📄 chemistry

The effect of boron content on the structure and dielectric properties of alkali-free boroaluminosilicate glass

This study systematically investigates the impact of varying B₂O₃ content (6–14 mol%) on the structure and properties of alkali-free boroaluminosilicate glass, revealing that a composition with 12 mol% B₂O₃ optimizes the network structure to achieve a minimum dielectric constant of 4.47 at 10 GHz and a thermal expansion coefficient of 4.17×10⁻⁶/°C, making it ideal for 5G/6G communication applications.

Original authors: Xian Wang, Jinlong Li, Weihong Zheng, Lincan Zhou, Ping Lu

Published 2026-08-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Xian Wang, Jinlong Li, Weihong Zheng, Lincan Zhou, Ping Lu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Modern life runs on invisible waves. From the smartphones in our pockets to the satellites guiding our travel, fifth and sixth-generation wireless networks are pushing data through the air at speeds that were once science fiction. But as these signals race toward us, they face a hidden obstacle: the materials that carry them. Inside the circuit boards and protective shells of our devices, glass fibers act as the highways for these signals. For the network to work efficiently, especially at the high frequencies used for future communication, these glass fibers must be exceptionally transparent to electricity. If the glass holds onto too much energy, it slows the signal down and turns that energy into heat, causing delays and dropped connections. Scientists have long known that adding boron, a common element found in glass, can make these materials better at letting signals pass. However, the exact amount needed to create the perfect balance between a strong, durable glass and one that is electrically invisible has remained a mystery.

A team of researchers at Wuhan University of Technology set out to solve this puzzle by creating a series of special glasses without any alkali metals, which are known to weaken electrical insulation. They focused on a specific type of glass made from silicon, aluminum, and boron, systematically changing the amount of boron in the mix while keeping everything else exactly the same. Their goal was to watch how the internal structure of the glass shifted and to see how those shifts affected its ability to handle heat and transmit high-frequency signals. By testing five different versions of this glass, ranging from low to high boron content, they mapped the relationship between the chemical recipe and the final performance, looking for the precise point where the material becomes ideal for next-generation communication.

The researchers began by melting raw powders of silicon, aluminum, and boron oxides in a platinum crucible at temperatures exceeding 1,500 degrees Celsius. To ensure the glass was pure and free of bubbles, they added a small amount of tin oxide and carefully controlled the cooling process to prevent internal stress. Once the glass cooled into solid blocks, they subjected it to a battery of tests. They used X-ray diffraction to confirm the material was truly glass and not a crystal, and they employed Raman spectroscopy and nuclear magnetic resonance to peer inside the atomic structure. These tools allowed them to count how many atoms were arranged in specific shapes, such as four-sided tetrahedrons versus flat triangles, which is crucial because the shape of these atomic clusters determines how the glass behaves.

As they increased the boron content from 6 percent to 14 percent, the internal architecture of the glass changed in a predictable but complex way. At lower levels, the boron atoms helped form a tight, three-dimensional network by connecting with silicon and aluminum atoms. However, as the boron increased, the available oxygen in the mixture became scarce. This shortage forced some of the boron and aluminum atoms to change their shape from stable, four-sided structures into looser, three-sided or six-sided forms. The researchers found that this structural shift was not a straight line; instead, there was a specific tipping point. When the boron content reached 12 percent, the glass achieved its most compact and stable arrangement, with the highest number of those strong, four-sided connections.

This structural peak translated directly into superior performance. The glass with 12 percent boron exhibited the lowest thermal expansion, meaning it would expand and contract the least when heated or cooled, a vital trait for preventing cracks in electronic devices. More importantly, this specific composition showed the lowest dielectric constant at 10 gigahertz, a frequency critical for millimeter-wave communication. In plain terms, this version of the glass allowed electrical signals to pass through with the least amount of delay and energy loss. The researchers also measured how the glass flowed when molten, finding that this 12 percent mixture maintained a viscosity that was high enough to be stable during manufacturing but low enough to be drawn into fine fibers without breaking.

The study revealed that simply adding more boron does not always lead to better results. Once the boron content exceeded 12 percent, the glass began to lose its structural tightness. The excess boron atoms could no longer form the strong four-sided shapes and instead reverted to flatter, less stable forms. This change made the glass slightly more prone to expanding with heat and slightly less efficient at transmitting signals. The team concluded that the optimal composition was not the one with the most boron, but the one where the boron and aluminum atoms worked in perfect harmony to create the densest possible network. This finding provides a clear blueprint for manufacturers aiming to produce the next generation of glass fibers, ensuring that the invisible highways of our future communication networks are as fast and reliable as possible.

Technical Summary: The Effect of Boron Content on the Structure and Dielectric Properties of Alkali-Free Boroaluminosilicate Glass

Problem Statement
The rapid advancement of 5G and 6G millimeter-wave communication technologies demands glass fiber substrates with low dielectric constants ( $\varepsilon_r$ ) and low dielectric loss ( $\tan\delta$ ) to minimize signal delay and energy dissipation. While alkali-free boroaluminosilicate glasses are promising candidates due to their insulation and durability, a critical challenge remains: optimizing the trade-off between processability (low melting and fiber-forming temperatures) and high-frequency dielectric performance. Previous research has not systematically clarified the independent role of $B_2O_3$ content on the network structure and high-frequency dielectric properties under fixed alkaline-earth oxide (RO) content, particularly regarding the complex "B-Al coordination competition" for free oxygen.

Methodology
This study systematically investigated the impact of varying $B_2O_3$ content (ranging from 6 to 14 mol%) on the structure and properties of alkali-free boroaluminosilicate glasses (SiO $_2$ -Al $_2$ O $_3$ -B $_2$ O $_3$ -RO). Five glass samples (BS1–BS5) were prepared where the $B_2O_3$ /SiO $_2$ ratio was systematically varied by increasing $B_2O_3$ at the expense of SiO $_2$ , while the contents of Al $_2$ O $_3$ , RO (CaO, MgO, SrO), and Y $_2$ O $_3$ were held constant. To minimize boron volatilization during melting, 0.6 mol% $Y_2O_3$ was introduced into the glass composition, where $Y^{3+}$ ions form stable yttrium borate species (e.g., YBO $_3$ ) to immobilize boron.

The study employed a multi-faceted characterization approach:

Structural Analysis: X-ray diffraction (XRD) confirmed the amorphous nature of the samples. Raman spectroscopy analyzed the evolution of siloxane ( $Q^n$ ) species. Magic-angle spinning nuclear magnetic resonance (MAS-NMR) of $^{11}$ B and $^{27}$ Al quantitatively determined the coordination states of boron ([BO $_3$ ] vs. [BO $_4$ ]) and aluminum ([AlO $_4$ ], [AlO $_5$ ], [AlO $_6$ ]).
Thermal Properties: Thermal dilatometry and Differential Scanning Calorimetry (DSC) measured the coefficient of thermal expansion (CTE), glass transition temperature ( $T_g$ ), crystallization peak ( $T_p$ ), and thermal stability ( $\Delta T$ ).
Dielectric Properties: Measurements were conducted at 1 MHz and 10 GHz using the cavity perturbation method to determine the dielectric constant and loss tangent.
Viscosity: High-temperature viscosity was measured between 1200–1600 °C and fitted to the Arrhenius equation to derive melting ( $T_m$ ) and fiber-forming ( $T_f$ ) temperatures.

Key Results
The study revealed a non-monotonic relationship between $B_2O_3$ content and the glass properties, driven by the competition for free oxygen between $B^{3+}$ and $Al^{3+}$ :

Network Structure Evolution: As $B_2O_3$ content increased from 6 to 12 mol%, the fraction of tetrahedral units ([BO $_4$ ] and [AlO $_4$ ]) increased, reaching a maximum at 12 mol% (Sample BS4). At 14 mol% (Sample BS5), free oxygen became insufficient, causing a conversion of [BO $_4$ ] to [BO $_3$ ] and [AlO $_4$ ] to higher-coordination [AlO $_5$ ]/[AlO $_6$ ] units. Raman spectra indicated an overall increase in network polymerization (higher $Q^3$ fraction) but a complex variation in $Q^4$ fractions due to this coordination competition.
Thermal Properties: The glass transition temperature ( $T_g$ ) decreased monotonically from 738 °C to 708 °C as the rigid [SiO $_4$ ] network was replaced by the less rigid borate network. The CTE exhibited a minimum value of $4.17 \times 10^{-6}$ /°C at 12 mol% $B_2O_3$ (BS4), corresponding to the peak in tetrahedral network compactness.
Dielectric Performance: At 10 GHz, the dielectric constant reached a minimum of 4.47 for Sample BS4 (12 mol% $B_2O_3$ ). This improvement is attributed to the lower polarizability of $B^{3+}$ compared to $Si^{4+}$ and the formation of a dense, highly connected 3D network via [BO $_4$ ] units. Dielectric loss followed a similar trend, minimized at BS4 due to the suppression of carrier migration in the dense network.
Viscosity and Processability: High-temperature viscosity generally decreased with increasing $B_2O_3$ content, attributed to the introduction of planar [BO $_3$ ] units and eutectic formation. However, the glass melting temperature ( $T_m$ ) and fiber-forming temperature ( $T_f$ ) displayed similar non-monotonic trends, with Sample BS4 exhibiting a relatively high viscous flow activation energy and moderate fiber-forming temperature ( $T_f \approx 1436$ °C), indicating a balance between fluidity and structural stability.

Significance and Claims
The paper claims to provide a theoretical and experimental basis for developing low-dielectric glass fibers suitable for 5G/6G communication. The primary contribution is the identification of an optimal composition (12 mol% $B_2O_3$ ) that achieves a superior balance of properties:

Low Dielectric Constant: 4.47 at 10 GHz.
Low CTE: $4.17 \times 10^{-6}$ /°C.
High Crystallization Resistance: Enhanced $\Delta T$ .
Moderate Processability: Suitable fiber-forming temperature.

The authors emphasize that the optimal performance is not achieved by simply maximizing $B_2O_3$ content, but by tuning the $B_2O_3$ /SiO $_2$ ratio to maximize the [BO $_4$ ]/[BO $_3$ ] and [AlO $_4$ ]/([AlO $_5$ ] + [AlO $_6$ ]) ratios. This specific composition (BS4) offers a dense network structure that minimizes polarization and thermal expansion while retaining sufficient viscosity for industrial fiber drawing, addressing the central challenge of balancing processability with high-frequency dielectric performance.

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