Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

Nepheline (Na₆K₂Al₈Si₈O₃₂) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na₂O–CaO–Al₂O₃–SiO₂ in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by ²⁹Si, ²⁷Al, and ²³Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na₂O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO₂–poor glass‐ceramics with (Na₂O+CaO)/Al₂O₃ > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.     

Structure of cesium–borosilicate glasses according to NMR spectroscopy

The local structure of cesium-borosilicate glasses with various Cs₂O and B₂O₃ proportions and constant SiO₂ content (50 mol %) is studied with ¹¹B, ²⁹Si, and ¹³³Cs MAS NMR spectroscopy. The portions of three- and four-fold Si atoms are determined, as well as the concentrations of various silicate Q ⁿ units. Our experimental data are compared with the predictions of Dell’s model and thermodynamic modeling for sodium–borosilicate analogs of the glasses studied.     

Improved Prediction of Young's Modulus of Fluorine‐Containing Glasses Using MAS‐NMR Structural Data

The model developed by Makishima and Mackenzie (M–M) may yield reasonable estimates for the E‐modulus of a range of glasses. In the M–M model the bonding enthalpy and packing densities present in the compounds that form the glass are taken as input for the calculation. This study shows that a more accurate estimate can be obtained by incorporating in the model structural information from MAS‐NMR data. Specifically, we have determined by means of the impulse excitation technique (IET) the E‐modulus for ionomer glasses with composition 4.5SiO₂–3Al₂O₃–1.5P₂O₅–3MO–2MF₂, where M denotes the alkaline earth metal (M = Mg, Ca, Sr, or Ba). The MAS‐NMR structural analysis shows that substitution of calcium by barium or strontium results in a disrupted network, whereas magnesium leads to a more packed network. In this study we will show how a higher coordination state of the aluminum as determined by ²⁷Al MAS‐NMR can be taken into account in the model. This leads to rather small corrections of the estimates for these particular glasses. In contrast, the ¹⁹F MAS‐NMR study shows the presence of Al–F–M(n) or Al–F and Si–F–M(n) types of environment in the glass network. Al–F and Si–F bonds are not accounted for in the E‐modulus estimate by the M–M model. We will show how by incorporating the new bonding of F with Al and Si a significantly improved estimate of the E‐modulus is obtained compared with the original model.     

Elucidating the Effect of Iron Speciation (Fe2+/Fe3+) on Crystallization Kinetics of Sodium Aluminosilicate Glasses

We report on the influence of Fe₂O₃ on the crystallization kinetics of nepheline (Na₂O·Al₂O₃·2SiO₂)‐based sodium aluminosilicate glasses. A series of glasses with varying Al₂O₃/Fe₂O₃ content were synthesized in the system 25Na₂O–(25–x) Al₂O₃–xFe₂O₃–50SiO₂ (x varies between 0 and 5 mol%) through melt‐quench technique. A systematic set of experiments were performed to elucidate the influence of iron speciation (Fe²⁺/Fe³⁺) on the crystallization kinetics of these glasses including: (1) obtaining the details of nonisothermal crystallization kinetics by differential scanning calorimetry, (2) determining the influence of heat treatment on the structure and iron coordination in glasses by X‐ray photoelectron spectroscopy and wet chemistry, and (3) following the crystalline phase evolution in glasses in air and inert environments by X‐ray diffraction and scanning electron microscopy. The crystallization of two polymorphs of NaAlSiO₄—carnegieite (orthorhombic) and nepheline (hexagonal)—was observed in all the glasses, wherein the incorporation of iron promotes the formation of nepheline over carnegieite while shifting the crystallization mechanism from surface to volume. The influence of environment (air versus inert) and iron content on the crystallization kinetics of these glasses is contextualized from the perspective of the devitrification problem usually observed in sodium‐ and alumina‐rich high level nuclear waste glasses.     

Influence of Al2O3/SiO2 ratio in multicomponent LNAS glasses and glass-ceramics on the crystallization behavior,microstructure and mechanical performance

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al₂O₃ for SiO₂. The relationships between glass network structure and crystallization behavior of Li₂O–Na₂O–Al₂O₃–SiO₂ (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al₂O₃. LNAS glasses with the addition of Al₂O₃ from 16 to 18 mol% exhibited increasing Q⁴ (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al₂O₃ content increasing to 19–20 mol%, the formation of highly disordered (Li, Na)₃PO₄ phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al₂O₃ showed high transparency ～84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m^1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays.     

Cation field-strength effects on ion irradiation-induced mechanical property changes of borosilicate glass structures

We examine the impact of the glass network-modifier cation field strength (CFS) on ion irradiation-induced mechanical property changes in borosilicate (BS) glasses for the ternary M₂O–B₂O₃–SiO₂ systems with M = {Na, K, Rb} and the quaternary [0.5M₍₂₎O–0.5Na₂O]–B₂O₃–SiO₂ systems with M = {Li, Na, K, Rb Mg, Ca, Sr, Ba}. ¹¹B nuclear magnetic resonance (NMR) experiments on the as-prepared BS glasses yielded the fractional population of four-coordinated B species (B^[4]) out of all {B^[3], B^[4]} groups in the glass network, along with the fraction of B^[4]–O–Si linkages out of all B^[4]–O–Si/B bonds. Both parameters correlated linearly with the (average) CFS of the M⁺ and/or {M⁽²⁾⁺, Na⁺} cations. Both the nanoindentation-derived hardness and Young's modulus values of the glasses reduced upon their irradiation by Si²⁺ ions, with the property deterioration decreasing linearly with increasing M^z+ CFS, that is, for higher M^z+⋅⋅⋅O interaction strength. The irradiation damage of the glass network also increased linearly with the fraction of B^[4]–O–Si linkages, which are the second weakest in the structure after the M^z+⋅⋅⋅O bonds. Our results underscore the advantages of employing BS glasses with high-CFS cations for enhancing the radiation resistance for nuclear waste storage.     

Structural and Luminescence Studies on Barium Sodium Borosilicate Glasses Containing Uranium Oxides

Barium sodium borosilicate glasses containing different amounts of uranium oxides were prepared by conventional melt quench method and investigated for their structural aspects by ²⁹Si and ¹¹B MAS NMR technique combined with steady‐state luminescence and lifetime measurements. Based on MAS NMR studies, it is confirmed that uranium ions act as network modifier up to 15 wt% and beyond which a separate uranium containing phase is formed. From the luminescence studies, it is inferred that uranyl species is in a highly distorted environment. For more than 15 wt% uranium oxide incorporation, weaker U–O–U linkages are formed at the expense stronger U–O–Si/B linkages, as suggested by the excited state lifetime value of the uranyl species as well as red shift in emission peak maximum. For glass samples containing more than 25 wt% uranium oxides, crystalline barium uranium silicate gets phase separated from glass matrix as confirmed by XRD studies.     

Structural variations of bioactive glasses obtained by different synthesis routes

Two bioactive glasses with different chemical compositions (mol%) 46.2SiO₂–26.9CaO–24.3Na₂O–2.6P₂O₅ (45S5) and 40SiO₂–54CaO–6P₂O₅ (A2) were synthesized by the use of sol–gel and melt–quenching techniques. The effect of synthesis method on glass structure was investigated using X-ray diffraction, FTIR, Raman, XPS, ²⁹Si and ³¹P MAS–NMR spectroscopic methods. The results show that the synthesis route has significant influence on the glass structure. Both melt–derived A2 and 45S5 glasses exhibit fully amorphous structure, while gel–derived ones, stabilised at 700 °C, reveal the presence of crystalline silicate and phosphate phases. Gel–derived glasses exhibit more polymerized structure compared to melt–quenched ones. Phosphorus is present in the orthophosphate type environment (Q⁰) together with some pyrophosphate (Q¹) species and it does not take part in the formation of Si–O–P bonds. This indicates that phosphorus acts as a glass structure modifier and forms phosphate-rich phase separated from a silica-rich one. The theoretically predicted network connectivity is consistent with the experimental determination only for melt–derived glasses, assuming silicon as the only network former.     

Crystallisation of processed nepheline syenite–magnesite glasses

Abstract

Transparent glasses were prepared from processed nepheline syenite–magnesite mixtures. Incorporation of TiO₂ in the base glasses changes the glass colour from white to amber or dark brown. Translucent porcelainous glass ceramics with white, creamy and a variety of bluish colorations were obtained in glasses containing non-magnetic nepheline syenite. However, dark marblelike glass ceramics were developed in glasses containing middling and tailing nepheline syenite. Aluminium diopside [Ca(Mg,Al)(Si,Al)₂O₆], nepheline, forsterite, magnesium titanate MgTi₂O₅ and hematite were developed by heat treatment of these glasses. SEM micrographs tend to show fine and uniform bulk with increasing Fe₂O₃ contents in the parent glass ceramic samples, however addition of TiO₂ enhances nucleation and the microstructure becomes of evenly good uniform fine structure in the sample with lowest iron content.     

Synthesis and NMR Characterization of Titanium and Zirconium Oxides Incorporated in SBA-15

Single oxides of Ti and Zr incorporated SBA-15 were prepared and characterized by N₂ adsorption, NMR, and XPS techniques. ²⁹Si MAS NMR results suggest the formation of Si–O–X linkages (X: Ti or Zr) by an increase in the ratio of Q ³/Q ⁴ in the presence of Ti or Zr. XPS analysis of Ti–SBA-15 catalysts indicate the presence of Ti–O–Si bonds in addition to Ti–O–Ti and Si–O–Si bonds, supporting the NMR evidence.     

Crystallization of fibre-coating compounds of potential use in fibre-reinforced oxide ceramics

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, BaAl₂Si₂O₈ and lanthanum hexaluminate, LaAl₁₁O₁₈) were prepared by hybrid sol–gel synthesis and their thermal crystallisation was monitored by thermal analysis, X-ray diffraction and multinuclear solid state MAS NMR. Both the gels convert to the crystalline phase below about 1200°C, via amorphous intermediates in which the Al shows an NMR resonance at 36–38 ppm sometimes ascribed to Al in five-fold coordination. Additional information about the structural changes during thermal treatment was provided by ²⁹Si and ¹³⁷Ba MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about 500°C but the Ba is still moving into its polyhedral lattice sites about 400°C after the sluggish onset of crystallization. Lanthanum hexaluminate crystallises sharply at 1230°C via γ-Al₂O₃.     

Effect of Niobium Oxide on the Structure and Properties of Melt‐Derived Bioactive Glasses

The effects of adding Nb₂O₅ on the physical properties and glass structure of two glass series derived from the 45S5 Bioglass^® have been studied. The multinuclear ²⁹Si, ³¹P, and ²³Na solid‐state MAS NMR spectra of the glasses, Raman spectroscopy and the determination of some physical properties have generated insight into the structure of the glasses. The ²⁹Si MAS NMR spectra suggest that Nb⁵⁺ ions create cross‐links between several oxygen sites, breaking Si–O–Si bonds to form a range of polyhedra [Nb(OM)_6?y(OSi)_y], where 1 ≤ y ≤ 5 and M = Na, Ca, or P. The Raman spectra show that the Nb–O–P bonds would occur in the terminal sites. Adding Nb₂O₅ significantly increases the density and the stability against devitrification, as indicated by ΔT(T_x ? T_g). Bioglass particle dispersions prepared by incorporating up to 1.3 mol% Nb₂O₅ by replacing P₂O₅ or up to 1.0 mol% Nb₂O₅ by replacing SiO₂ in 45S5 Bioglass^® using deionized water or solutions buffered with HEPES showed a significant increase in the pH during the early steps of the reaction, compared using the rate and magnitude during the earliest stages of BG45S5 dissolution.     

Structure-property relations in crack-resistant alkaline-earth aluminoborosilicate glasses studied by solid state NMR

The effect of the average ionic potential ξ = Ze/r of the network modifier cations on crack initiation resistance (CR) and Young's modulus E has been measured for a series of alkaline-earth aluminoborosilicate glasses with the compositions 60SiO₂–10Al₂O₃–10B₂O₃–(20−x)M₍₂₎O–xM’O (0 ≤ x ≤ 20; M, M’ = Mg, Ca, Sr, Ba, Na). Systematic trends indicating an increase of CR with increasing ionic potential, ξ, have been correlated with structural properties deduced from the NMR interaction parameters in ²⁹Si, ²⁷Al, ²³Na, and ¹¹B solid state NMR. ²⁷Al NMR spectra indicate that the aluminum atoms in these glasses are essentially all four-coordinated, however, the average quadrupolar coupling constant <C_Q> extracted from lineshape analysis increases linearly with increasing average ion potential computed from the cation composition. A similar linear correlation is observed for the average ²⁹Si chemical shift, whereas the fraction of four-coordinate boron decreases linearly with increasing ξ. Altogether the results indicate that in pure alkaline-earth boroaluminosilicate glasses the crack resistance/E-modulus trade-off can be tailored by the alkaline-earth oxide inventory. In contrast, the situation looks more complicated in glasses containing both Na₂O and the alkaline-earth oxides MgO, CaO, SrO, and BaO. For 60SiO₂–10Al₂O₃–10B₂O₃–10MgO–10Na₂O glass, the NMR parameters, interpreted in the context of their correlations with ionic potentials, are consistent with a partial network former role of the MgO component, enhancing crack resistance. Altogether the presence of MgO in aluminoborosilicate glasses helps overcome the trade-off issue between high crack resistance and high elasticity modulus present in borosilicate glasses, thereby offering additional opportunities for the design of glasses that are both very rigid and very crack resistant.     

Effects of Al/Na and heat treatment on the structure and properties of glass ceramics from molten blast furnace slag

Glass ceramics with different Al/Na molar ratio from blast furnace slag were prepared using conventional melting-casting method. The structure and properties of glasses or glass ceramics were investigated by DSC, Raman, MAS NMR, XRD, and SEM. The DSC results indicated that the thermal stability (ΔT = T_c-T_g) and crystallization temperature (T_c) of the parent glass firstly increased and then decreased when Al/Na exceeded 1.21. The Raman and ²⁷Al MAS NMR spectra analysis revealed that [AlO₆] increased positively with Al₂O₃/Na₂O. The calculation of Qⁿ ([SiO₄] units with bridging oxygen atoms number of n) suggested an obvious decline of (Q⁰+Q²)/(Q¹+Q³) and that [SiO₄] mainly existed in the form of Q¹ when Al/Na exceeded 1.21, which accorded closely with T_c variation. The crystallization results determined by XRD showed that as Al/Na increased, the main crystal phase was transformed from akermanite to gehlenite and nepheline disappeared. Glass ceramics with Al/Na of 1.48 nucleated at 780 °C for 2 h and crystallized at 880 °C for 3 h exhibited the maximum value of flexural strength. Orthogonal experiment (L9(3⁴)) were carried out to investigated the optimum heat treatment of glass ceramics with a Al/Na of 1.48. The analyses indicated that nucleation time variation has little influence on the flexural strength, and the optimum heat treatment was determined as 760 °C – 1 h–900 °C – 1 h and the flexural strength was characterized as 81.310 MPa.     

The effects of SiO2 and K2O on glass forming ability and structure of CaOTiO2P2O5 glass system

The effects of SiO₂ and K₂O were investigated on the glass forming ability (GFA) and structural characteristics of CaOTiO₂P₂O₅ system. Differential thermal analyzer (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR and ³¹P magic angle spinning NMR methods were applied for characterizations of the system.Unwanted crystallization in the initial three components base glass composition was observed by adding SiO₂ and crystalline phases such as TiP₂O₇, rutile (TiO₂) and cristobalite (SiO₂) were formed in it.The results showed that K₂O prevents crystallization of glasses and promotes the formation of glass. FT-IR and X-ray diffraction showed that the addition of K₂O caused the formation of phosphate–silicate network as POSi, and formation of isolated droplet phases (rich of Si and P) separated from the phosphate matrix.The optimum amounts of SiO₂ and K₂O in phosphate structure were respectively 6 and 2 wt.%, 0 in accordance with glass forming ability (GFA) parameters. Despite addition of SiO₂ along with K₂O; the ³¹P MAS NMR and infrared spectrums of glasses show that no Q² sites were in the phosphate network. The Q¹ and the pyrophosphate groups was the predominant structural unit in these glasses.     

Assessment of interatomic parameters for the reproduction of borosilicate glass structures via DFT-GIPAW calculations

Borates and borosilicates are potential candidates for the design and development of glass formulations with important industrial and technological applications. A major challenge that retards the pace of development of borate/borosilicate based glasses using predictive modeling is the lack of reliable computational models to predict the structure-property relationships in these glasses over a wide compositional space. A major hindrance in this pursuit has been the complexity of boron-oxygen bonding due to which it has been difficult to develop adequate B–O interatomic potentials. In this article, we have evaluated the performance of three B–O interatomic potential models recently developed by Bauchy et al [J. Non-Cryst. Solids, 2018, 498, 294–304], Du et al [J. Am. Ceram. Soc. https://doi.org/10.1111/jace.16082 ] and Edèn et al [Phys. Chem. Chem. Phys., 2018, 20, 8192–8209] aiming to reproduce the short-to-medium range structures of sodium borosilicate glasses in the system 25 Na₂O x B₂O₃ (75 − x) SiO₂ (x = 0-75 mol%). To evaluate the different force fields, we have computed at the density functional theory level the NMR parameters of ¹¹B, ²³Na, and ²⁹Si of the models generated with the three potentials and the simulated MAS NMR spectra compared with the experimental counterparts. It was observed that the rigid ionic models proposed by Bauchy and Du can both reliably reproduce the partitioning between BO₃ and BO₄ species of the investigated glasses, along with the local environment around sodium in the glass structure. However, they do not accurately reproduce the second coordination sphere of silicon ions and the Si–O–T (T = Si, B) and B-O-T distribution angles in the investigated compositional space which strongly affect the NMR parameters and final spectral shape. On the other hand, the core-shell parameterization model proposed by Edén underestimates the fraction of BO₄ species of the glass with composition 25Na₂O 18.4B₂O₃ 56.6SiO₂ but can accurately reproduce the shape of the ¹¹B and ²⁹Si MAS-NMR spectra of the glasses investigations due to the narrower B–O–T and Si-O-T bond angle distributions. Finally, the effect of the number of boron atoms (also distinguishing the BO₃ and BO₄ units) in the second coordination sphere of the network former cations on the NMR parameters have been evaluated.     

Submixture model to predict nepheline precipitation in waste glasses

High-alumina high-level waste (HLW) glasses are prone to nepheline precipitation during canister-centerline cooling (CCC). If sufficient nepheline forms, the chemical durability of the glass will be significantly impacted. Overly conservative constraints have been developed and used to avoid the deleterious effects of nepheline formation in U.S. HLW glasses. The constraints used have been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A 90-glass study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. The CCC crystallinity data from these glasses were combined with 657 glasses found in the literature. The trends showed significant effects of Na₂O, Al₂O₃, SiO₂, B₂O₃, CaO, Li₂O, and potentially K₂O on the propensity for nepheline formation. A pseudo-ternary submixture model was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo-ternary with axes of SiO₂ + 1.98B₂O₃, Na₂O + 0.653Li₂O + 0.158CaO, and Al₂O₃ was found to divide glasses that precipitate nepheline during CCC from those that do not. Application of this constraint is anticipated to increase the loading of Hanford high-alumina HLWs in glass by roughly one-third.     

Effect of Al Speciation on the Structure of High‐Al Steels Mold Fluxes Containing Fluoride

To design suitable mold fluxes for the casting of high‐Al steels, the structure of mold fluxes based on CaO–SiO₂, CaO–SiO₂–Al₂O₃, and CaO–Al₂O₃ was examined by Raman spectroscopy and magic‐angle spinning nuclear magnetic resonance. The results showed that Si atoms are replaced by Al atoms as the network formers with the increase in Al₂O₃ in the mold fluxes. This converts the silicate slags (CaO–SiO₂ mold fluxes) into aluminosilicates slags (CaO–SiO₂–Al₂O₃ or CaO–Al₂O₃ mold fluxes). The F^? ions in the mold flux containing Al₂O₃ are classified into three categories, according to function: Bridging F's, Nonbridging F's, and Free‐F's. The Al³⁺ ion holds three distinct coordination environments: ^IVAl, ^VAl, and ^VIAl. The addition of F affects the coordination environment of Al³⁺ to form AlO₃F and AlO₂F₂ that accommodate the network structure of slags. The network structure in the CaO–SiO₂ mold fluxes is mainly connected through Si–O–Si linkage. However, the network structure of the mold fluxes containing elevated content of Al₂O₃ is mainly connected through Si–O–Si, Al–O–Al, Al–O–Si, and Al–F–Al linkages. Hence, the structural characteristics of high‐Al steels mold fluxes must be considered during the designing step of the mold fluxes.     

Thermal history driven molecular structure transitions in alumino‐borosilicate glass

The glass network structure governs various thermos‐physical properties such as viscosity, thermal, and electrical conductivities, and crystallization kinetics. We investigated the effect of temperature on structural changes in a Na₂O‐CaO‐Al₂O₃‐SiO₂‐B₂O₃ glass system using ²⁷Al MAS NMR spectroscopy. Around the glass transition temperature, most of aluminate structures exist as AlO₄, acting as a glass former. When the temperature is above the melt crystallization temperature, the AlO₄ structure is drastically decreased and glass structures are mainly composed of AlO₅ and AlO₆, acting as glass modifiers. Thermodynamic assessment based on Gibbs energy minimization was used to confirm the dependency of aluminate structure's amphoteric characteristic on temperature by calculating the site fraction of aluminate molecular structures at different temperatures. Temperature‐induced aluminate structural variation can also influence silicate and borate structural changes, which have been confirmed by the ²⁹Si and ¹¹B NMR spectra.     

Energetics and Structure of Polymer‐Derived Si–(B–)O–C Glasses: Effect of the Boron Content and Pyrolysis Temperature

The structure and properties of polymer‐derived Si–(B–)O–C glasses have been shown to be significantly influenced by the boron content and pyrolysis temperature. This work determined the impact of these two parameters on the thermodynamic stability of these glasses. High‐temperature oxide melt solution calorimetry was performed on a series of amorphous samples, with varying boron contents (0–7.7 at.%), obtained by pyrolysis of precursors made by a sol–gel technique. Thermodynamic analysis of the calorimetric results demonstrated that at a constant pyrolysis temperature, adding boron makes the materials energetically less stable. While the B‐containing glasses pyrolyzed at 1000°C were energetically less stable than the competitive crystalline components, increasing the pyrolysis temperature to 1200°C led to their enthalpic stability. ²⁹Si and ¹¹B MAS nuclear magnetic resonance (NMR) spectroscopy measurements on selected samples confirmed a decrease in the concentrations of mixed Si‐centered SO_iC_4?i and B‐centered BO_jC_3?j bonds at the expense of formation of SiO₄ and B(OSi)₃ species (indicating a tendency toward phase separation) when the boron content and pyrolysis temperature increased. In light of the findings from calorimetry and NMR spectroscopy, we propose a structure–energetic relationship in Si–(B–)O–C glasses.