Synthesis Of Mullite Whiskers And Preparation Of Phosphate-based Binder With Different Curing Agents

As one of the most commonly used inorganic binders,chromium-aluminum phosphate binder is usually prepared by mixing phosphoric acid with hydroxide or oxide.Phosphate binder is been valued and used because of its low curing temperature,non-toxic and high temperature stability.Phosphate binders have been widely used in coatings,high-temperature composites,bonded metals or glass.Mullite whisker belongs to a special form of needle-shaped mullite crystal.Its internal structure is almost flawless and its strength is close to the theoretical accuracy of complete crystal;Mullite whisker has the advantages of high temperature resistance,good thermal shock resistance,low thermal expansion coefficient,low creep resistance at high temperature.It is often used in high temperature resistant ceramic base,metal base and other composite materials to improve the comprehensive properties of materials,generally in the aspects of toughening and improving strength.The addition of a phosphate-based binder as a curing agent can reduce the curing temperature on the one hand and enhance the bonding strength of the adhesive on the other hand.In this paper,first of all,the molten salt method was used to derive the mullite whiskers,the influence of raw material ratio and reaction temperature on product structure and morphology were systematically studied.Then,the effect of adding different curing agents on the phase change process were discussed.The main research contents and the research results were as follows:（1）With kaolin and aluminum sulfate as raw materials,sodium sulfate as the reaction medium,this research made use of molten salt method prepared mullite whiskers,the influence of reaction temperature and insulation time on the structure and morphology of mullite whiskers were studied;Aluminum fluoride was used to replaced part of aluminum sulfate,and sodium sulfate was used as reaction medium to prepared mullite whisker by molten salt method;The structure and morphology of the synthetic samples were characterized by X-ray diffraction analysis,scanning electron microscopy analysis,and differential scanning calorimetry.The results show that the optimum reaction temperature and time for mullite whisker were 1000℃and 2 h;Mullite whiskers with length-diameter ratio of10～80 and length greater than 1μm were prepared with ten percent aluminum fluoride doped,and fifty percent was the optimum molten salt content for synthesizing whisker.（2）Used phosphoric acid（H₃PO₄）,aluminum hydroxide（Al（OH）₃）and chromium trioxide（Cr₂O₃）as raw materials,the matrix of chromium-aluminum phosphate binder was prepared by constant temperature water bath method.Added 7%calcium hydroxide（Ca（OH）₂）into the matrix as curing agent,the system of calcium hydroxide and chromium-aluminum phosphate binder was prepared.The prepared phosphate binder was calcined at different temperatures,and the structure and phase of the samples after calcined were analyzed and characterized.The experimental results show that the main form of chromium-aluminum phosphate binder was(Ca₁₀（PO₄）₆（OH）₂)at low temperature,and the other components are amorphous.With the increase of temperature,hydroxyapatite(Ca₁₀（PO₄）₆（OH）₂)undergo a phase transition to generate calcium phosphate,and Al PO₄begins to appear and transition from orthogonality to monoclinic.When the temperature rises to 700℃,the diffraction peaks of monoclinic type ring Type B aluminum metaphosphate(Al₂P₆O₁₈)increase.（3）Calcium hydroxide-mullite-chromium aluminum phosphate system binder is prepared by used chromium aluminum phosphate adhesive as the matrix,calcium hydroxide（Ca（OH）₂）as the curing agent,and 5%mullite whiskers.The prepared binder was baked at different temperatures,and the morphology and structure of the baked samples were analyzed and characterized,the following conclusions are drawn:in low temperature environment,mullite whiskers mainly play an enhanced role in chromium aluminum phosphate binder;With the increase of temperature and the decomposition of mullite whiskers,silicon phosphate（Si P₂O₇）appears in the system,and the viscosity and compactness of the adhesive are further improved.