May 2004 Mullite(3Al,03 2SiO2/ -Aluminum Phosphate (AlPO4, Oxide, Fibrous Monolithic Composites M M 1 mm 1 mm Fig. 1. Optical micrographs of mullite-AlPOA monofilament rods(M: matrix(mullite); 1: interphase(AlPO4)):(a)25 monofilament rod which was first extruded through a die with an orifice diameter of 4.0 mm; (b)150 monofilament rod which was first extruded through a die with an orifice diameter of 1.5 58433, and 1 12 000 g, respectively. The reason for using and outer mullite layer formulations were warm pressed into half mixture of three different eva grades as a binder was to facilitate tubular shapes, with the aid of a thickness-controllable mold the binder removal process. Dioctyl phthalate(Aldrich, 99%)and at a temperature of 150C and a pressure of 34.5 MPa. Th stearic acid(Aldrich, 95%) were used as a plasticizer and a center rod and AlPO interphase, half tubular shapes were lubricant, respectively ranged into a cylindrical mold which was heated to 90C, having Ceramic powder, binder, plasticizer, and lubricant were mixed 23 mm diameter and 150 mm length, and extruded into a die of 1.5 using a computer-controlled, high-shear mixer(Model 2100, C w. 2.0, or 4.0 mm diameter, depending on subsequent applications Brabender, NJ). The mixtures were made following the formula- This is called the"first extrusion" and the extrudate is referred to tions in Table I. In the case of mullite. to facilitate the extrusion of as a two-layer monofilament rod Because the"warm"extrusion the outer r layer so as to have lower viscosity, a lower amount of was made at 90C, the"seamless"half tubular shapes were bonded solids content of 50 vol% and a higher amount of lower-molecular- around the center rod after the first extrusion: 25.93. or 150 weight EVa binder were used; 40 vol% of powder loading was two-layer monofilament rods were arranged into a cylindrical used for mixing the AlPO4 interphase. The Brabender mixing mold which was heated to 90oC and extruded again into a die of 2 perature for the mullite inner rod and AlPO4 interphase layer mm diameter. This is called the second extrusion"and the 150%C. The mixing temperature for the mullite outer layer was extrudate is called a two-layer multifilament rod. To study the C because of its lower viscosity effects of decreasing the thickness of the porous and weak AlPO The inner mullite rods were extruded into cylindrical shapes interphase layer on the strength and work of fracture ing diameters of 11, 13.5, 16, and 20 mm, depending monolithic composites, monofilament rods with green rent subsequent configurations. The extrusion rate was 50 thicknesses of 0.33, 0. 19, and 0.073 mm were extruded n /min. The Brabender-mixed, AlPO4, interphase formulations to make fibrous monolithic composites. For the composi m mm 1 mm (a) 2. Optical micrographs of the mullite-AIPO4 multifilament rod (M: matrix(mullite); 1; interphase(AlPO4)):(a)25 multifilament rod, (b)15058 433, and 112 000 g, respectively. The reason for using a mixture of three different EVA grades as a binder was to facilitate the binder removal process. Dioctyl phthalate (Aldrich, 99%) and stearic acid (Aldrich, 95%) were used as a plasticizer and a lubricant, respectively. Ceramic powder, binder, plasticizer, and lubricant were mixed using a computer-controlled, high-shear mixer (Model 2100, C. W. Brabender, NJ). The mixtures were made following the formula￾tions in Table I. In the case of mullite, to facilitate the extrusion of the outer layer so as to have lower viscosity, a lower amount of solids content of 50 vol% and a higher amount of lower-molecular￾weight EVA binder were used; 40 vol% of powder loading was used for mixing the AlPO4 interphase. The Brabender mixing temperature for the mullite inner rod and AlPO4 interphase layer was 150°C. The mixing temperature for the mullite outer layer was 120°C because of its lower viscosity. The inner mullite rods were extruded into cylindrical shapes having diameters of 11, 13.5, 16, and 20 mm, depending on different subsequent configurations. The extrusion rate was 50 mm/min. The Brabender-mixed, AlPO4, interphase formulations and outer mullite layer formulations were warm pressed into half tubular shapes, with the aid of a thickness-controllable mold,47,48 at a temperature of 150°C and a pressure of
34.5 MPa. The center rod and AlPO4 interphase, half tubular shapes were ar￾ranged into a cylindrical mold which was heated to 90°C, having 23 mm diameter and 150 mm length, and extruded into a die of 1.5, 2.0, or 4.0 mm diameter, depending on subsequent applications. This is called the “first extrusion” and the extrudate is referred to as a two-layer monofilament rod. Because the “warm” extrusion was made at 90°C, the “seamless” half tubular shapes were bonded around the center rod after the first extrusion; 25, 93, or 150 two-layer monofilament rods were arranged into a cylindrical mold which was heated to 90°C and extruded again into a die of 2 mm diameter. This is called the “second extrusion” and the extrudate is called a two-layer multifilament rod. To study the effects of decreasing the thickness of the porous and weak AlPO4 interphase layer on the strength and work of fracture of fibrous monolithic composites, monofilament rods with green interphase thicknesses of 0.33, 0.19, and 0.073 mm were extruded and used to make fibrous monolithic composites. For the composites with a Fig. 1. Optical micrographs of mullite–AlPO4 monofilament rods (M: matrix (mullite); I: interphase (AlPO4)): (a) 25 monofilament rod which was first extruded through a die with an orifice diameter of 4.0 mm; (b) 150 monofilament rod which was first extruded through a die with an orifice diameter of 1.5 mm. Fig. 2. Optical micrographs of the mullite–AlPO4 multifilament rod (M: matrix (mullite); I: interphase (AlPO4)): (a) 25 multifilament rod, (b) 150 multifilament rod. May 2004 Mullite (3Al2O3
2SiO2)–Aluminum Phosphate (AlPO4), Oxide, Fibrous Monolithic Composites 795