Temperature-dependent measurements of the X-ray structure factor of molten Na2B4O7 reveal a continuous structural transition. We demonstrate that the thermodynamic model of ideal associated solutions is capable of predicting this evolution of melt structure, between a low density, depolymerized melt at ≳300 K above the liquidus, toward a dense, polymerized melt close to the glass transition. This temperature-dependent nature of melt structure is predicted to be strongly composition-dependent, with the B–O coordination number depending on temperature only in the range 20–50 mol % Na2O, which appears to be manifest in the broad maximum observed in the glass-transition temperatures. We discuss the ramifications of these findings for the application of topological constraint theory, with relevance to industrial glass design and manufacture, crystal growth from melts of nonlinear optical materials, geochemistry, and the understanding of melt fragility and the glass transition.

The triboelectric effect, charge transfer during sliding, is well established but the thermodynamic driver is not well understood. We hypothesize here that flexoelectric potential differences induced by inhomogeneous strains at nanoscale asperities drive tribocharge separation. Modelling single asperity elastic contacts suggests that nanoscale flexoelectric potential differences of ±1-10 V or larger arise during indentation and pull-off. This hypothesis agrees with several experimental observations, including bipolar charging during stick-slip, inhomogeneous tribocharge patterns, charging between similar materials, and surface charge density measurements.

Using high energy x-ray diffraction, the structure factors of glassy and molten B2O3 were measured with high signal-to-noise, up to a temperature of T  =  1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B3O6] boroxol rings, which are abundant in the glass, whilst the high-T (>~1500 K) liquid can be more closely described as a random network structure based on [BO3] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B2O3, for which the temperature evolution of the 808 cm−1 Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence. Our conclusions are supported by both first-principles and polarizable ion model molecular dynamics simulations which are capable of giving good account of the experimental data, so long as steps are taken to ensure a ring fraction similar to that expected from Raman spectroscopy. The mean thermal expansion of the B-O bond has been measured directly to be αBO  =  3.7(2)  ×  10−6 K−1, which accounts for a few percent of the bulk expansion just above the glass transition temperature, but accounts for greater than one third of the bulk expansion at temperatures in excess of 1673 K.

Borate melts containing <20 mol% Na 2 O have been studied using high-energy synchrotron X-ray diffraction. Temperature dependencies of the mean B-O bond lengths are shown to vary strongly with soda content, by comparison to previous measurements on liquid B 2 O 3 and Na 2 B 4 O 7 . Whereas in liquid B 2 O 3 linear thermal expansion of the BØ 3 units is observed, with coefficient a BO = 3.7(2) 9 10 À6 K À1 , this expansion is apparently slightly suppressed in melts containing <20 mol% Na 2 O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B-O coordination number, where the reaction BØ 4 À + BØ 3 ⇌ BØ 3 + BØ 2 O À shifts to the right with increasing temperature. The empirical bond-valence relationship is used to convert measured bond lengths, r BO , to coordination numbers, n BO , including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of n BO from peak areas in the radial distribution functions. Gradients of Dn BO / DT = À3.4(3) 9 10 À4 K À1 close to the diborate composition, and Dn BO / DT = À0.3(1) 9 10 À4 K À1 for a 13(3) mol% Na 2 O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.