硼酸盐热力学基元贡献模型及在MgBO2(OH)纳米晶须水热合成过程分析中应用

以MgCl₂·6H₂O、H₃BO₃、NaOH为原料,控制摩尔比Mg:B:Na=2:3:4,经室温共沉淀及水热转化(220～240℃,6.0～30.0 h)制得了单斜相MgBO₂(OH)纳米晶须。借鉴机械混合模型、基团贡献法思想,根据质量守恒原则将硼酸镁盐按照复盐分子式进行基元拆分,通过引入校正因子k,利用复盐基元的基础热力学数据拟合建立了Δ_fH_m及Δ_fG_m等硼酸盐热力学基础数据估算的基元贡献模型,估算了共沉淀产物Mg₇B₄O₁₃·7H₂O及水热产物MgBO₂(OH)不同温度下的Δ_fH_m、Δ_fG_m热力学数据,得到了25～100℃下共沉淀反应及25～250℃下水热转化的Δ_rH_m、Δ_rG_m。结果显示,共沉淀反应在室温条件下可自发进行,水热转化须借助一定温度条件方能自发实现,且反应趋势随水热温度升高时间延长而增大。该热力学基元贡献模型有望为其他结构复杂复盐化合物纳米结构的湿化学法控制合成分析及预测提供借鉴。

Subunit contribution model for thermodynamic properties of borates and its application in hydrothermai synthesis of MgBO2(OH) nanowhiskers

Pure monoclinic MgBO₂(OH) nanowhiskers were synthesized by room temperature co-precipitation followed by hydrothermal treatment at 220—240℃ for 6.0—30.0 h, using MgCl₂·6H₂O, H₃BO₃, and NaOH as raw materials with molar ratio of Mg:B:Na=2:3:4.Enlightened by mechanical mixture model and group contribution method, a novel subunit contribution model for the assessment of thermodynamic properties such as Δ_fH_m and Δ_fG_m of borates was developed, by introducing correction factor k and reasonably decomposing the complex double salt metal borates into constitutional subunits according to mass conservation.The as-developed model was utilized to assess Δ_fH_m and Δ_fG_m at various temperatures for precipitate Mg₇B₄O₁₃·7H₂O and hydrothermal product MgBO₂(OH), based on which the values of Δ_rH_m and Δ_rG_m of the co-precipitation of Mg₇B₄O₁₃·7H₂O and hydrothermal conversion to MgBO₂(OH) were obtained.The results show that, the room temperature co-precipitation can occur spontaneously whereas the hydrothermal conversion can only be performed spontaneously at high temperature. The feasibility for the co-precipitation as well as the hydrothermal conversion turns to be higher with the increase in the temperature and time.The subunit contribution model is beneficial for the analysis and prediction of controllable synthesis of other complex double salt nanostructures via the soft-chemistry based method.