Journal of the Ceramic Society of Japan 118 [7] 617-619 2010 Note Novel synthesis method of Ca3Al2O6 using an ethanol solution technique Weining LIU*,** and Jiang CHANG*,³ *State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China **Graduate School of the Chinese Academy of Sciences, 319 Yueyang Road, Shanghai 200050, China A novel method was developed for preparation of Ca3Al2O6 which is one composition of Portland cement-based mineral trioxide aggregate for endodontic treatment. OnlyAl(NO3)3·9H2O and Ca(NO3)2·4H2O as raw materials and ethanol as solvent were needed in the whole process. The consequent dry Ca3Al2O6 precursor was examined by thermal analysis and the as-burnt powders calcined at different temperatures for 3 hours were measured by X-ray diffractometry. Pure phase Ca3Al2O6 with particlesize of 1­10 µm was obtained at 1100°C and highly pure Ca3Al2O6 (>99.5%) was obtained at 1350°C. Therefore, this method is usefulfor preparation of pure Ca3Al2O6 for biomedical applications. ©2010 The Ceramic Society of Japan. All rights reserved. Key-words : Tricalcium aluminate, Synthesis, Ethanol, Pure phase, Biomedical applications [Received November 24, 2009; Accepted April 15, 2010] Al(NO3)3·9H2O and Ca(NO3)2·4H2O for the reaction, without 1. Introduction any aidof other components or repeated calcination for the Tricalcium aluminate (Ca3Al2O6) isanimportant constituent of synthesis. A combination of techniques (TG/DSC, XRD, SEM 1) Portland cement. Besides, Ca3Al2O6 isalso a major phase of and BET) was introduced to monitor and analyze Ca3Al2O6 mineral trioxide aggregate (MTA) which is a Portland cement- precursors, intermediate calcium aluminate compounds and final like biomaterial with numerous exciting clinical applications pure phase Ca3Al2O6. in endodontics, however, also with complex composition and consequent high cost.2)­6) In order to form a simpified MTA, each 2. Experimental individual phase of MTA should be evaluated so that several All reagents were of analytical grade, purchased and used as principal phases which mostly contribute to the good physico- received without further purification. All experiments were chemical and biological properties of MTA will be selected. conducted under air atmosphere. The starting materials were Therefore, it is necessary to prepare individual phases of MTA Al(NO3)3·9H2O and Ca(NO3)2·4H2Owith an initial CaO/Al2O3 with high purity, like Ca3Al2O6, for further biomedical applica- molar ratioof 3 in order to obtain 3CaO·Al2O3 or Ca3Al2O6. tions. First, 0.02 mol Al(NO3)3·9H2O was added in excessive ethanol Ca3Al2O6 is conventionally formed by a solid-state reaction which afterwards turned into green under continuous stirring, between CaO and Al2O3 through high temperature and repeated followed by the addition of 0.03 mol Ca(NO3)2·4H2O. After calcinations. However, it becomes easy to fabricate pure phase achieving complete dissolution, the solution was maintained Ca3Al2O6 even at relatively low temperatures when wet chemical at 80°C for 24 h to form a very viscous gel precursor. Such methods are introduced, though some problems also exist. Ca3Al2O6 precursors derived from ethanol solutions were further 7),8) Stephan et al. prepared fine Ca3Al2O6 particles viaasol­gel dried at 120°C overnight and then calcined at different temper- process, though 3 cycles of burning above 1260°C for up to 14 h atures between 700 and 1100°C for 3 h. The resultant powders with intermediate grindings were necessary. Lee et al.9) produced were ground and sieved through 300-mesh (<52 ¯m) for further Ca3Al2O6 through a solution­polymerization route at 1200­ characterizations. 1300°C for merely 2 h, though the additional aidof poly(vinyl The thermogravimetric analysis (TG) and differential scanning alcohol) was required. Yuan and Xu10) successfully synthesized calorimetric (DSC) curves were taken with a rising temperature Ca3Al2O6 using citrate precursor method at 1100°C for 2 h, rate of 10°C/min in flowing air by a STA 409/PC simultaneous though the extra combination of nitrate salts and citricacid was thermal analyzer (Netzsch, Germany) in order to monitor the needed as well as the subsequent ignition of resultant gels around decomposition and the oxidation process of Ca3Al2O6 precursors. 180°C. Therefore, further facile and economic methods are The as-prepared powders calcined at different temperatures were expected. In Yuan’s method, citricacid and aluminium nitrate examined by XRD (Geigerflex, Rigaku, Japan) with Cu (K¡) nonahydrate (Al(NO3)3·9H2O) were dissolved in ethylene glycol, radiation, operating at 40 kV and 100 mA. In addition, morpho- so it was likelytodissolve nitrate salts in ethanol which is fairly logical analysisof Ca3Al2O6 powders obtained at 1100°C was cheap and readily available. Thus, the aimof this study was to performed using a scanning electron microscope (SEM; JSM- explore the possibility offormation of the pure phase Ca3Al2O6 6700F, Japan) and the specific surface area of the powders at a relatively low temperature onlyusing ethanol to dissolve was determined by nitrogen absorption (BET, ASAP 2020M Analyzer, Micromeritics). Finally, the content offree CaO in ³ Corresponding author: J. Chang; E-mail address: jchang@mail. the samples calcined at different temperatures all for 3 h was sic.ac.cn determined by the glycol­ethanol method.11) ©2010 The Ceramic Society of Japan 617 JCS-Japan Liu et al.: Novel synthesis method of Ca3Al2O6 using an ethanol solution technique / f l i f l Fig. 1. TG DSC curves o Ca3A 2O6 precursors der ved rom ethano Fig. 2. XRD patterns of various calcium aluminate compounds l i so ut ons. obtained at different calcination temperatures for 3 h. 3. Results and discussion The TG­DSC curves of the dry Ca3Al2O6 precursor are shown in Fig. 1. The endothermic peak around 65°C in DSC accounting for about 2% of the initial weight loss in TG was assigned to the evaporation of residual ethanol. Thereafter, two endothermic peaks at 185 and 217°C in DSC, accompanied by the first sharp weight loss in the TG curve, were attributed to the dehydration of two different kinds of nitrate salts. The second step of weight loss from 217 to 556°C inTGmight be mainly caused by the formation of nitrite salts derived from corresponding nitrate salts, which accordinglyrelated to two endothermic peaks at 288 and 366°C in DSC, respectively. The last sharp weight loss from 556 to 695°C in TG resulted from the further and complete Fig. 3. SEM photograph of the as-prepared Ca3Al2O6 powders calcined decomposition of nitrite salts, which meantime led to the two at 1100°C for 3 h. endothermic peaks at 556 and 584°C in DSC. In addition, the other two exothermic peaks at 695 and 955°C in DSC with almost no change inweight loss were probablyarisen from the Table 1. BET specific surface area and free CaO contents of sample intermediate calcium aluminate compounds in the formation of powders l i l i ifi pure phase Ca3A 2O6, respect ve y, wh ch was ver ed by Temperature (°C) 1000 1100 1350 f ll i l f o ow ng XRD ana yses. There ore, 700°C was chosen as the 2/ ® a b ® i i i l i l BET (m g) 3.251 vs 3 n t a test temperature at wh ch the Ca3A 2O6 precursor was Content of CaO (%) 17.13 « 2.505 3.11 « 0.957 0.32 « 0.163 calcined for 3 h. aAs-prepared powders; bLee9) As shown in Fig. 2, the Ca3Al2O6 phase began to form at 1000°C accompanied by three other phases, namelyCa12Al14O33 (JCPDS 09-0413), Ca5Al6O4 (JCPDS 70-0801) and CaO. Then the pure phase Ca3Al2O6 (JCPDS 70-0839) could be obtained at Lee et al. (See Table1). In addition, Table 1 shows the content 1100°C onlywith a small overlapping peak both for Ca3Al2O6 offree CaO in powders by chemical analysis. In the case of a and free CaO. However, there was no evidence of the formation calcination temperature at 1000°C, large percentage (17.1%)of of Ca3Al2O6 below 1000°C in XRD patterns. After initially free CaO was found in the resultant powders. When the heating the Ca3Al2O6 precursor at 700°C for 3 h, Ca12Al14O33 calcination temperature increased to 1100°C, the Ca3Al2O6 with and CaO were identified in the pattern as well as CaAl12O19 little free CaO (only3%) formed, which was well crystallized as (JCPDS 32-0151) whose peak intensity was much lower relative shown in the XRD pattern (Fig. 2). When the calcination to other phases present. In the case of 850°C, a new phase of temperature was up to 1350°C, the obtained Ca3Al2O6 phase was Ca5Al6O4 appeared together with Ca12Al14O33 and CaO. There- highly pure, with very little amount offree CaO (0.32%). fore, three intermediate calcium aluminate compounds such as Previous studies have shown that,1),10),12) in the synthesisof Ca12Al14O33,Ca5Al6O4 and CaAl12O19 appeared herein in the Ca3Al2O6, some alumina-rich intermediate compounds rather crystallization process of Ca3Al2O6 below 1100°C, which was than Ca3Al2O6 first formed below 1100°C due to the fast 1),10) 2+ demonstrated by many other studies. conversion of alumina by diffusion of Ca to Ca12Al14O33 or Figure 3 illustrates the morphology of Ca3Al2O6 crystalline Ca5Al6O4.Since the diffusion is temperature dependant, certain powders obtained by calcination at 1100°C for 3 h. The particle amount of CaO exists when calcination temperature is low. With size was about 1­10 ¯m, which accordingly led to a relatively the increase of the calcination temperature, the diffusion of high BET specific surface area comparable to that reported by calcium ions through the layers of reaction products is enhanced, 618 Journal of the Ceramic Society of Japan 118 [7] 617-619 2010 JCS-Japan and these intermediate calcium aluminate compounds slowly 12) 4.