Y. He et al. /Ceramics Intemational 34(2008)1399-1403 deionized water until no chlorine ion was detected by AgNO olution. Then it was washed with ethanol and dried in 50C The process is similar to that of [3]. 2. 2. Preparation of Mgo whiskers by conversion of magnesium hydroxide whiskers ≥ The preparation process includes three reactions as follows 6MgCl2+ 5Ca(OH)2+8H2O 2Mg3(OH)sCI. 4H20 5CaCI (1) Mg3(OH)sCI- 4H2O + NaOH 3Mg(OH)2+ NaCl+ 4H2O Mg(OH)2= MgO H2O Fig. 1. XRD pattern of Mgo whiskers prepared by hydrolysis of melts 5.93 g calcium hydroxide powder was added bit by bit to 250 mL 4.0 M aqueous magnesium chloride solution with a Fig. 3 shows the SEM images of the product prepared by molar ratio of 1 Ca(Oh)2 to 10 MgCl2 under constant stirring. hydrolysis of melts, revealing that it mainly consists of Then the suspension was agitated in 45C bath. Suspension magnesia whiskers with diameter of 1-3 um and length mainly became a gel after several hours. It was found through an of 130-300 um. However, the morphology of products varies optical microscopy the gel contained lots of fibers after 30 h. The precipitation was washed with deionized water, ethanol (a) and isopropanol, respectively and filtered with suction to remove the unreacted magnesium chloride and gel. Product after filtering was dried in 35C under normal pressure. Dried product thus obtained was suspended in 256 mL mixture solu- tion with a volume ratio of 3 ml ethanol to 1 mL water and hen reacted with 85 mL I M aqueous sodium hydroxide. The suspension was heated to and held at 62C for 3 h under g constant stirring. It was reported that a 3: 1 mixture of ethanol ind water seems to be the best solvent for the preservation of the whisker morphology and relative high reaction temperature (102) around 60C)and short reaction time helps to preserve the morphology [5]. At last the suspension was filtered and washed with ethanol several times and dried at 50C. It was found that the solid products were kept in fibrous shape with an optical microscopy. The composition of final product was identified by XRD At last, the dried powder was first heated to 150C (3C/ min), and then slowly from 200 to 800C (1.5C/min) before the temperature was ramped to 1000C in flowing N2. The owder was calcined at 1000C for an hour to finish the onversion to magnesia whiskers 3. Results and discussion 3.1. Characterization of the products The composition of products was identified by XRD. Fig. 1 shows that the product prepared by hydrolysis is only composed f magnesia phase. Fig. 2 presents the evidence of the conversion from Mg(OH) whiskers(brucite)to MgO whiskers It can be seen that the final product was also composed of ig 2. x rD yern cation ot co versio rs;(b) XRD patterns of magnesia phase. A little impurity is attributed to forsterite, Mgo whiskers converted from Mg(oH) wh the peaks is attributed to probably introduced by the silica tube the impurity forsterite denoted by (*deionized water until no chlorine ion was detected by AgNO3 solution. Then it was washed with ethanol and dried in 50 8C. The process is similar to that of [3]. 2.2. Preparation of MgO whiskers by conversion of magnesium hydroxide whiskers The preparation process includes three reactions as follows: 6MgCl2 þ 5CaðOHÞ2 þ 8H2O ¼ 2Mg3ðOHÞ5Cl 4H2O þ 5CaCl2 (1) Mg3ðOHÞ5Cl 4H2O þ NaOH ¼ 3MgðOHÞ2 þ NaCl þ 4H2O (2) MgðOHÞ2 ¼ MgO þ H2O (3) 5.93 g calcium hydroxide powder was added bit by bit to 250 mL 4.0 M aqueous magnesium chloride solution with a molar ratio of 1 Ca(OH)2 to 10 MgCl2 under constant stirring. Then the suspension was agitated in 45 8C bath. Suspension became a gel after several hours. It was found through an optical microscopy the gel contained lots of fibers after 30 h. The precipitation was washed with deionized water, ethanol and isopropanol, respectively and filtered with suction to remove the unreacted magnesium chloride and gel. Product after filtering was dried in 35 8C under normal pressure. Dried product thus obtained was suspended in 256 mL mixture solu￾tion with a volume ratio of 3 mL ethanol to 1 mL water, and then reacted with 85 mL 1 M aqueous sodium hydroxide. The suspension was heated to and held at 62 8C for 3 h under constant stirring. It was reported that a 3:1 mixture of ethanol and water seems to be the best solvent for the preservation of the whisker morphology and relative high reaction temperature (around 60 8C) and short reaction time helps to preserve the morphology [5]. At last the suspension was filtered and washed with ethanol several times and dried at 50 8C. It was found that the solid products were kept in fibrous shape with an optical microscopy. The composition of final product was identified by XRD. At last, the dried powder was first heated to 150 8C (3 8C/ min), and then slowly from 200 to 800 8C (1.5 8C/min) before the temperature was ramped to 1000 8C in flowing N2. The powder was calcined at 1000 8C for an hour to finish the conversion to magnesia whiskers. 3. Results and discussion 3.1. Characterization of the products The composition of products was identified by XRD. Fig. 1 shows that the product prepared by hydrolysis is only composed of magnesia phase. Fig. 2 presents the evidence of the conversion from Mg(OH)2 whiskers (brucite) to MgO whiskers. It can be seen that the final product was also composed of magnesia phase. A little impurity is attributed to forsterite, probably introduced by the silica tube. Fig. 3 shows the SEM images of the product prepared by hydrolysis of melts, revealing that it mainly consists of magnesia whiskers with diameter of 1–3 mm and length mainly of 130–300 mm. However, the morphology of products varies Fig. 1. XRD pattern of MgO whiskers prepared by hydrolysis of melts. Fig. 2. XRD verification of conversion from Mg(OH)2 whiskers to MgO whiskers: (a) XRD patterns of Mg(OH)2 whiskers; (b) XRD patterns of MgO whiskers converted from Mg(OH)2 whiskers, the peaks is attributed to the impurity forsterite denoted by ($). 1400 Y. He et al. / Ceramics International 34 (2008) 1399–1403