Customized structural and mechanical characteristics of Eu³⁺ imbued boro-telluro-dolomite glasses: effect of Dy³⁺ co-doping

Abstract

Inorganic oxide glasses with high mechanical strength and durability became demanding. Thus, a new type of Eu³⁺/Dy³⁺ co-doped boro-telluro-dolomite glass system with customized structures and mechanical properties were prepared by the melt-quenching method and characterized. The densities, FTIR and Raman spectra of the glasses revealed a significant modification in the network structure with the increase of Dy³⁺ contents. The values of microhardness and elastic moduli of the glasses were increased with the increase of co-doping contents. In addition, a correlation between the glass hardness and applied load was established. The proposed glasses were shown to withstand about 300 g applied force without any distortion, confirming their usefulness to design mechanically stable screen shield and high strength surfaces.

KEYWORDS:

• Boro–telluro–dolomite glasses
• co-doping
• network structures
• mechanical properties
• microhardness

1. Introduction

Nowadays, distinct class of oxide glasses obtained by blending various natural minerals with usual synthetic glass formers has drawn much attention [1–3]. The ever-growing interest on these glasses is mainly due their enhanced chemical and thermal stability, high rare-earth ions (REIs) solubility, and moderate mechanical strength [4,5]. It was reported that any glass host incorporated with natural minerals by virtue of their excellent hardness, high Young’s modulus, high fracture toughness together with high resistance against scratches and sharp contact damage find countless applications in the protective covers of electronic displays such as smartphones, laptops, tablets, and wearable devices [6]. Ideally, the cover glasses are the outermost layer of the electronic displays and remain the most vulnerable part of these wearable devices under external force. Extreme hardness and higher Young’s modulus can reduce the glass thickness while retaining sufficient durability [7]. Despite the promises of this kind of glass system, nevertheless the selection of abundant minerals that can suitably be amalgamated into the existing synthetic based glass formers to form a new class of glass matrix for the aforementioned usage remains challenging.

Repeated studies showed that the structures, mechanical and optical properties of these glasses can appreciably be improved by europium (Eu³⁺) and dysprosium (Dy³⁺) ions co-doping. Besides, efforts have been made to incorporate the dolomite mineral (calcium rich stone) and other by-products such as wollastonite, cement dust, and ashes from sugarcane, coconut husk, palm oil, soda lime silica, and white rice husk into different oxide glass hosts for improving their structural, mechanical and optical performance wherein Abdellaoui et al. [8] studied the influence of natural raw barite and dolomite minerals on the glass forming ability of borate system. The results showed an increase in the glass density, molar volume, and the ratio of BO₄ to BO groups with the increase of BaO contents. Furthermore, the glass transition temperature and optical band gap were decreased, and the UV cut-off was moved towards higher wavelength with the increase of BaO contents. The gamma ray shielding features of lithium borate glasses doped with dolomite, hematite and goethite minerals were also studied [9]. The results revealed much higher increase in the glass density, transparency, exposure buildup factor and radiation shielding properties with the inclusion of dolomite than hematite and goethite.

The elastic and optical properties of CeO₂-doped borotellurite glasses containing two dolomite minerals (pebble and marble) was reported, indicating their usefulness for the development of active filter in optical devices [10]. Zakaly et al. [11] prepared a series of dolomite-borate glasses using the melt-quenching method and determined their physical, structural, and optical characteristics. A comprehensive overview of recent literatures showed that majority of the studies focused on the thermo-luminescence features of these glasses [12–15]. The incorporation of dolomite in the lead-zinc borate glasses was observed to improve their gamma radiation shielding capacity. Despite an excellent shielding potency shown by the glass composed of optimum dolomite content, yet their physical and mechanical properties still need further improvement wherein REIs co-doping may be viable strategy.

The amalgamation of boron oxide having high phonon energy and tellurium oxide with low thermal stability as foundational glass former can impart desired practical properties to the final product like low phonon energy, high thermal stability and negligible hydroscopic nature [16–18]. Moreover, the physical, structural and mechanical properties of La³⁺ infused zinc borate [19], Ag/Eu³⁺-magnesium zinc sulfophosphate [20] and Nd₂O₃:SrO-Bi₂O₃-B₂O₃ [21] glasses were meaningfully enriched due to the integration of REIs. However, studies on the structural and mechanical properties of Eu³⁺/Dy³⁺ incorporation in boro-tellurate glass system containing dolomite mineral to the best of authors knowledge has not been reported.

Based on the aforementioned facts, we made a series of Eu³⁺/Dy³⁺ co-doped boro-telluro-dolomite glasses and determined their structural and mechanical characteristics. The main purpose was to develop a high strength glass system with improved structural and mechanical properties suitable for mobile devices screen protector applications. As-quenched samples were characterized using different analytical instrument to determine some essential parameters like density, molar volume, N₄ ratio, microhardness and elastic moduli. Furthermore, the effects of various Eu³⁺/Dy³⁺ co-doping concentrations on the network structures and mechanical traits of the boro-telluro-dolomite glasses were examined. The obtained experimental results were analyzed, discussed, and compared with similar findings reported in the state-of-the-art literature.

2. Materials and methods

2.1. Preparation of glass samples

In this study, analytical grade chemical reagents (99.99% purity, Sigma Aldrich) of tellurium dioxide (TeO₂), boron oxide (B₂O₃) europium oxide (Eu₂O₃), dysprosium oxide (Dy₂O₃) oxide and dolomite (CaMg(CO₃)₂) were acquired. Dolomite minerals (pebble and marble) were collected from various sites of Johor (Malaysia) and crushed into fine powder using the (100$\times$60) Mini-Jaw Crusher. Eu³⁺/Dy³⁺ co-doped boro-telluro-dolomite (BTD) glasses of composition $(34-b)$ B₂O₃-TeO₂-20Dp-15Dm-1Eu₂O₃-$b$Dy₂O₃ (where $b=0.3$, 0.3, 0.5, 0.7 and 0.9 mol%, Dp = Dolomite pebble and Dm = Dolomite marble) were made using the conventional melt-quenching route and coded as BTDEu $b$Dy (Table 1). First, about 15 g of the glass constituents (called batch composition) were weighed and homogeneously mixed in an alumina crucible, and melted at 1200°C for 1 h 30 min in a high temperature electrical furnace. Next, the molten fluid was dispensed onto a pre-heated stainless-steel mold and subjected to annealing in another furnace at 400°C for 3 h to release the residual strain that may cause glass embrittlement.

Table 1. Glass code, compositions, density ($\rho )$ and molar volume ($V_m$) of the studied glasses.

	Compositions (mol%)						$\rho$ (g/cm^3)	$V_m$ (cm^3/mol)
Glass codes	B2O3	TeO2	Dp	Dm	Eu2O3	Dy2O3	$\pm$0.003	$\pm$0.003
BTDEu0.1Dy	33.9	30	20	15	1.0	0.1	2.310	60.434
BTDEu0.3Dy	33.7	30	20	15	1.0	0.3	2.659	52.859
BTDEu0.5Dy	33.5	30	20	15	1.0	0.5	2.716	51.961
BTDEu0.7Dy	33.3	30	20	15	1.0	0.7	3.024	46.863
BTDEu0.9Dy	33.1	30	20	15	1.0	0.9	3.465	41.078

2.2. Characterizations of samples

2.2.1. XRD and SEM-EDX analyses

The amorphous nature of the as-quenched samples was confirmed using X-ray diffraction analyses (Siemens X-Ray Diffractometer D5000 that used Cu-K $\alpha$ line of wavelength ≈ 1.54 $\text{Å}$ at 40 kV and 30 mA). The samples’ XRD patterns were recorded in the angular (2$\theta$) range of 10${}^{\circ }$ to 80${}^{\circ }$ at scan rate of 2${}^{\circ }$ per minute. The microstructures, surface morphology and presence of chemical elements in the dolomite minerals and samples were examined using the energy dispersive X-ray (EDX, SwiftED3000) spectrometer attached to a table top scanning electron microscope (SEM, TM3000 Hitachi). All the prepared samples were characterized at room temperature.

2.2.2. Density measurement

The density ($\rho$) of the studied samples was measured using Archimedes principle with distilled water (density, ${\rho }_w=1$ gcm⁻³) as immersion liquid and calculated using [22]: (1) $\rho =\frac{W_a}{W_a-W_b}\times {\rho }_w$(1) where $W_a$ and $W_b$ are the sample’s weight in air and water, respectively. The molar volume ($V_m$) in terms of molecular weight $(M_w)$ of the glass samples were calculated using [23]: (2) $V_m=\frac{M_w}{\rho }$(2)

2.2.3. FTIR and Raman spectral analyses

The FTIR spectra of the prepared samples were recorded in the range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹. Small amounts of powdered samples were mixed thoroughly with KBr at a ratio of 1:100 mg. The resultant mix was then hard-pressed by 5 ton per square inch pressure for one minute to get a transparent pellet of approximately 1 mm thick. The obtained pellet was used to record the FTIR spectra. The Raman spectra of the glasses in the range of 200–2000 cm⁻¹ at a resolution of 1 cm⁻¹ were recorded (HR800 UV Horiba Jobin Yvon Raman Spectrometer). FTIR and deconvoluted Raman spectra were used to estimate the N₄ ratio [24]: (3) $N_4=\frac{A_4}{A_4+A_3}$(3) where $A_4$ and $A_3$ are the area of the IR and Raman bands in the range of 800-1200 cm⁻¹ and 1200-1700cm⁻¹ due to the vibration of $B{O}_4$ and $B{O}_3$ units, respectively.

2.2.4. Vicker’s microhardness and ultrasonic velocity test

Micro-indentation measurement was performed on each sample with the applied loads of 50, 100, 200, 300 and 500 g (at a retention time of 15 s) using a digital micro-hardness tester (Shimadzu HVM-2). The values of glass microhardness were estimated using [5]: (4) $H_V=1.854\frac{F}{l^2}$(4) where $H_V$ is the Vickers hardness (in Hg/mm²), F is the applied force (in Newton) and l is the average length (in metres) of the indentation’s diagonals.

The longitudinal ($V_L$) and shear ($V_S$) ultrasonic velocities in the glasses at resonant frequency of 4 MHz were measured by a Ultrasonic flaw detector (RIGOL DS2202 digital storage oscilloscope). The values of $V_L$ and $V_S$ was calculated using [25]: (5) $\begin{array}{rl}& V_L=\frac{2d}{\mathrm{\Delta }t}\end{array}$(5) (6) $\begin{array}{rl}& V_S=\frac{2d}{\mathrm{\Delta }t}\end{array}$(6) where d is the glass thickness and $\mathrm{\Delta }t$ is the time interval (time taken between the start and retrieval of the pulse appear on the screen during round-trip).

The obtained values of $\rho$, $V_L$ and $V_S$ were used to estimate the Lame’s constants ($\lambda$ and $\mu$) [26]: (7) $\begin{array}{rl}& \lambda =\rho ({V_L}^2-2{V_S}^2)\end{array}$(7) (8) $\begin{array}{rl}& \mu =\rho {V_S}^2\end{array}$(8)

The values longitudinal (L), shear (G), bulk (K) and Young (E) modulus, Poison ratio ($\sigma )$ and fractal bond connectivity (D) were evaluated via [26]: (9) $\begin{array}{rl}& L=\lambda +2\mu \end{array}$(9) (10) $\begin{array}{rl}& G=\mu \end{array}$(10) (11) $\begin{array}{rl}& L=\lambda +2\mu \end{array}$(11) (12) $\begin{array}{rl}& K=\lambda +\frac{2}{3}\mu \end{array}$(12) (13) $\begin{array}{rl}& E=\mu \frac{3\lambda +2\mu }{\lambda +\mu }\end{array}$(13) (14) $\begin{array}{rl}& \sigma =\frac{\lambda }{2(\lambda +\mu )}\end{array}$(14) (15) $\begin{array}{rl}& D=\frac{4G}{K}\end{array}$(15)

3. Results and discussion

3.1. Physical and structural characteristics of glasses

Figure 1 shows the XRD patterns of the Eu³⁺/Dy³⁺ co-doped as-quenched samples which did not show any distinct crystalline peaks, rather revealed a broad hump at lower Bragg angles, affirming their amorphous nature. This observation is in agreement with the one reported by Saddeek et. al [27].

Figure 1. XRD pattern of the as-quenched samples.

Figure 2 displays the EDX spectraand maps of BTDEu0.5Dy glass, indicating the uniform distribution of elements like B, Te, O, C, Ca, Al, Mg, Eu and Dy existed as glass constituents [28]. The white spots in the SEM image (inset of Figure 2(a)) were the artifacts of measurement. The weight percent of the chemical elements were matched to the nominal glass composition (inset table of Figure 2(a)). Furthermore, the existence of Al in the studied glass confirmed the successful incorporation of dolomite pebble minerals into the BT glass network.

Figure 2. (a) EDX spectra (Inset SEM image and weight percent table) and (b) EDX maps of BTDEu0.5Dy glass.

Figure 3 illustrates the glass density and molar volume as a function of Dy₂O₃ contents. To determine the degree of the geometric configuration, network structures compactness, interstitial space facets and co-ordination number of the glasses, the density was measured [29]. The REIs concentration dependent variation in the glass density provided a basic insight related to the structural transformation of BO₃ units into four-fold BO₄ units. The numbers of BO₄ structural units were more than BO₃ units, responsible for increasing the network binding and density (more compact structure) of the glasses. The observed increase in the glass density with the increase of Dy₂O₃ contents was mainly due to the replacement of B₂O₃ with lower molecular weight (69.62 g $\text{mo}{\text{l}}^{-1}$) by Dy₂O₃ with much higher molecular weight (372.99 $\text{gmo}{\text{l}}^{-1}$) in the glass matrix. Conversely, the molar volume of the glasses was decreased with an increase of doping levels. Clearly, the inclusion of dopants with higher molecular weight than the host was enabled to increase the density of oxygen packing, transforming the loose network structures of the glass into more compact one [29] useful for the improvement of mechanical performance and durability. The obtained enhancement in the glass density and network structure packing with the increase of Dy₂O₃ contents can be ascribed to the conversion of Sp² hybridized planar (triangular) BO₃ units into more stable Sp³ hybridized BO₄ tetrahedral units. Compared to the planar BO₃ units, BO₄ tetrahedral units are strongly connected together, suggesting a shrinking of the amorphous network structure and thus improvement in the glass density. In addition, the co-dopants (Eu₂O₃ and Dy₂O₃) can occupy the lattice interstitials, leading to a complete modification of the glass network structures [30].

Figure 3. Variation of density and molar volume of the glasses against Dy₂O₃ contents.

Figure 4 shows the FTIR spectra of the glasses in the range of 380–4000 cm⁻¹. The IR absorption spectral analyses allowed us to distinguish the presence of various chemical functional groups in the proposed glass network. The measured absorbance of the glasses showed the bonding vibrations corresponding to the network former (B₂O₃), conditional network former (TeO₂) and natural minerals (dolomite) network modifier (MgCa(CO₃)₂) used in the compositions, wherein the inclusion of co-dopants was found to influence the glass network structures appreciably. However, the IR spectral band positions were not affected significantly with the variation of Dy₂O₃ concentrations. Various literature reports suggested that for any glass host made with B and Te show the IR bands of TeO₄, TeO₃, BO₄ as well as BO₃ units around 600–650, 650–700, 800–1200 and 1200–1400 cm^−1, respectively [31]. Herein, the bands around 403–412 cm⁻¹ were due to the bending vibration of Te-O-Te or O-Te-O linkages formed by sharing the corners of TeO₄ and TeO₃₊₁ polyhedral units with TeO₃ groups [32]. The FTIR bands at 486–510, 689–726, 951–981, 1144–1200, and 1344–1375 cm⁻¹ corresponded to the vibrations of TeO₄, TeO₃, BO₄ and BO₃ structural units [33]. Earlier study reported that pure borate shows a sharp IR peak at 806 cm⁻¹ due to the vibration of boroxol ring [34]. However, we did not observe the IR band corresponding to the boroxol ring vibration, indicating the dominance of BO₃ and BO₄ structural units in the proposed glass network. The band at 1630–1667 cm⁻¹ was due to the stretching vibration of B-O bonds from isolated pyroborate groups. The IR band at 3444–3469 cm⁻¹ was to the stretching vibration of H–O linkage from hydroxyl groups [35].

Figure 4. FTIR spectra of the prepared glasses.

Figure 5 displays the Raman spectra of some selected samples in the range of 2000–200 cm⁻¹. The Raman analysis complemented the IR results because some of the forbidden transitions in the IR absorbance (zero net dipole moment) became allowed in the Raman spectra due to nonzero net polarizability [36]. Table 2 shows the Raman peak positions and their assignments. The overall Raman profiles for different glasses remain similar wherein the shift in the Raman peak positions with the increase of Dy₂O₃ content was insignificant. The observed peak at 349 cm⁻¹ was due to the vibrations of the Te-O bonds from TeO₄ tetrahedra characteristic of α-TeO₂ [37]. The weak peak (shoulder) at 682-696 cm⁻¹ was due to the vibrations of Te–O linkages among TeO₄ units and bridging oxygen (BOx) [38]. In addition, the intense Raman peak at 770 cm⁻¹ was due to the symmetrical and asymmetrical vibration modes of (Te_eq-O) in TeO_3 + 1 or TeO₃ units [39]. The weak Raman peak at 922 cm⁻¹ was due to the stretching vibration of BO₄ unit from diborate (B₂O₅⁻⁴) groups [40]. The extremely weak peak at 1220 cm⁻¹ for all the studied glasses corresponded to the stretching vibration of BO₄ groups. The Raman peak at1357 cm⁻¹ was due to the asymmetrical stretching vibration of BO₃ groups in different borate units [41]. In brief, both FTIR and Raman spectra of the glasses showed the existence of functional chemical groups of boron and tellurium in the network structures and did not reveal any spectrum related to Dy₂O_3.

Figure 5. Raman spectra of the selected glasses.

Table 2. Comparison of the obtained Raman peak positions and assignments with other studies.

Raman shift (cm^−1)	Reported values (cm^−1)	Assignment	Ref.
349 (i)	345	Oscillations of TeO bonds inTeO4 tetrahedra characteristic of α-TeO2.	[42]
687 (ii)	670–684	Symmetric stretching vibrations of TeO in TeO4 units or Te–O–Te linkages between two fourfold coordinated Te atoms.	[43]
770 (iii)	750–760	Stretching vibrations of Te–O in TeO3 units and/or TeO3+1 groups.	[43]
922 (iv)	906	B-O stretching vibration in the BO4 units.	[44]
1220 (v)	1136	Vibrations of pyroborate groups (B2${\text{O}}_{\text{5}}^{\text{4 - }}$).	[45]
1357 (vi)	1311	Vibrations of BO3 segments	[45]

Some significant FTIR and Raman spectral peaks were deconvoluted into many Gaussian functions (Figures 6 and 7) to assess the relative area (A) of each component proportional to the structural group abundance [46]. The fraction of the 4-coordinated boron atoms (N₄) was evaluated using the expression described in Section 2.5. Together with the N₄ ratio, the vibrational spectral deconvolution provided a deeper insight into glass network structure whether densely packed or loosely bound matrix. The relative areas of the bands due to the IR vibration of BO₄ units in the range of 800–1200 cm⁻¹ (A₄), BO₃ units in the range of 1200–1600 cm⁻¹ (A₃), TeO₄ units (in the range of 600–650 cm⁻¹) and TeO₃ units (in the range of 650–800 cm⁻¹) were calculated. Table 3 shows the achieved relative areas and N₄ ratio of the glasses.

Figure 6. Deconvoluted FTIR spectra of (a) BTDEu0.3Dy, (b) BTDEu0.5Dy and (c) BTDEu0.7Dy glass fitted to Gaussian-Lorentzian function.

Figure 7. Deconvoluted Raman spectra of a selected glass.

Table 3. Relative area of the deconvoluted FTIR and Raman bands alongside N₄.

	Relative area of the deconvoluted FTIR bands centred at
Glass codes	865 cm^−1	1140 cm^−1	1339 cm^−1	N4 ratio
BTDEu0.1Dy	4.55	15.94	49.28	0.294
BTDEu0.3Dy	3.67	16.30	48.63	0.291
BTDEu0.5Dy	6.47	55.18	7.79	0.888
BTDEu0.7Dy	7.01	58.53	3.71	0.946
BTDEu0.9Dy	6.93	55.18	7.10	0.897
	Relative area of the deconvoluted Raman peaks
Glass codes	(i)	(ii)	(iii)	(iv)	(v)	(vi)	N4 ratio
BTDEu0.1Dy	6.00	53.54	29.62	1.61	1.17	8.07	0.64
BTDEu0.5Dy	7.30	53.96	27.72	1.19	0.76	9.08	0.66
BTDEu0.9Dy	8.69	55.77	25.76	0.76	0.82	8.19	0.68

Figure 8 illustrates the Dy₂O₃ contents dependent N₄ and TeO₄ ratio in the glasses. The N₄ ratio was increased gradually with the increase of Dy₂O₃ contents in the glass, indicating an improvement in the glass network compactness through BO₃+ O → BO₄ and TeO₃+ O → TeO₄ isomerization reaction with the generation of more BOx at the expense of non-bridging oxygen (NBOx). Comparable observations were made for the fluoride borate glasses [47] and nickel-doped bismuth borate glasses [48]. It was shown that by tuning the doping contents, the relative fraction of BO₄ and TeO₄ units in the studied glasses can be increased, enabling better control on the glass network structure, density and mechanical performance.

Figure 8. Dy ions contents dependent (a) TeO₄ ratio and (b) N₄ ratio of the studied glasses obtained from FTIR and Raman analysis.

3.2. Mechanical characteristics of glasses

The glass-based screen protectors of various portable electronic devices must be thin, high strength, durable, and light-weight [49]. These glasses should be resilient to external applied stress wherein their resistance to external pressure (shock shielding capacity) can be gauged through the mechanical hardness and elastic moduli measurement. Microhardness test provides basic knowledge on the glass resistance to permanent distortion stimulated via harder materials when subjected to stress. In the present study, the capacity of the proposed glass system in resisting plastic or permanent distortion under intense mechanical loading was examined using the Vickers hardness test [50]. The hardness ($H_V$) of the glasses were measured at various applied loads (50, 100, 200, 300 and 500 g) with retention time of 15 s. The glass hardness was increased with the decrease of Dy₂O₃ contents as shown in Figure 9. This observed increase in microhardness can be ascribed to the enhancement of glass network rigidity due to the reduction of NBO.

Figure 9. Glass hardness against applied load.

The results (Figure 9) of hardness against applied load can be interpreted by diving into two regions AB (with lower loads) and BC (with higher loads). The region AB is predominantly an active zone wherein the stress–strain behaviour is nonlinear. Conversely, the region BC is essentially a plateau or saturation zone in which the stress–strain response is somewhat independent of the loads variation. The observed non-linear response at lower load was mainly due to the increase of glass resistance toward additional indentations wherein some newly formed dislocations may be nucleated at the indentation region, thus enhancing the microhardness. The plastic zone (active region) was extended up to 3 N of load matching the plateau limit of the glass hardness. Furthermore, an additional increase in the applied load could lead to the phonon modes softening of the glass, making the microhardness constant. This result indicated that the proposed glass composition can resist up to 300 g (3 N) of applied load without being permanently deformed, making them potential candidates for high performance glass protector in mobile devices.

Deeper insight into the elastic modulus of various glass hosts became significant for the design and fabrication of high strength and durable screen shield [51]. Thus, various elastic parameters of the studied glasses were recorded and compared with the existing-state-of-the-art glass hosts reported in the literature (Table 4). The values of $V_L$ and $V_S$ of the glasses were increased with the increase of Dy₂O₃ concentrations (Figure 10), confirming the increase of glass network compactness and rigidity. In is worth noting that with the increase of glass density, the values of $V_L$ and $V_S$ for most materials generally show a significant increase [52], thus the proposed glasses followed the expected trend. This observation was supported by the improvement of the glass elastic moduli (L, G, K, and E) with the increase of Dy₂O₃ contents (Figure 11). Yet again, the achieved improvement in the elastic moduli of the prepared glass system indicated their good mechanical stability which agreed with the other reported findings on borotellurite glass [53]. It was argued that the observed increase in both ultrasonic velocities and elastic moduli of the glasses with the increase of doping levels was mainly due to the continuous conversion of TeO₃, and BO₃ groups into TeO₄ and BO₄ groups and thereupon the generation of more BOx at the cost of NBOx reduction. In short, the BOx atoms generated from the conversion of BO₃ into BO₄ structural units in turn strengthen the network bonding and density of the cross-links among different constituent atoms in the glass matrix, significantly improving the glass network rigidity and mechanical qualities. These data supported the results of density, FTIR and Raman spectra.

Figure 10. Variation of glass ultrasonic velocities against Dy₂O₃ contents.

Figure 11. Variation of glass elastic moduli against Dy₂O₃ contents.

Table 4. Ultrasonic velocities, elastic modulus, Poisson ratio and fractal bond connectivity of the present glass system compared to other reported works.

	Ultrasonic velocities (m/s)		Elastic moduli (GPa)				Poison ratio	Fractal bond connectivity
Glass system	$V_L$	$V_S$	L	G	K	E	$\sigma$	D	Ref.
BTDEu0.1Dy	5051	2575	58.934	15.317	38.511	40.572	0.324	1.591	Present
BTDEu0.3Dy	5125	2640	69.840	18.532	45.131	48.902	0.319	1.643	Present
BTDEu0.5Dy	5340	2855	77.448	22.138	47.931	57.160	0.300	1.847	Present
BTDEu0.7Dy	5512	2967	91.876	26.621	56.381	69.003	0.296	1.889	Present
BTDEu0.9Dy	5589	3010	108.236	31.393	66.379	81.354	0.295	1.892	Present
Borotellurite	3662	2200	43.02	22.32	15.53	37.81	0.218	$-$	[54]
Bismuth borotellurite	4328	2417	75.1	23.4	43.9	59.7	0.27	$-$	[55]
Silica borotellurite	6774	4183	182.6	69.7	89.7	186.0	0.192	$-$	[56]
Tellurite	3482	1987	68.88	36.14	20.80	52.36	0.25	$-$	[57]

The variation in the Poisson’s ratio ($\sigma )$ with the increase of Dy₂O₃ contents clearly reflected the glass network structures modification (called dimensionality changes) in terms of cross-linking density. Eevon et al. [58] reported that the Poisson’s ratio changes can be a good indicator for the glass network structure alterations. A glass system with $\sigma$ values in the range of 0.1 to 0.2 is characterized as dense cross-linked network. Conversely, a glass system with $\sigma$ values in the range of 0.3 to 0.5 is characterized as low density cross-linked network structure. According to Bergman and Kantor [59], fractal bond connectivity (D) can elucidate the effectual dimensionality of a specific material (for example inhomogeneous random mix of fluid states and solid backbones) near the percolation threshold. Interestingly, the value of D for the network structures in 1D, 2D and 3D are approximately 1.0, 2.0 and 2.0, respectively. In the current glasses, the values of $\sigma$ was $\ge$0.3, indicating their low cross-linking network density. In addition, the values of D of the studied glasses were found to be 2.0, confirming their 2D network structures.

Figure 12 depicts the correlation between fractal bond connectivity and Poisson’s ratio of the glasses. In pure B₂O₃ glass, most of the triangular BO₃ units are organized as boroxol rings wherein 3 O atoms are within the ring and 3 O atoms occur at the exterior of the ring. When the BO₃ units undergo constant transformation into BO₄ units due to the addition of Dy₂O₃ as substitute of B₂O₃, these boroxol rings suffer random interconnections. Consequently, the values of Poisson ratio decrease and dimensionality of glass network increase (Table 4). This observation is consistent with the proposed semi-empirical relation $\mu =A-$zD [59]. The obtained $\sigma -D$ correlation was fitted to $\sigma =0.474-0.095D$, with correlation constant of 0.09, wherein the constants A and z validated the earlier claim made from the study on the WO₃–B₂O₃–MgO–TeO₂ glass system with $\sigma =0.466-0.09D$ [60], oxyfluoro-zinc-tellurite with $\sigma =0.45-0.083D$ [61] and oxyfluoride tellurite AlF3-ZnO-TeO₂ with $\sigma =0.54-0.085$ [62].

Figure 12. Correlation between the fractal bond connectivity and Poisson’s ratio of the produced BTDEubDy glass system.

4. Conclusion

The possibility of customizing the structure and mechanical properties of some newly developed boro-telluro-dolomite glasses by co-doping with Eu₂O₃/Dy₂O₃ was determined for the first time. These glasses of composition $(34-b)$ B₂O₃-TeO₂-20Dp-15Dm-1Eu₂O₃-$b$Dy₂O₃ (where $b=$0.1, 0.3, 0.5, 0.7 and 0.9 mol%) were made using melt-quenching method and thoroughly characterized. The incorporation of Dy₂O₃ into the glass matrix was found to appreciably enhance the glass stiffness without affecting the borate entities, leading to an increase in the density and network compactness, thus improving the mechanical qualities useful for high performance screen shield. The FTIR and Raman spectral deconvolution confirmed a significant improvement in the structural and mechanical characteristics of the glassed due to Eu₂O₃/Dy₂O₃ concentration tuning, wherein the N₄ ratio was shown to increase. This structural enhancement of the glasses was ascribed to the isomerization process of $\text{B}{\text{O}}_3+\text{ NBOs}\leftrightarrow \text{B}{\text{O}}_4$ and TeO₃+ O → TeO₄. The high elastic modulus and strong correlation between fractal bond connectivity and Poisson’s ratio of the glasses affirmed their high resistance against permanent deformation under external applied pressure, making them protective for mobile devices screen protector.

Disclosure statement

No potential conflict of interest was reported by the author(s).