Electrical and Mechanical Properties of Some High-Alumina Compositions Obtained by the Casting Process of Aqueous Suspensions

In this paper the choosen material compositions, the manner of preparing and shaping of composites are presented. Measurements of the mechanical resistance and electrical resistivities of cast samples obtained by pouring in plaster moulds are presented comparatively with the same properties obtained on mould samples by other processes.


Introduction
Alumina has two important industrial properties: hardness and electrical insulation.The hardness qualities of alumina contribute to the use of this material as a wear resistant coating or ceramic body.Electrical insulation properties of alumina are widely appreciated.Also, in addition to its reasonably high thermal conductivity in some special conditions alumina ceramics can be created.
High mechanical resistance and good electrical properties characterize ceramic materials based on alumina, as a raw material.For many applications in ceramic processing it is desirable to sinter at relatively low temperatures and, at the same time, to obtain fully dense and fine-grain microstructures [1].
One method by which these goals can be accomplished is by using the casting process of aqueous suspensions.In order to obtain a uniformly consolidated compact structure, it is also advantageous to use dispersed colloidal suspensions which contain more than 62 wt.% solids, while still maintaining relatively low viscosities of 30-90 mPas.
In general, the fine dispersed suspensions can be stabilized by electrostatic, steric or electrosteric stabilization mechanisms [2].Electrostatic stabilization is accomplished by generating a common surface charge on the particles.
Steric stabilization is achieved by adsorption of polymeric additives, which serve to form protective colloids.Electrosteric stabilization requires the presence of an adsorbed polymer or a polyelectrolyte and significant electrical double-layer repulsion.
Polyelectrolytes such as sodium carboxymethylcellulose (Na-CMC) and arabic gum are used for deflocculation.The use of Na-CMC in the slip casting of alumina, which is a nonplastic with inherently poor casting characteristics, offers several advantages because it is both a synthetic polyelectrolyte and an effective organic binder [3].Arabic gum in water solutions is charged negatively [4].
Ceramic products obtained from aqueous suspensions by casting in plaster moulds are sometimes more dense, without defects and therefore better than pressed objects.

Experimental
Reactive α-Al 2 O 3 from Oradea (Romania) with an average particle size of 4.9 µm, TiO 2 of Merck quality (Germany) and Harghita clay (Romania) were used for preparing aqueous suspensions.For good suspension stability the raw material mixtures were subjected to wet grinding in a porcelain ball mill.The powder mixture:ball:water ratio was 1:1:0.2.Grinding was carried out until the average particle size reached 2.1-2.2µm.The defloculant binders used were Arabic gum and sodium carboxymethylcellulose in a ratio of 1:1.5.The compositions of best casting slips are shown in Table I.
The casting slip compositions are selected among a large series in which additives (0.065-0.8 wt.%) and water (20-37 %) were varied.The choice of the optimal compositions took rheological properties into account, namely viscosity and tixotropy.

Results
The usual determinations of technological parameters: pH, density and tixotropy index after 30 minutes of repose were performed on the aqueous suspensions prepared in accordance with the procedure mentioned above.Characteristics of the slip casting mixtures are presented in Table II.At the same time the behavior when pouring into the plaster mould was followed.The cast samples were dried in air and then burnt in an electric kiln at 1450-1500ºC for 1-2 h.
Some electrical properties such as: dielectric rigidity, volume resistivity and bending resistance were made in accordance with the Romanian Standard 1292-82: Dielectric rigidity was determined on samples in the shape of a disc, in TR-30 oil, using silver electrodes; the upper electrode was a sphere of 10 mm in diameter, the inferior electrode was a plate of 25 mm in diameter, the rate of voltage increase was 1 kV/s.Volume resistivity at 20ºC was determined on samples in the shape of a disc with 100 mm in diameter and 3-5 mm thickness; the electrodes were formed from a continuous silver film.Dielectric loss angle tangent at 1 MHz was determined on samples in the shape of a disc with 30-40 mm in diameter and 1-3 mm thickness, electrodes from a continuous silver film, with a TF 1245 A apparatus.
The bending resistance was made on cylindrical samples 10 mm in diameter and 120 mm in length with a FPZ 100/1 machine.
Electrical and mechanical properties measured on the synthesized samples are presented in Table III.Table III also contains literature data [5] for comparison with the results obtained for present samples.

Conclusions
The superaluminous ceramic material features Al 2 O 3 as the main constituent (over 94 %).High mechanical resistance and good electric properties characterize it.From Table III it can be seen that the electrical and mechanical properties of the cast products are better than those obtained by pressing.The cast products are denser than the pressed ones and therefore

Table I
Casting slip composition

Table II
Characteristics of the slip casting mixtures

Table III
Electrical and mechanical properties of samples