簡單化學熱分解反應之主導曲線初步研究

Abstract

主導曲線模型(MCM)是一個可以準確預測各種動力學反應變化的全新模型－只要由實際的實驗數據得到其反應的主導曲線與一個最佳視活化能值，即可用來預測此種動力學反應變化的過程。相較於一般反應動力學必須先得知反應活化能Q、反應級數n、以及碰撞頻率K0等三個參數，再代入假設的反應數學式才能描述其反應的變化過程要簡單且客觀得多。 本研究團隊已證實主導曲線模型(MCM)可以用到複雜如陶瓷燒結的反應，不僅能準確預測微米及次微米級粉末的常壓燒結緻密化過程，更可應用在奈米陶瓷燒結緻密化過程之描述。本研究工作則進一步驗證主導曲線模型(MCM)是否也能準確預測簡單化學熱分解反應之反應過程。 本研究所選用的材料為碳酸鈣、硫酸銅、硫酸鈣以及碳酸鈉等四種粉末，每種粉末均以幾組不同升溫速率經由熱重分析儀TGA進行熱分解反應。其中碳酸鈣、硫酸銅及硫酸鈣三種粉末，在未經過研磨過篩之情況下進行之實驗分析，發現無法擬合出理想的主導曲線，但經過研磨過篩的粉末則可以得到較佳的主導曲線。顯示出未研磨之較大顆粒粉體可能會出現表面與內部反應不均一的問題。此外由於碳酸鈉之熱分解反應溫度太高，本文只探討其低溫之脫附水反應；由於脫附作用不是一個穩定的化學動力學反應，因此如預期地無法用主導曲線模型來分析與預測。 本研究之初步結果，證實主導曲線模型(MCM)的確可以找出碳酸鈣、硫酸銅與硫酸鈣等粉末的熱分解反應之溫度、時間與反應變化百分比的簡單關係，並準確預測簡單熱分解反應之反應過程。但是預測的準確性至少會受到兩個因素的影響：一個是前述的粉體顆粒的粒徑，另一個則是升溫速率。一般而言，較快的升溫速率會得到較大的視活化能，因此在使用主導曲線模型來分析預測熱分解反應時，應考慮升溫速率的影響，才不會造成太大的誤差。

Master Curve Model (MCM) is probably a universal kinetic model, and can be used to predict the variations of various kinetic reactions as long as we can derive the master curves and best apparent activation energy from experimental data. Unlike general chemical kinetic model, which needs all three parameters, i.e. activation energy Q, reaction order n, and collision frequency Ko, to construct the model, MCM is a much simpler and more objective method. Our previous work has shown that MCM can be used to describe and predict complicated ceramic sintering reactions, not only for micron-or submicron powders, but also for nanocrystalline ceramic powders. The purpose of this research is to test the applicability of MCM on some simple chemical thermal decomposition reactions. Four powders, including CaCO3, CuSO4·5H2O, CaSO4·2H2O, and Na2CO3, had been thermally decomposed at various heating rate by thermo gravimetric analysis (TGA). Without grinding and sieving process, no acceptable master curves can be derived from the powders, but after grinding and sieving, three powders (except Na2CO3) gave much better master curves. It indicates that the reaction rates on the surface of the particles are different from that inside the bulk. Below 600oC, Na2CO3 only showed physical desorption reaction, which is not a stable chemical kinetic reaction, therefore no kinetic model is expected to be able to adequately describe the reaction. With some limitations, the preliminary results show that MCM indeed can be used to interpret and predict the thermal decomposition reactions of simple compounds. At least two factors will influence the accuracy of predictions of MCM, i.e. the particle sizes as we have mentioned earlier and the heating rates. In general, a faster heating rate gives a larger apparent activation energy. Therefore, heating rate is a must considered factor when using MCM on the analysis of thermal decomposition reactions.