reaction order
Study on Synthesis of the High Aspect Ratios Nesquehonite Whiskers
Pingke Yan a , Bin Wang b and Yujuan Gao c
1College of Resource and Environment Engineering, Liaoning Technical University,
Fuxin, Liaoning, 123000, China
a yanpingke@126,
b wangbin851229@126,
c gaoyujuan0122@163
Keywords: Nesquehonite, High Aspect Ratios, Whisker, Synthesis
Abstract. In this paper, nesquehonite whiskers were synthesized by low-temperature aqueous solution method, and the impacts of reaction temperature, reaction time and surfactant dosage and other factor
s on the maximum whisker length and high aspect ratios of nesquehonite whiskers were also investigated. Results showed that under the conditions that the reaction temperature was 40 – 50 °C the reaction time was 50 – 60min and the amount of surfactant dosage was 1% (by mass), high aspect ratios nesquehonite whisker products can be synthesized. On this basis, growth mechanism of the nesquehonite whiskers was discussed.
Introduction
Nesquehonite (MgCO 3·3H 2O) whiskers are single crystals of magnesium carbonate, not only development of the crystals are completed, but also the crystals are colorless and transparent with few defects and high strength. Nesquehonite whiskers can not only be used for preparation of high purity magnesium oxide, magnesium hydroxide, basic magnesium carbonate, magnesium sulfate, magnesium nitrate and other magnesium salt products, but also they can be directly used in enhancing and modifying plastics, rubber, coatings and inks, etc. because of their good mechanical properties.  With special physical and chemical properties and practical applications, nesquehonite whiskers have attracted wide interest of researchers [1-4]. Japanese Sanzuixingping [5] et. al. synthesized regular magnesium carbonate columnar particles by hydro themal method, of which the diameters were mainly 1– 3µm, and the lengths were 10 – 50µm; while the ranges of the nesquehonite
whiskers’ diameters and lengths that the domestic researchers [6-9] have successively synthesized were 0.5 – 3µm and 10 – 100µm, respectively.
In summary, studies on nesquehonite whisker have made some progress, but there are still some problems, such as, the whisker length is shorter and the high aspect ratios is smaller than which is wanted. In this paper, low-temperature aqueous solution method was used and through strictly controlling the reaction temperature, reaction time, surfactants and other main factors, the optimal synthesis conditions of high aspect ratios nesquehonite whiskers were explored.
Experimental
Experimental  Reagents and Experimental Instruments. All of the drugs used in the experiment were analytical reagents, they were MgCl 2·6H 2O, NH 4HCO 3, NH 3·H 2O, Na 2HPO 4.
Instruments used in synthesis experiment were multi-functional reactor (Henan Yuhua Instrument Co.,Ltd.),DHG-9140A electrical thermostatic vacuum drying oven (Shanghai Precision Experimental Equipment Co., Ltd.), pHS-2C precision acidimeter and so on.
Experimental methods. Weighed MgCl 2·6H 2O and NH 4HCO 3 of which the molar ratio was 1:2 and
prepared solutions with certain concentration respectively; added MgCl 2 solution to the multi-functional reactor, heated to a certain temperature and kept constant; in the stir process, added NH 4HCO 3 solution of the same temperature to the MgCl 2 solution by a constant flow, and adjusted the solution pH to 9.0 using aqueous ammonia; added a certain amount of Na 2HPO 4 surfactant to the
washed in the deionized water till chloride ion can not be detected in the washing liquid (detected by silver nitrate solution); then dried the washed filter cake in vacuum at 50°C for 6h, the nesquehonite whisker samples would be obtained. Detected the samples’ structures using X-ray diffractometer, and observed their morphology, diameter, length and length-diameter ratio with a scanning electron microscope.
Results and Discussions
The impact of reaction temperature on products. In order to investigate the optimum temperature for whisker formation, experimental conditions were set as follows: concentration of MgCl 2·6H 2O and NH 4HCO 3 was 0.4 mol/L, 0.8mol/L respectively, the adding amount of Na 2HPO 4 was 0.5% (by mass) of the total system mass, and the reaction time was 1h.
The results (Fig. 1) showed that size of whisker was better at the temperature of 45°C, while both the high aspect ratios and the size of whisker reached maximum at 50°C (Fig. 2); when the temperature was higher than 50°C, the whisker size began getting smaller, and the trend that the length-diameter r
atio became smaller was obvious, of which the reasons were that the properties of nesquehonite whisker became unstable at a temperature higher than 50°C, and there was a trend of conversion to basic magnesium carbonate; it would completely convert to basic magnesium carbonate (Mg 5(CO 3)4(OH)2·4H 2O) when the reaction temperature was higher than 65°C and accordingly its morphology also converted from needle-like shape to pieces. Taking whisker morphology, reaction yield and other factors together into account, high aspect ratios whisker can be obtained when the
reaction temperature range was controlled at 45 – 50°C.
Fig. 1 Effects of reaction temperature                    Fig.2  SEM photograph of  MgCO 3·3H 2O on the morphology of  MgCO 3·3H 2O whisker                whisker at the  temperature of 50°C
Through XRD analysis we can see that (Fig. 3), when the reaction temperature was between 30and 50°C, the resulting product was MgCO 3·3H 2O, and it had high purity with no other impurity. When the reaction temperature was 50°C narrow XRD diffraction peak and high intensity indicated that the MgCO 3·3H 2O was well-developed. As the temperature increasing, the resulting hydrated magnesium carbonate was stable Mg 5(CO 3)4(OH)2·4H 2O. It can be seen that when MgCl 2·6H 2O and NH 4HCO 3 reacted, they would firstly synthesize MgCO 3·3H 2O at a lower temperature, but there was only a relatively stable temperature range, when temperature rose, MgCO 3·3H 2O began converting to Mg 5(CO 3)4(OH)2·4H 2O, the reaction processes were:
MgCl 2+ 2NH 4HCO 3 +2H 2O=MgCO 3·3H 2O↓+2NH 4Cl+CO 2↑.                                                    (1) 5MgCO 3·3H 2O=Mg 5(CO 3)4(OH)2·4H 2O↓+CO 2↑+10H 2O.                                                            (2)
Fig.3 XRD of magnesium carbonate hydrate obtainedat the temperatures from 30 to 65°C The impact of reaction time on products. In order to investigate the appropriate reaction time for whisker formation, experimental conditions were set as follows: concentration of MgCl 2·6H 2O, NH 4HCO 3 was 0.4 mol/L and 0.8mol/L respectively, the adding amount of Na 2HPO 4 was 0.5% (by mass) of the total system mass, and the reaction temperature was 50°C.
The results (Fig. 4) showed that when reaction time reached 50 min, the whisker formed a better morphology (Fig. 5), but taking yields and other factors into account, the reaction time should be more than 60 min.
Fig.4 Effects of reaction time  whisker                        Fig.5  SEM photograph of
on the morphology of MgCO 3·3H 2O whisker                  MgCO 3·3H 2O at 50 minutes
The impact of surfactant adding amount on products. In order to investigate the appropriate surfactant adding amount for whisker formation, experimental conditions were set as follows: concentration of MgCl 2·6H 2O, NH 4HCO 3 was 0.4 mol/L and 0.8mol/L respectively, the reaction time was 60min and the reaction temperature was 50°C.
The results (Fig. 6) showed that the surfactant adding amount reached 1% (by mass) the total system mass, diameter of the nesquehonite whisker reached its maximum 160µm, whisker diameter was 4.3µm and the high-aspect ratio was 37.2 (Fig. 7). That was because the additives added to the reaction solution selectively adsorbed on the nesquehonite crystal surfaces in the directions of [100] and [001], preventing the growth of crystal surfaces, so that the nesquehonite crystals grew in the direction of [010] in the process of crystallization, thus high aspect ratios whiskers formed. With the adding amount increasing, the high-aspect ratio decreased, of which the main reasons were the redundant Na 2HPO 4 in the solution would alter reaction system’s pH, and the reaction environment formed was not conducive to the crystallizing and growth of nesquehonite whisker, resulting to a 1120Advanced Materials
Fig.6 Effects of addition amount of surfactant          Fig.7 SEM photograph of  MgCO 3·3H 2O  on the morphology of MgCO 3·3H 2O whisker        whisker when adding one per cent of Na 2HPO 4
Fig.8  the simulated morphology of  MgCO 3·3H 2O by the chemical bond inviewpoint
Discussion of the Growth Mechanism of Nesquehonite Whisker
MgCO 3·3H 2O belongs to monoclinic system, structural research on crystals shows that Mg atoms and O atoms in the crystal compose MgO 6 octahedrons, Mg atoms are not in the MgO 6 octahedral center, among the six O atoms that connect with the Mg atoms, three are from molecules of water, and other three are from carbonate ions. These distorted MgO 6 octahedrons form infinite long-chains by common vertex connection. In these chains, each carbonate connects with three MgO 6 octahedron units, of which two are connected by common vertex connection, the other one is connected by common side  [10-12].
Since nesquehonite crystals themselves have highly anisotropic structure, so in the process of liquid phase growth, it shows that growth rate of nesquehonite crystals in the direction of [010] is greater than that of [001], resulting in nesquehonite crystals grow in a single direction and form whiskers. When surfactants are added, they will preferentially adsorb on the crystal surfaces in the directions of [101] and [001], and prevent interaction between ions and crystal surfaces in the solution, resulting in the growth of crystal surfaces in the directions of [101] and [001] slows or even stops, so that the [010] crystal surfaces can interact fully with ions in the solution and grow continually, ultimately, high aspect ratios whisker products will form.
Conclusions
By studying the impacts of reaction temperature, reaction time and surfactant adding amount on the growth of hydrated magnesium carbonate crystals in NH 4HCO 3 and MgCl 2·6H 2O reaction process, following conclusions are obtained: Advanced Materials Research Vols. 239-2421121
MgCO 3·3H 2O whisker crystals can relatively stably exist when the environmental temperature is below 60°C when the temperature is above 60°C, the hydrated magnesium carbonate obtained is stable Mg 5(CO 3)4(OH)2·4H 2O.
The optimum conditions for synthesis of the high aspect ratios magnesium carbonate whisker are: the reaction temperature is 50°C, reaction time is 60min, surfactant adding amount is 1%( by mass), the MgCO 3·3H 2O whisker length synthesized is 140 – 160µm, and the length-diameter ratio is 25 – 32.
Acknowledgements
Authors wish acknowledge the National Natural Science Foundation of China for financial  support of  this work under project No.51004064.
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Advanced Materials
doi:10.4028/www.scientific/AMR.239-242
Study on Synthesis of the High Aspect Ratios Nesquehonite Whiskers
doi:10.4028/www.scientific/AMR.239-242.1118

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