (HAS) process, which exploits the hydrolysis of the AlN for the solidification of the ceramic suspensions, the hydrolysis has to be prevented at room temperature but initiated at elevated temperatures. In this work a systematic study of AlN powder reactivity in water and other aqueous environments is presented. The AlN hydrolysis was investigated by measuring the pH of diluted suspensions and by analysis of the reaction products. The results indicate possible solutions for control of the reaction with water in order to exploit it or to prevent it to enable aqueous AlN powder processing.

Hydrolysis-assisted solidification of alumina slurry dispersed in silica sol without de-airing process

This work reports a novel hydrolysis-assisted solidification (HAS) casting technology without de-airing process developed to prepare mullite-based ceramics. Concentrated alumina slurry dispersed in silica sol with high solid volume loading but low viscosity was successfully prepared at about pH 2.0. Based on effect of pH values on stabilization behavior of the slurry, in situ consolidation was realized utilizing the self-hydrolysis of formamide to shift the pH value from 2.0 to above pH 2.8. High compact and uniform green body was obtained without entrapped air by HAS casting technology without de-airing process.

The Densification and Microstructure of Y‐TZP Ceramics Formed Using the Hydrolysis‐Assisted Solidification Process

Hydrolysis‐assisted solidification (HAS) was used in the slurry forming of yttria‐stabilized tetragonal zirconia (Y‐TZP) green bodies. AlN powder, a total of 1–5 wt% with respect to the solids, was added to a highly loaded aqueous zirconia (Y‐TZP) slurry containing 48.5 vol% of solids prior to shaping by aqueous injection molding. Aluminum hydroxide was formed during the hydrolysis of the AlN, which resulted in the production of dried green bodies with strength values ranging from 1.2 to 2.7 MPa. In the presence of alumina, formed by the thermal decomposition of the aluminum hydroxide in the green body, the densification and grain growth of the HAS‐formed material were enhanced. The fractional density, in the range 98.0–99.3%, and the bending strength, in the range 876–890 MPa, of a sintered Y‐TZP ceramic containing lower amounts (1 and 2.5 wt%) of AlN in the starting powder mixture did not differ significantly from the values obtained with conventionally formed reference material. The indentation toughness and Vickers hardness were both observed to increase, although this approximately 10% increase fell within a single standard deviation. The resistance to low‐temperature degradation of the HAS‐formed material, in terms of the thickness of the transformed monoclinic layer after aging for 24 h at 140°C in water, was found to be about four times lower and was statistically significant.

Analytical Electron Microscopy Study of Green Ceramics Formed from Aqueous Suspensions Using the Hydrolysis-Assisted Solidification Process

During the hydrolysis-assisted solidification (HAS) of aqueous ceramic suspensions, aluminum hydroxides are formed that bind the ceramic particles into a stiff wet body. Transmission electron microscopy investigations of HAS-processed Al2O3 and ZrO2 green parts after drying revealed that the secondary phase is amorphous and distributed uniformly around the host ceramic particles. The estimated thickness of this layer was 3–5 nm. Moreover, areas of a few tens of nanometers in size were found at three-particle junctions that contained an amorphous phase and individual nanocrystals of boehmite.

Hydrolysis assisted solidification of silicon carbide ceramics from aqueous suspension

The hydrolysis assisted solidification (HAS) process was used in the slurry forming of SiC green bodies. AlN powder was added to aqueous SiC slurry prior to shaping. The addition of small amount of AlN powder causes a dramatic increase in the suspension viscosity or even solidification in the case of highly loaded suspension. Green bodies were successfully produced. Measurements of the changes in viscosity, the pH and the zeta potential measurements were performed to evaluate the process. Effects of the addition of AlN and solidification temperature on the process and the ceramic suspension were followed.

Preparation of alumina ceramics from aqueous suspensions employing the hydrolysis of aluminum nitride

This paper summarizes the effects of selected processing parameters on the behavior of aqueous alumina suspensions during hydrolysis-assisted solidification and on the properties of the green and sintered parts prepared by this technique. Measurements of the changes in viscosity, the pH, and the impedance of aqueous suspensions with time were performed to evaluate the process. Using impedance spectroscopy, the effects of the addition of AlN, the solids content in the ceramic suspension, and the effect of the solidification temperature on the progress of solidification were followed. It was demonstrated that the solidification temperature significantly affects not only the solidification rate but also the properties of the green and sintered parts. A transmission electron microscopy analysis of dry green parts revealed that the host ceramic particles are bound by a predominantly amorphous AlOOH phase, which results in a high strength for the green parts.

Principles of the hydrolysis assisted solidification (HAS) process for forming ceramic bodies from aqueous suspension

Abstract Hydrolysis-assisted solidification (HAS) is a new method for net-shaping ceramic green parts from aqueous suspensions in non-porous molds. The process exploits the thermally activated and/or accelerated hydrolysis of aluminium-nitride powder, which has been added to ceramic suspensions. Several simultaneously occurring effects accompany the hydrolysis of AlN: an increased solids content in the suspension due to internal water consumption, a decreased ζ-potential due to the pH change caused by ammonia formation, and the formation of Al-hydroxides. The overall result is a dramatic increase in the suspension's viscosity, which ultimately leads to the formation of a solid body. The contributions of individual mechanisms were studied and are discussed in this paper. At very high solids loading, each of these mechanisms can lead to a significant increase in the viscosity of the suspension. At lower solids loadings, the precipitation of aluminium hydroxide seems to play the major role in the suspension's solidification.

Interactions in aqueous Al 2 O 3 –AlN suspensions during the HAS process

Abstract Hydrolysis-assisted solidification (HAS) is a new method for net-shaping ceramic green parts from aqueous suspensions. It is based on the thermally activated hydrolysis of aluminium nitride powder added to a concentrated suspension of a ceramic powder. During hydrolysis, water is consumed and ammonia is formed. The cumulative effect of water consumption and the change in zeta-potential due to a pH change leads to an exponential increase of viscosity until the fluid character of the suspension is lost and a moist solid is obtained. In the present paper the contribution of changing interparticle forces arising from the pH shift is described. The electrokinetic properties of AlN powder and their influence on the viscosity of acidic or alkaline aqueous alumina suspensions before and during hydrolysis are presented and discussed.