Linnow Kirsten, Zeunert Anke, Steiger Michael
Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany.
Anal Chem. 2006 Jul 1;78(13):4683-9. doi: 10.1021/ac0603936.
Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. To investigate such deleterious processes, appropriate in situ techniques are required. This paper describes the use of X-ray diffractometry under controlled conditions of temperature and relative humidity (RH-XRD) for the direct observation of phase transition reactions in a porous substrate. An improved environmental chamber without temperature gradients is presented and applied to the investigation of phase transformations in the system Na2SO4 + H2O. This salt is generally considered as particularly damaging and frequently used in accelerated weathering tests. It is demonstrated that RH-XRD can be successfully applied for the direct observation of several relevant phase transitions in glass frits used as porous substrates. The conversion of Na2SO4(III) to Na2SO4(V) and the hydration of Na2SO4(V) both proceed fairly rapidly as true solid-state reactions without deliquescence of the educt phases. In contrast, crystallization from solution is kinetically hindered as there is a strong tendency of aqueous Na2SO4 to form supersaturated solutions also in narrow pores. The important implications of this behavior of the salt are also briefly discussed in the paper.
在受限空间中生长的晶体能够产生应力，并且是多孔材料损坏的主要原因。为了研究此类有害过程，需要合适的原位技术。本文描述了在温度和相对湿度可控的条件下使用X射线衍射法（RH-XRD）直接观察多孔基质中的相变反应。介绍了一种改进的无温度梯度的环境舱，并将其应用于研究Na2SO4 + H2O体系中的相变。这种盐通常被认为具有特别大的破坏性，并且经常用于加速老化试验。结果表明，RH-XRD能够成功地用于直接观察用作多孔基质的玻璃料中的几种相关相变。Na2SO4(III)向Na2SO4(V)的转变以及Na2SO4(V)的水合作用均作为真正的固态反应相当迅速地进行，且反应物相不会潮解。相比之下，从溶液中结晶在动力学上受到阻碍，因为在狭窄孔隙中，Na2SO4水溶液也有形成过饱和溶液的强烈趋势。本文还简要讨论了这种盐的这种行为的重要影响。
