碱铝磷酸盐玻璃的近程结构及化学稳定性

Short-Range Structure and Chemical Stability of Alkaline Aluminum Phosphate Glass

为了揭示碱金属、玻璃结构及化学稳定性之间的内在联系,以及对未来核废料基质玻璃的发展及化学稳定性的评估,本工作通过熔融淬冷法制备了一系列不同碱金属组成的铝磷酸盐玻璃,其组成为[x Na_(2)O–y K_(2)O–(41.6–x–y)Cs_(2)O]–16.7Al_(2)O_(3)–41.7P_(2)O_(5)。采用产品一致性测试B方法研究了这些玻璃的化学稳定性,并使用先进的固体核磁共振技术对玻璃的原子尺度结构进行了精确解析。结果表明,碱金属种类的改变不仅通过调节对非桥氧的电场吸引力影响了玻璃网络的致密性,而且明显改变了玻璃的网络结构。在这些玻璃中,不同配位数的铝都只被磷氧四面体(PO_(4))所包围。随着碱金属离子场强的增大,高配位铝的含量增加,导致铝与磷氧四面体的化学键连接增多。这种结构变化进一步增强了玻璃网络的致密性,最终提高了其化学稳定性。

Introduction The chemical stability of phosphate glass is vital for its applications in various fields,i.e.,biomedicine,laser gain media,sealing,and high-level nuclear waste solidification.Alkali metals affect the chemical stability of phosphate glass.Some studies indicate that alkali metals with a higher ionic field strength can enhance the chemical stability of glass via reinforcing the ionic crosslinking between alkali ions and non-bridging oxygen atoms.However,a few studies indicate that certain phosphate glasses containing low-field strength alkali metals exhibit a better chemical stability,compared to the glasses containing high-field strength alkali metals.These results cannot be explained based on the simple field strength.It is thus necessary to further investigate the influence of alkali metals on the chemical stability of phosphate glass.In this study,we utilized various advanced solid-state nuclear magnetic resonance(SSNMR)techniques to analyze the network structure of the aluminum phosphate glasses containing different alkali metals in an atomic scale.The intrinsic correlation among alkali metals,glass structure,and chemical stability was discussed.Methods The glasses with molar compositions of[x Na_(2)O–y K_(2)O–(41.6–x–y)Cs_2O]–16.7Al_(2)O_(3)–41.7P_(2)O_(5)were synthesized by melt quenching methods.The values of(x,y)used were(41.6,0),(0,41.6),(20.8,0),and(0,20.8),respectively.The raw materials required for glass preparation were Al(OH)_(3)(purity>99%),Al(PO_3)_(3)(purity>99%),NaPO_(3)(purity>95.0%),KPO_(3)(purity>99%,),Cs_2CO_(3)(purity>99%),and NH_(4)H_(2)PO_(4)(purity>99%).Each batch,consisting of 20 g of mixed raw materials,was loaded into a platinum crucible and subjected to a high-temperature furnace at 1250–1300℃for 30 min to melt raw materials.Thereafter,the melt was poured onto a steel plate and quickly pressed to obtain a bulk glass.The chemical stability of these glasses was assessed by the Product Consistency Test B(PCT-B)method.1 g of dried glass powder with particle sizes of 75–150μm was mixed with 10 mL of neutral deionized water in a sealed polytetrafluoroethylene(PTFE)container.The container was then placed in a constant temperature environment at 90℃for 7 d.After the corrosion experiments,the concentrations of leached elements in the leachates were measured by inductively coupled plasma-atomic emission spectroscopy(ICP-AES),thus calculating the normalized mass loss(NL)of each element within the different glasses.All the SSNMR measurements were performed on a model Bruker AvanceⅢHD 500 MHz spectrometer in a magnetic field of 11.7 T at room temperature.The measurements included^(27)Al magic-angel spinning(MAS),^(31)P MAS,^(27)Al{^(31)P}rotational echo double resonance(REDOR),^(31)P{^(27)Al}rotational echo adiabatic passage double resonance(REAPDOR),and^(31)P 1D refocused INADEQUATE.The Raman spectra were determined by a model Renishaw inVia spectrometer,with a laser wavelength of 488 nm.Results and discussion The NL of phosphorus(P)in the glasses containing alkali metals Na,K,and Na/Cs are 0.7,15.2,and 21.1 g/m^(2),respectively.The glass containing K/Cs exhibits a complete corrosion without detectable leachate obtained.These findings indicate that the chemical stability of the aluminum phosphate glasses with varying alkali metal compositions follows a decreased order of Na,K,Na/Cs,K/Cs.The P units within the different glasses are predominantly Q^(1)units(Q^(n),“n”represents the number of P–O–P bonds per P unit),indicating a consistent proportion of P—O—P bonds in these glasses.The variation in alkali metals does not affect the P—O—P bonds within the glasses.However,the variation in alkali metals significantly affects the Al species within the glasses.Specifically,the glasses containing alkali metals with a high ionic field strength exhibit a higher proportion of high-coordinated Al.In these glasses,the Al species with different coordinations are exclusively surrounded by phosphorus oxygen tetrahedra(PO_(4)),indicating that all Al species exist solely in the form of Al—O—P bonds.The compactness of the glass network increases with the proportion of Al—O—P bonds.The compact glass network can impede the dissolution and leaching of glass elements,as well as the diffusion of hydrogen species from solution into glass,thereby enhancing the chemical stability of the glass.The compactness of the glass network can be characterized by the reciprocal of the molar volume(1/V_(mol)).A higher value of 1/V_(mol)indicates a greater number of atoms per unit volume,corresponding to a higher compactness of the glass network.The values of 1/V_(mol)(×10^(4)mol/L)in these glasses containing alkali metals Na,K,Na/Cs,and K/Cs are 2.6,2.2,2.1,and 2.0,respectively.Correspondingly,the proportions of Al—O—P bonds are 54.5%,52.6%,50.7%,and 50.5%,respectively.A decreasing order of Na,K,Na/Cs,K/Cs is consistent with a decreasing order of the chemical stability.These results provide an evidence that the proportion of Al—O—P bonds increases as the ionic field strength of alkali metal increased,resulting in an enhanced compactness of the glass network and improved chemical stability.Furthermore,it is noteworthy that the proportion of Al–O–P bonds in the glass containing Na/Cs is comparable to that in the glass containing K/Cs.The former demonstrates a denser glass network.This can be attributed to the smaller ionic radius and higher ionic field strength of Na,compared to K.The smaller ionic radius results in narrower structural gaps within the glass network,while the higher ionic field strength enhances its electric field attraction to non-bridging oxygen.Conclusions The chemical stability of the aluminum phosphate glasses containing different alkali metals decreased in an order of Na,K,Na/Cs,K/Cs.The chemical stability increased as the ionic field strength of alkali metals increased.The change in alkali metals affected the compactness of the glass network via modulating the electric field attraction to non-bridging oxygen,and substantially modified the network structure of the glass.In these glasses,the Al species with varying coordination numbers were exclusively surrounded by phosphorus oxygen tetrahedra(PO_(4)).The proportion of high-coordinated Al increased with increasing the ionic field strength of alkali metals,resulting in an augmentation of the chemical bond connections between Al and PO_(4).This structural adjustment further enhanced the compactness of the glass network,ultimately improving its chemical stability.