Electrodiffusion versus Chemical Diffusion in Alkali Calcium Phosphate Glasses: Implication of Structural Changes

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• 作者：Anneli Hein ; Johannes Martin ; Martin Schäfer ; Karl-Michael Weitzel
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：February 16, 2017
• 年：2017
• 卷：121
• 期：6
• 页码：3203-3211
• 全文大小：398K
• ISSN：1932-7455

文摘

A long-term transport experiment has been performed on a bioactive calcium phosphate glass of the molar composition 30CaO*25Na₂O*45P₂O₅ using the technique of bombardment induced ion transport (BIIT) with potassium as foreign bombarder ion. Ion transport due to gradients of the electrical potential and the concentration lead to incorporation of K⁺ and depletion of both Na⁺ and Ca⁺⁺ by electrodiffusion in the forward direction. The resulting concentration profiles have been quantitatively analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The concentration profiles of the P⁺ and PO_x⁺ signals (x = 1–4) resemble those of the K⁺, Na⁺, and Ca⁺⁺ signals, indicating a characteristic change of the local bonding situation. This is interpreted as an indirect hint of a change of local structure of the glass network. Because the concentration profiles imprinted by the BIIT constitute pronounced concentration gradients, these depletion profiles further evolve on a much longer time scale due to chemical diffusion (absence of electric potential gradients). The former depletion zone is partially refilled by chemical diffusion. At the same time, the structural changes of the glass network are demonstrated to be reversible. Numerical simulations on the basis of the coupled Nernst–Planck–Poisson equations allow one to derive the diffusion coefficients of sodium, potassium, and calcium for both cases, that is, electrodiffusion and chemical diffusion. The two experiments are sensitive to different aspects of the diffusion coefficients and thus are complementary. The analysis is sensitive to the concentration dependence of D(Na⁺) and D(Ca⁺⁺) for the electrodiffusion and of D(K⁺) for the chemical diffusion. For the chemical diffusion of Na⁺ and Ca⁺⁺ in the backward direction, D(Ca⁺⁺) is larger than D(Na⁺), indicating that the extra sites occupied by Ca⁺⁺ in the preceding electrodiffusion are energetically high-lying.