Study on Thermodynamics and Kinetics for the Reaction of Magnesium Diboride and Water by Microcalorimetry

An exothermic reaction between MgB2 and water was observed in our laboratory at high temperature, although no obvious reaction occurred at room temperature. The reaction process of MgB2 and water was therefore studied by using microcalorimetry. The results showed that the reaction enthalpies of MgB2 with water and the formation enthalpies of MgB2 at T = (323.15, 328.15, 333.15 and 338.15) K are (–313.15, –317.85, –322.09, –329.27) kJmol, and (–238.96, –237.73, –236.50, –234.30) kJmol, respectively. The standard enthalpy of formation and standard molar heat capacity of MgB2 obtained by extrapolation method are –245.11 kJmol and 246 J molK, respectively. The values of activation energy E, pre-exponential factor A and the reaction order for the reaction of MgB2 and water over the temperature range from 323.15 K to 338.15 K are 50.80 kJmol, 10 s and about 1.346, respectively. The positive values of G  and and negative value of indicate that the reaction can take place easily above 314.45 K. H   S  

Boride is an important component of fuel-rich propellant, but it has difficulties in the ignition and combustion.One of the most effective methods to improve the performance of boron is to add combustible metals by chemical combination.In solid fuel ramjet's development, the propulsion application obtained by the reaction of Mg and water is the most popular topic of the water ramjet.For a deeper investigation of the potential applications of MgB 2 in special solid propellants, the reaction of MgB 2 and water was carried out by our laboratory to understand the physiochemical properties of MgB 2 in solid propellants.We found for the first time that the MgB 2 and water do not react visibly under room temperature, but when the temperature rises, an exothermic reaction happens obviously.The aim of this work is to describe the thermodynamic and kinetic properties of the reaction, and the investigation will offer valuable suggestions for the application of MgB 2 on chemical propulsion aspect.

Sample and Equipment
The sample (MgB 2 ) used in the experiment was prepared by Northwest Institute for Non-ferrous Metal Research.Its purity was improved from 95% up to 99.4% after recrystallized by our laboratory.The water was twice-distillated before use with its electrical conductivity being (0.8 -1.2) × 10 4 Sm -1 .
All measurements were made using a RD496-2000 Calvet microcalorimeter.Two replicates of each sample were tested.The enthalpy of dissolution of KCl (spectrum purity) in distilled water measured at 298.15 K was 17.234 kJmol -1 , which was an excellent accord with the literature value 17.241 kJmol -1 [10], showing that the device for measuring enthalpy used in this work was reliable.

The Reaction of MgB 2 and Distilled Water at a
Heating Rate of 0.2 Kmin -1 Certain amount of MgB 2 and distilled water were mixed in standard vessel at 298.15 K.The whole device was put into the furnace.A heating rate of 0.2 Kmin -1 was employed from 298.15 K to 348.15 K.The product was dried under 313.15K and the element analysis was then carried out.

Element Analysis
VarioE III element analysis equipment from German was employed to detect the content of H after the reaction.S4 Pioneer X-ray fluorescence spectrometer was used to determine the contents of both Mg and B, and the field emission scanning electron microscopy from FEI Company in Holand was also used to determine the contents of Mg, B and O.

The Reaction of MgB 2 and Distilled Water at
Different Temperatures The reaction of MgB 2 and distilled water was carried out at 323.15, 328.15, 333.15 and 338.15 K, respectively.The dependence of the heat flow of the reaction on reaction temperature (reaction time) was shown in Figure 1.

Results and Discussion
From Figure 1, one can see that the reaction do not proceed at 298.15 K.When the temperature rises up to 312.15 K, the reaction of MgB 2 and water begins and the reaction rate reaches a maximal point when the temperature rises to 314.45 K.

Element Analysis
The element H was determined by VarioELⅢ element analysis equipment for three times, and the results were 3.00%, 3.02% and 3.00%, respectively.The content of Mg was 36.63% by using the X-ray fluorescence spectrometer.The element analysis results obtained by the field emission scanning electron microscopy were shown in both Figure 2 and Table 1.
From the results of the element analyses, the reaction can be deduced as The results of the element analyses are close to the theoretical calculation: B 19.81%, Mg 33.43%, O 44.01%, H 2.75%.

The Standard Enthalpy of Formation of MgB 2
According to the Equation ( 2), we get the enthalpy of formation of MgB 2 as in Equation ( 3).
where Δ f H m is the enthalpy of formation of each compound; Δ r H m is the enthalpy of reaction.
The enthalpy of formation of MgB 2 could be obtained by Equation(4) [11].
Where c is the molar heat capacity and is the standard enthalpy of formation of each compound.
The determined reaction enthalpy of MgB 2 and water at different temperatures were listed in Table 2. Other necessary data for calculation were listed in Table 3.
Additionally, the standard enthalpy of formation, molar heat capacity and specific heat capacity of MgB 2 is -245.11kJmol -1 , 246 Jmol -1 K -1 and 5.36 J•g -1 •K -1 , respectively, which indicates that MgB 2 has high thermal capacity compared to other compounds.

The Kinetic Parameters of the Reaction of MgB 2 and Water
By putting the original data in Table 4,  , into the kinetic equation ( 5) [12], the values of n and lnk listed in Table 5 are obtained, where n is the reaction order and k the reaction rate constant.
From Table 5, one can see that the values of n are close at different temperatures, and lnk increase slightly with temperature rising.
The Equation ( 6) was applied to calculate the values of activation energy E and pre-exponential factor A by the slope and the intercept of the linear.
The value of E is 50.80 kJ•mol -1 and A is 10 4.78 s -1 .
With the data of E and A, the entropy of activation ( S   ), enthalpy of activation ( H   ), and Gibbs free energy of activation ( G   ) of the reaction processes of MgB 2 and water under different temperatures were obtained by Equations ( 7) and ( 8) and shown in Table 6.
Table 6 The values of , and The positive values of G   and H   , and the negative value of S   show that the reaction can easily take place when the temperature is high enough.

Conclusions
1) The reaction of MgB 2 and water will not happen until the environment temperature reaches 312.15 K.
3) The activation energy E and pre-exponential factor A of the reaction were obtained as 50.80 kJmol -1 and 10 4.78 s -1 respectively.The values of n are close under different temperatures, and lnk increases slightly with temperature rising.The entropy of activation ( S   ), enthalpy of activation ( H   ), and Gibbs free energy of

3. 1 .
The Result of the Reaction of MgB 2 and Distilled Water at a Heating Rate of 0.2 Kmin -1

Figure 1 .
Figure 1.The heat flow curve of the reaction of MgB 2 and water at a heating rate of 0.2 K•min -1 .

Figure 2 .
Figure 2. The electronic picture of the sample after the reaction.

Table 6 . The values of
reaction processes of MgB 2 and water under different temperatures show that the reaction can easily take place when the temperature is high enough.