Dielectric and thermal properties of aluminoborosilicate glasses doped with mixed rare-earth oxides

Highlights

• Glasses have the most compact structure with the same amount of La₂O₃ and CeO₂

• There dips in dielectric constant and loss at the most compact structure

• Thermal stability changes little with more La₂O₃ replacing CeO₂

Abstract

This study discusses aluminoborosilicate glasses with La₂O₃ and CeO₂. Structural, thermal, and dielectric properties were investigated by FTIR, DSC, and impedance analyzer. Results indicate that more Q³ and Q⁴ would form at the sample with the same amount of La₂O₃ and CeO₂, meaning the most compact structure, while more La₂O₃ would lead to a loosen network. Meanwhile, glasses obtained the best dielectric properties here, and the dips in dielectric constant and loss are ascribed to the structural changes in the glass network. The glass transition temperature, T_g ascends first and then declines, due to the variation of bond strength and non-bridge oxygen content, as well as changes in field strength. Parameters for thermal stability decrease firstly and then increase slightly, indicating small changes in thermal stability. The glass density gradually increases, which is mainly from the small field strength of La³⁺ ions and the larger molar mass of La₂O₃.

Introduction

The fast pace of 5G communication indicates a new level of high frequency and speed in communication technology for electronic devices, which also sets superior requirements for all communication equipment [1, 2]. The signal propagation loss originates from dielectric constant and dielectric loss showed in the following equation [3, 4]:

$$\mathit{\alpha }\propto \mathit{f}·\mathit{tan}\mathbf{\delta }·\sqrt{{\mathit{\varepsilon }}_{\mathit{r}}}$$

in which α means the signal propagation loss, f is the frequency, tanδ refers to dielectric loss, and ε_r is the dielectric constant. Increments in the dielectric constant and loss cause augment in signal loss [5], in which case it is necessary to improve the dielectric properties for all devices.

The dielectric properties for communication devices count mainly on the printed circuit board (PCB), whose properties in the dielectric area rely on the copper-clad plates (CCL). While the CCL plates are made from glass fibers and resin with the former largely made of alkali-free aluminoborosilicate glasses, it is best to optimize the dielectric properties for these glasses [6], [7], [8].

There are several pieces of research about improving the dielectric properties for aluminoborosilicate glasses. Results from dielectric measurements depicted the declines in both dielectric constant and dielectric loss. Zhang et al [9] studied influences from the ratios between Ca and Mg in aluminoborosilicate glasses, with results showing that when Ca was replaced by Mg, the glass network structure turned more compact, resulting in lower dielectric constant and loss. Du et al [10] also found the introduction of MgO in calcium aluminoborosilicate glasses optimized the properties in the dielectric field, which was due to the compact network structure in glasses.

Besides the above studies, rare earth elements possess high field strength and can be introduced into the glass network, filling the gaps, and acting as network modifiers [11, 12]. They can also result in free oxygen ions to make up the ‘open structure’ in glasses. Based on research from Liu et al [13], the lower dielectric constant and loss occurred when the ratio of Ca/La reached 44.83 and 1.86. Changes in these two properties followed the same trend with increasing firstly and then declining, where the increment in dielectric constant and loss is owing to the high polarization ability of rare-earth ions. Ni et al [14] studied the impact of Sm₂O₃ on the structure of aluminoborosilicate glasses. The Sm³⁺ ions are network modifiers and Sm₂O₃ provided free oxygens to promote the transformation of [BO₃] to [BO₄], leading the glass structure to tighter, which will be beneficial in reducing dielectric constant and loss. Similar results also occurred to Jia et al [15], where the Sm³⁺ ions resulted in a compact structure, enhancing the chemical and dielectric properties.

In our previous studies, effects from Lanthanum, Cerium, and Yttrium on dielectric and thermal properties were discussed. Zhang [16] proposed that dielectric properties of aluminoborosilicate glasses were enhanced with more La₂O₃. Lu [17] discussed the effects of La₂O₃ and CeO₂ when they are introduced separately. Glasses with La³⁺ possess a peak in the degree of polymerization (DOP), while the DOP increased continuously with more Cerium. The dielectric constant and loss depicted minima with 0.5% Lanthanum while they showed the opposite trend with the introduction of Cerium oxide. Zhang et al [18]studied changes of glasses from doping Yttrium, the results showing that the structure turned compact and then loose as the concentration of Y₂O₃ increased from 0 to 3.0 mol%, in which case, the dielectric constant and loss reached minima at 1.5 mol% and 2.0 mol% of Yttrium. On the other hand, several kinds of research focused on a mixture of two or more rare earth elements. Wang et al [19] studied the introduction of Gd₂O₃ and Y₂O₃ together, and found that the co-doping of Y₂O₃ and Gd₂O₃ rose the fraction of NBO (the non-bridging oxygen) and led to a decrease in connectivity number. However, the depolymerization effect of co-doping on silicate glasses is less than that from the separate introduction of Gd and Y ions. Zhao et al [20]found a kind of phenomenon that is similar to the mixed alkali effect in dielectric constant and loss when introducing Y₂O₃ and La₂O₃ into the glass network.

In the field of electronic glass fiber, we will not only consider the dielectric properties but also effects from other properties. Thermal stability is also of significance for glass melts, which is, for example, defined by ΔT=T_p - T_g. Herein, T_p refers to the crystalline peak for the exothermal peak from the DSC results. The larger the ΔT is, the more difficult the crystals will form in the melts during the cooling, namely the more stable the melt will be. As several kinds of rare earth elements influenced the thermal properties of aluminoborosilicate glasses with Lanthanum, Cerium, and Yttrium alone [16], [17], [18]. To the best of our knowledge, there is no study on effects from introducing both CeO₂ and La₂O₃ into aluminoborosilicate glasses. This paper studies the effect of substitution of CeO₂ by La₂O₃ on the structure, density, and dielectric properties of the glass.