470894 Investigations on the Mechanical Forces Required for Mechanochemical Synthesis of Hydroxyapatite
Mechanochemistry is the field of science which studies chemical reactions induced by mechanical energy1. This paper focuses on the synthesis of hydroxyapatite, a bone substitute biomaterial, by mechanochemical synthesis, and in particular in determining the mechanical forces required for the solid state reaction to occur.
In recent years, mechanochemistry has become a popular alternative for executing chemical reactions, as opposed to traditional solvent processes, which make use of high temperatures and pressures in addition to high volumes of toxic solvents. Mechanochemical processing exhibits many advantages over traditional solvent heavy processes. The solid state reactions carried out in a solvent free environment are a greener alternative which improves the safety of the chemical reaction and limits the occurrence of undesired side reactions. Furthermore, the reaction may be carried out at ambient temperatures and pressures, with no further processing or solvent recovery required.
The mechanochemical process requires the input of mechanical energy to stimulate a chemical reaction. The mechanical energy initiates the process in which defects and dislocations develop in the material, which may cause a solid state change in the material. When mechanical activation has occurred, the mechanical energy supplied may potentially cause a change in solid state such as the formation of a metastable polymorphic form, the amorphous form, or alternatively, chemical reaction may occur.
For chemical reactions, the activation energy of the system, also known as mechanical activation, is the threshold energy that reactants must acquire for chemical reaction to occur. When the threshold energy level has been obtained, the reactants will convert to a higher energy transition state, prior to converting to a lower energy state with the formation of a product, with lower potential energy and higher stability. In the case of an exothermic reaction, when the activation energy of the reaction has been attained for a particular reaction and chemical reaction has initiated, energy is released, to form a product with a less energy, as described in figure 1. In contrast, an endothermic reaction involves the absorption of energy for chemical transformation to occur. Figure 2 demonstrates the absorption of energy for an endothermic reaction when the activation energy of the system has been achieved. The energy absorbed is utilised to form the final product with a higher energy state.
An exothermic reaction features a release in energy when mechanical activation has been achieved.
An endothermic reaction features absorption of energy when mechanical activation has been achieved.
Hydroxyapatite was synthesised from calcium hydroxide and diammonium phosphate in a Retsch Mixer Mill MM 400, varying both the reaction time and milling frequency. The solid state endothermic reaction was undertaken using molar ratios of calcium hydroxide and diammonium phosphate according to the following reaction equation:
The reaction time for the mechanochemical synthesis of hydroxyapatite was measured at various milling frequencies between 5Hz and 25 Hz. The synthesis was carried out in a 25mL stainless steel milling jar, using a powder loading of 1g with one stainless steel milling ball.
Alternatively, an exothermic reaction investigating the use of calcium oxide as an alternative starting reactant was studied. Hydroxyapatite was synthesised using calcium oxide and diammonium phosphate under identical milling conditions defined above, according to the following reaction equation:
The formation of hydroxyapatite and reaction yield was analysed by powder x-ray diffraction (XRD) and Raman spectroscopy, which monitored the progression of hydroxyapatite with increasing reaction time and milling speed.
Scanning electron microscopy (SEM) was carried out to monitor a change in morphology and size of the milled powder. The change in morphology and size of the powder was analysed with respect to increasing reaction time and mill speed. Figure 1 below illustrates a change in particle size between the starting reactants calcium oxide and diammonium phosphate and the resulting milled powder containing calcium oxide and diammonium phosphate ball milled for 2 hours at a milling frequency of 20 Hz.
Physical Mixture of calcium oxide and diammonium phosphate prior to milling.
Ball milled powder mixture calcium oxide and diammonium phosphate milled for 2 hours at 20 Hz
Mechanochemical synthesis of hydroxyapatite involves the use of mechanical forces to induce a chemical change, as opposed to traditional methods which make use of solvents. The impaction forces present in the milling jar were estimated to determine the mechanical energy required for the mechanochemical reaction to occur. The impaction forces in the ball mill were estimated using Newtons Second Law of motion, which investigates the change in velocity or acceleration of an object, when subjected to a change in force. This may be applied to the ball milling process observed in the Retsch Mixer mill, in which the energy supplied is used to oscillate the milling jar in a lateral direction. This in turn transfers the milling ball in the mill jar, which promotes the mixing of powder. The impaction of the powder and milling ball occurs at the wall of the milling jar.
Newtons Second Law of Motion was applied to the milling process used in this study to estimate the impaction forces at the wall of the mill jar, which were calculated based on the mass and acceleration of the milling media in the mill jar, according to the following equation:
The acceleration of the milling ball was calculated using the milling speed, the dimensions of the milling jar and the pattern of the milling media inside the mill jars. The mass of the mill ball remained constant for the purpose of this study. The acceleration of the mill ball varied depending on the mill speed utilised. The size of the milling jar used also remained constant throughout the study, and therefore had no impact on the acceleration of the milling ball.
Mechanical activation, or the activation energy of the reaction, denotes the mechanical energy essential to initiate a chemical reaction. In the case of the ball milling process, mechanical energy supplied from the ball mill is absorbed and mechanical activation will occur. Both the reaction rate constant and activation energy of the system are reaction specific.
The activation energy required in the formation of hydroxyapatite was estimated using the Arrhenius equation defined below:
The reaction rates for the reactions specified previously were measured at a range of different temperatures in order to estimate the activation energy for the reaction.
The objectives of the study described aims to gain a better understanding for the mechanochemical process, in particular for mechanochemical reactions and mechanochemical synthesis of materials. The mechanical activation and mechanical energy necessary for initiating a mechanochemical reaction were estimated using the Arrhenius equation and Newtons Second Law of Motion. The reaction activation energy, enthalpy of reaction and impaction forces calculated characterise the mechanochemical process, and aid towards understanding the mechanochemical process.
1. Balá, P. et al. Hallmarks of mechanochemistry: from nanoparticles to technology. Chem. Soc. Rev. 42, 7571637 (2013).
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