Exploring Structural Origin of Mixed-Alkali Effect on Thermal Conductivity in Borosilicate Glass

doi:10.16552/j.cnki.issn1001-1625.2025.1057

Abstract

Abstract:

Thermal transport in amorphous materials is primarily governed by two vibrational modes： propagons， which resemble lattice waves in crystals， and diffusons. However， the mechanism by which glass composition modulates these two modes remains insufficiently understood. To address this gap， this study investigated a series of borosilicate glasses with the composition 65.0SiO₂·5.0B₂O₃·7.5CaO·4.9MgO·xK₂O·（17.6-x）Na₂O （x=0~17.6）， to elucidate the impact of Na-K substitution on thermal conductivity and uncover its atomic-scale structural origins. Thermal conductivity measurements demonstrate that this glass series exhibits a typical mixed-alkali effect， characterized by a nonlinear decrease in thermal conductivity as potassium ions gradually replace sodium ions. To clarify the physical nature of this nonlinear behavior， the total thermal conductivity was decomposed into contributions from propagons and diffusons based on the semi-empirical model proposed by Agne et al. The decomposition results indicate that the contribution from propagons remains essentially constant throughout the substitution process. In stark contrast， the thermal transport contribution from diffusons exhibits a pronounced nonlinear decline that mirrors the trend of the total thermal conductivity. This confirms that the suppression of diffuson-mediated thermal transport is the dominant factor driving the mixed-alkali effect in this glass series. Detailed Raman spectroscopic characterization was employed to probe the structural evolution underpinning these thermal property changes. The spectral analysis reveals that the coexistence of dissimilar alkali ions （Na⁺ and K⁺） induces local structural strain due to ionic radius and field strength mismatch. This strain manifests as a variation in the distribution of Si—O—Si bond angles， evidenced by the shift of the characteristic Raman band near 1 090 cm^-1. This bond angle distortion enhances the short-range disorder of the glass network， thereby disrupting the vibrational synergy and reducing the energy transfer efficiency of the diffusons. In conclusion， this study not only deepens the fundamental understanding of thermal transport in amorphous solids but also provides a theoretical basis for designing glass materials with tailored thermal conductivities via composition engineering.

Key words: borosilicate glass, mixed-alkali effect, thermal conductivity, diffuson, propagon, structural disorder

CLC Number:

TQ171

ZHOU Qunfeng, ZHOU Hemin, ZHU Yongchang, CUI Zhu, QIAO Ang, TAO Haizheng. Exploring Structural Origin of Mixed-Alkali Effect on Thermal Conductivity in Borosilicate Glass[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(3): 787-793.

Figures/Tables 4

Fig.1 Calorimetric curves， Tg， and CTE for Na-K substitution borosilicate glass series as a function of R

Fig.2 κ， κdiff， and κprop as a function of R for Na-K substitution borosilicate glass series

Table 1 ρ， vL， vT， vs， κ， and κdiff of Na-K substitution borosilicate glass series at 25 ℃

R	ρ/（g·cm^-3）	v_L/（m·s^-1）	v_T/（m·s^-1）	v_s/（m·s^-1）	κ/（W·m^-1·K^-1）	κ_diff/（W·m^-1·K^-1）
0	2.530	6 240	3 430	4 367	0.919	0.689
0.2	2.522	6 230	3 420	4 357	0.912	0.676
0.4	2.517	6 160	3 390	4 313	0.898	0.660
0.5	2.514	6 130	3 370	4 290	0.883	0.653
0.6	2.511	6 100	3 340	4 260	0.874	0.644
0.8	2.501	5 990	3 290	4 190	0.843	0.627
1.0	2.475	5 780	3 180	4 047	0.812	0.600

Fig.3 Raman spectra and variation of characteristic peak position （ν） near 1 090 cm-1 as a function of R for Na-K substitution borosilicate glass series

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