For higher amounts of boehmite, as for A80A, the solids loading affects the setting time apparently. Moreover, the influence of the nano-powder on the consolidation is much more obvious, as it yields to longer setting times. Aluminium hydroxide exhibits a comparably high specific surface with quantitatively more hydroxide groups -OH on its surface than alumina. Therefore, high amounts of boehmite act as buffer against the pH shift from acidic to alkaline Equation 4 resulting in longer setting times, which was verified by titration experiments.
In Figure 7 pH evolutions of A80A suspensions with 1. For an unaged suspension processed by applying ultrasound continuously during the closed loop process a starting zeta potential of In contrary, a zeta potential of The zeta potential indicates the degree of stability of a suspension. Flocculation and, thus, destabilization of suspensions caused by attractive interparticle forces due to hydrogen bonds between boehmite particles are prevented through energy input via ultrasound and immediate processing without aging.
As a result, destabilization of aluminium hydroxide-loaded alumina suspensions occur when either no ultrasound treatment is performed or stabilized suspensions are aged.
This observation supports the assumption that attractive forces are present among boehmite particles. Thus, it could be revealed that the destabilization of suspensions due to gelling of boehmite has a significant impact on the setting time. Besides, the influence of ultrasound treatment after adding of the setting agent to the suspension was investigated. A70A with 1. This observation shows that an inhomogeneously distributed setting agent also affects the coagulation kinetics considerably. The influence of the setting temperature T set was also found to be a further crucial parameter influencing the coagulation kinetics, which was also reported in literature In Figure 8 the effect of T set on the setting time for A90A suspensions is illustrated.
As expected, higher setting temperatures result in faster coagulation of the suspensions due to temperature-induced elevated reaction rates of the chemical reactions expressed in Equation This is in agreement with the observations made for A90A suspensions. The amount of setting agent m AlN was also found to affect the coagulation kinetics remarkably. In Figure 9 the evolution of pH versus time during consolidation and the resulting setting time in dependency on m AlN for A80A suspensions are shown. An increasing amount of AlN results in shorter setting times and also increases the strength of the green parts, which will be discussed in the next chapter.
Setting times are reduced, as ammonia formation and boehmite precipitation during hydrolysis of the setting agent are increased Equation 3. For instance, the setting time decreases for A80A suspensions with In contrary, for 1. This leads to the conclusion that the influence of the solids loading on setting time can be diminished for suspensions with boehmite, when the amount of setting agent is chosen properly. Properties of green parts. For instance, volumetric shrinkage of 3. Comparable results were found in literature 12, In Figure 10 the dependency of green density and drying shrinkage on solids loading and amount of nano-powder are presented.
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In comparison, A90A and AA with identical solids loadings exhibit Obviously, higher solids loadings result in lower volumetric drying shrinkage. After drying, the aluminium hydroxide nano-powder phase and, additionally, the precipitated amorphous aluminium hydroxide phase Equation 5 connects the host alumina particles and cause a considerable increase in green density and strength.
A core-shell arrangement is built up consisting of fine boehmite particles surrounding coarser alumina particles. As a result, the nano-phase acts as a kind of gap-filler within the alumina particle network. This observation could be verified by scanning electron microscopy of fracture surfaces of green parts with varying amount of AlOOH Figure As a binder phase within the alumina network, boehmite also results in enhanced mechanical properties of green parts Table 3 , which was also reported in literature In Figure 12 strength and density values of green parts with The characteristic strength correlates well with the relative density values of the green parts Table 3.
Weibull moduli were calculated and exhibited scattered values in the range of 3 to 12, which might be enhanced, when finer sieves in the range of few microns would be used for filtering potential contaminations and residual agglomerations before degassing and casting In literature a similar behaviour for the evolution of the density of extruded alumina-boehmite green parts was reported Though, a maximum in green density could not be observed in our investigations.
It is supposed that the applied processing method for the preparation of the suspensions has a considerable effect on compaction and mechanical properties. A different processing method was used by Ananthakumar and Warrier 25 for the preparation of alumina-boehmite suspensions. In contrary, in this study suspensions were fabricated by controlling the electrochemistry of the suspension carefully that resulted in a core-shell arrangement of alumina and aluminium hydroxide, which could be transmitted to the green state successfully Figure The favourable gap-filling characteristic of the boehmite nano-powder yielding to an increase of green density and green strength seems to be limited to low AlOOH amounts only, when the suspension is not processed properly, which is not the case for the proposed NPAC method.
For this, NPAC is considered to be a favourable direct casting method for advanced ceramics with tailored microstructure, as the precipitation of boehmite on an alumina core phase during coagulation results in an additional AlOOH shell phase, which is transformed to alumina after sintering.
However, NPAC should not be limited to alumina-based materials only, when appropriate combinations of coarse and fine powders as well as setting agents are considered. Various processing parameters were investigated influencing the rheology and coagulation kinetics of alumina-aluminium hydroxide boehmite suspensions as well as the properties of green parts fabricated via nanoparticle assisted coagulation NPAC. The colloidal suspensions exhibited shear thinning behaviour and showed pronounced rheopectic anti-thixotropic flow properties for increasing amount of boehmite due to an increase in gelling susceptibility.
The effect of boehmite nano-powder on the packing characteristics was investigated, revealing a decrease of the maximum solids loading with increasing amount of boehmite. Destabilization of suspensions as a result of non-absorbed hydrophilic boehmite could be prevented by continuous ultrasound treatment during powder addition and after addition of the setting agent. It was found that boehmite causes a buffering effect regarding the pH shift of the suspension from acidic to the isoelectric point IEP , which, in turn, causes longer setting times.
Elevated setting temperatures resulted in faster coagulation due to temperature induced higher reaction rates of the chemical reactions during hydrolysis of the setting agent aluminium nitride AlN. The amount of setting agent was also found to affect the coagulation kinetics remarkably. Higher concentrations of the setting agent resulted in shorter setting times, as ammonia formation and boehmite precipitation during hydrolysis is increased.
Increasing green density with increasing shrinkage at the same time was observed for suspensions with constant solids loading but increasing amount of aluminium hydroxide due to the gap-filling characteristic of the hydrophilic nano-powder. The mechanical properties of dried samples correlated well with the evolution of the relative green density as a function of the amount boehmite.
An increase of the amount of boehmite resulted in a significant increase in green density and green strength. In this study it could be demonstrated that with NPAC improved microstructures of green parts can be obtained fabricated with alumina-aluminium hydroxide suspensions.
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NPAC enables aqueous colloidal suspensions with homogeneous core-shell arrangement of submicron alumina surrounded by nanometer-scaled aluminium hydroxide, which can be transmitted to the green state successfully yielding to an enhanced microstructure. Besides the successful application to the fabrication of all-oxide continuous fiber-reinforced ceramic matrix composites, direct casting of ceramics via NPAC may also attract attention in the field of functional and structural ceramics for the manufacturing of components with tailored microstructures.
Combinations of different materials with various functional or structural properties could be enabled to achieve customized material properties. The authors wish to thank Prof. Walter Krenkel University of Bayreuth, Germany for providing the opportunity to use the infrastructure of the department of Ceramic Materials Engineering. The authors also want to thank Marc Schrinner Physical Chemistry I, University of Bayreuth for his support in cryo transmission electron microscopy, Werner Reichstein and Bernd Martin both Ceramic Materials Engineering, University of Bayreuth for their support in scanning electron microscopy and technical issues.
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Combined study of coagulation kinetics and close-range aggregate structure
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- Coagulation Kinetics and Structure Formation : H. Sonntag : ?
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Combined study of coagulation kinetics and close-range aggregate structure. You have access to this article. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Article type: Paper. DOI: Download Citation: J. Combined study of coagulation kinetics and close-range aggregate structure A. Lips and R. Duckworth, J.