Synthesis and Characterization of Aluminum Borate Ceramic Whiskers

Abstract

Aluminum borates are synthesized via a precipitation process. Aluminum nitrate and calcium hexaboride (CaB 6 ) as a source of B 2 O 3 were used. Various proportions of aluminum nitrate were mixed with CaB 6 powder and to a solution of ammonium hydroxide. The resulting mixtures were evaporated and calcined up to 6501C. A porous ceramic with a framework structure of aluminum borate whiskers was synthesized in situ by firing the green compact bodies up to 14001C under argon gas avoiding the oxidation of the CaB 6 . The structure and morphology were investigated by XRD and SEM. The results indicate that whiskers exhibit a well-crystallized structure with an average diameter distribution of about 500 nm and lengths ranging from 5 to 8 mm. Porous aluminum borate ceramic consisted of whiskers with a porosity of 9-21% for samples sintered up to 14001C. Increasing the sintering temperature to 15001C and CaB 6 addition (15 wt%) deteriorated both the flexural strength and modulus of elasticity of the ceramic bodies.

Preparation and characterization of aluminum borate whiskers reinforced ceramics with MnO2 addition

“…Continuing to increase the heating temperature, the Al 2 O 3 was surrounded by B 2 O 3 and then reacts through a solid‐liquid (S‐L) reaction mechanism. Due to the addition of excess H 3 BO 3 , (Al 2 O 3 ) x (B 2 O 3 ) y nucleation was formed and wrapped by liquid B 2 O 3 , followed by a second liquid‐solid (L‐S) reaction, resulting in the growth of ABWs 17,20 . The reactions in the sintering process are as follows 22 : $$\begin{equation}2{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} \to {{\rm{B}}_2}{{\rm{O}}_3}({\rm{s}}) + 3{{\rm{H}}_2}{\rm{O(g)}} \uparrow \end{equation}$$ $$\begin{equation}{{\rm{B}}_2}{{\rm{O}}_3}(s) \to {{\rm{B}}_2}{{\rm{O}}_3}\left( {\rm{l}} \right)\end{equation}$$ $$\begin{equation}{\rm{xA}}{{\rm{l}}_2}{{\rm{O}}_3}({\rm{s}}) + {\rm{y}}{{\rm{B}}_2}{{\rm{O}}_3}\left( {\rm{l}} \right) \to {\left( {{\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}} \right)_{\rm{x}}}{\left( {{{\rm{B}}_2}{{\rm{O}}_3}} \right)_{\rm{y}}}\end{equation}$$ …”

“…This demonstrates that the addition of H 3 BO 3 and MnO 2 results in increased porosity. 17,20 The reactions in the sintering process are as follows 22 :…”

“…The mass fraction of potassium sulfate powders is 40 wt%, and thermostatic time is 4 h in which uniform whiskers could be obtained. Sayed H. Kenawy 17 composed ABWs via a precipitation process. Aluminum nitrate and calcium hexaboride as a source of aluminum and boron were used, respectively.…”

“…The whiskers exhibit a good crystalline structure with an average diameter distribution of about 500 nm and a length ranging from 5 to 8 μm by firing the green compact bodies up to 1400°C. With an increase in the sintering temperature to 1500°C, the flexural strength reduced 17 . However, there are few reports on the improvement of mechanical properties by the addition of MnO 2 in solid‐phase sintering reaction for the synthesis of ABWs.…”

Boron alumina: qualitative investigation of the surface acidity by FTIR measurements of CO adsorption

“…Boron alumina (Al 18 B 4 O 33 or 9Al 2 O 3 , 2B 2 O 3 ) is well known as ceramic material that possesses excellent electrical conductivity and chemical inertness properties at high temperatures [1][2][3]. It is also known as a thermostable light-weighted ceramic (density = 2.9 kg dm 3 ) that can be used as refractory materials up to 1700 °C [4,5].…”