Structure and Properties of Mixed Former Aluminum Borate Glasses Modified With Silver Oxide

doi:10.21203/rs.3.rs-1202986/v1

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Structure and Properties of Mixed Former Aluminum Borate Glasses Modified With Silver Oxide

https://doi.org/10.21203/rs.3.rs-1202986/v1

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The current study focuses on glass preparation and characterization in the xAl₂O₃ (35-x) Ag₂O.65B₂O₃ system (0≤x≤35 mol%), where Ag₂O is replaced with Al₂O₃. To examine a wide range of both structure and morphology of the produced glasses, nuclear magnetic resonance (NMR) of ²⁷Al nuclei, X-ray diffraction (XRD) spectroscopy, and transmission electron microscopy (TEM) are used. Changing the Al₂O₃ and Ag₂O molar ratios reveals a substantial change in material structure. In Al₂O₃-rich glass, the well-formed AlO₆, AlO₅, and AlO₄ structured groups are the well-formed units. In samples of (20 and 30 mol % Al₂O₃), tetrahedral AlO₄ and traces from AlO₆ units may be detected. At lower concentrations of Al₂O₃ (10 mol%), the dominant unit is only AlO₄ groups containing non-bridging oxygen bonds (NBO). The XRD and EDP spectra confirm the amorphous nature of the glasses of Al₂O₃ ˂ 20 mol%. Glasses of higher Al₂O₃ concentrations contain crystalline Ag₂Al₂B₂O₇ which are formed due to the higher oxygen packaging of the mixed AlO₅ and AlO₄ compared with that of glasses containing only AlO₄ species. The amount of higher coordinated Al species AlO₅ and AlO₆ are gradually increased in response to an increase in the ratios of Al₂O₃/Ag₂O. The morphology of crystalline units is confirmed from TEM to differ from that of an amorphous composition. The conductivity decreases and the activation energy for ionic conduction increase with increasing Al₂O₃. The hardness number of the studied glasses is highly increased with increasing Al₂O₃ content. The increase of activation energy and the hardness number of the glasses led to an increase in the durability of the investigated glasses.

Inorganic Chemistry

Organic Chemistry

Aluminum Borate glass

NMR

coordination of aluminum atom

Conductivity

Crystallization process

In recent decades, glass or glass-ceramics containing Al₂O₃, such as Al₂O₃-B₂O=, Al₂O₃-SiO₂, or Al₂O₃-B₂O₃-SiO₂, and Al₂O₃-P₂O₅ systems, have gotten a lot of attention [1- 4]. This is because the presence of Al₂O₃ in glass systems improves chemical stability [5] and leads to good mechanical properties [6, 7]. Physical characteristics of Al₂O₃-containing glasses, such as micro-hardness, conductivity, and chemical durability, are strongly linked to their Al coordination, which shifts from six (AlO₆) to five (AlO₅) to four (AlO₄) coordination [7–10]. Then, for adjustments of the structure to get appropriate physical properties, it is important to control both the boron and aluminum coordination in the investigated glasses.

The relation between adding network modifiers and changing in both boron and aluminum coordination had been documented in earlier reports [9, 11]. Increasing the molar ratio of modifier oxide to aluminum oxide should lower or suppress the concentration of higher coordinated Al species such as AlO₆ and AlO₅ in glasses. Reverse behavior was identified with increasing Al₂O₃ concentration in the glass network since higher-coordinated AlO₆ and AlO₅ can simply be formed because their cationic field strength is sufficiently large.

Recently, there are some available researches on alumina-based glasses and glass ceramics but little research attention has been paid to crystalline Ag₂O- Al₂O₃-B₂O₃ nano-composites. Studies concerning this issue are our main aim. The addition of Ag₂O to aluminum borate glasses can expand the glass formation region and generate the intended amounts of AlO₆ beside AlO₅, proportional to the ratio of Al₂O₃ to Ag₂O. In addition, at high Al₂O₃/Ag₂O₃ ratios, crystalline structural units can be formed in the range of nanoscale. Al₂O₃ oxide reinforced crystallized glasses [12–15] give the material a good advantage to be used as compatible dental fillers to enhance the hardness and other mechanical properties of dental material [12–14].

Borate glasses containing silver, strontium, zinc, and/or cerium oxides can simply exhibit good compatibility with tissues and support cell proliferation [15–17]. But there is a problem in the case of releasing a high amount of boron ions during the dissolution process which may become toxic. To solve such a problem, mixing of B₂O₃ with an additional glass former such as P₂O₅, SiO₂ or Al₂O₃ can regulate the process of ion release in the solution and prevent the toxicity to be performed. The mixed ion effect can also optimize the dissolution and boron release rate. Therefore, our aim in this work is to study the effect of mixing specific concentrations from Al₂O₃ with B₂O₃ in silver borate glasses. The Al₂O₃ can enhance the hardness and toughness of the glass and makes the material durable against corrosions. The Ag, as well as Cu ions, are known to promote the proliferation of endothelial cells, which are important processes for wound healing. In addition, Ag⁺ ions in glasses have a good ability to be used as antibacterial agents giving the mixed former glasses more advantages toward biomedical applications [18].

2.1. Preparation of Glasses

The chemically pure AgNO₃, H₃BO_3, and Al₂O₃ were used to prepare all glass samples from the starting materials. Stoichiometric powders were carefully mixed and melted for 50 minutes in a platinum crucible at 1340-1450°C. Subsequently, melts were quenched on a metal plate that was pre-heated to 350^oC to avoid cracking progress. The resulting glasses had formulations of xAl₂O₃.(35-x) Ag₂O.65 B₂O₃ (x= 0-35 mol%).

2.2. Measurements

X-ray diffraction measurements are carried out with Shimadzu X-ray diffract meter (Dx-30, Metallurgy institute, El Tebbin-Cairo). The peak position and intensity values used to identify the type of material phases were compared with patterns in the international powder diffraction file (PDF) database compiled by the joint committee for powder diffraction standards (JCPDS). NMR measurements were carried out at ambient temperature on a JEOL RESONANCE GSX-500 spectrometer operating at a high external magnetic field (11.747 T). ²⁷Al NMR spectra were measured at the resonance frequency of 130.2 MHz, using a 3.2 mm MAS NMR probe operated at a rotor frequency of 15 kHz. Typical pulse lengths were 2.5 µs and 60 seconds delay time was sufficient to enable relaxation. Spectra are referenced to Al(NO₃)₃ (0 ppm)... 27Al spectra were fit with Delta NMR software, version 5.04 (Japan) has been used for NMR analysis, simulation, and integration of the area under each species peak.

3.1 XRD and TEM-EDP

The formation of the crystalline aluminum-borate structure in some of the investigating compositions is very important in the broad field of applications. The ordered species from Ag₂Al₂B₂O7 [18] would play a good role in the improvement of hardness and compactness of the material network and consequently enhance its corrosion resistance. In addition, both boron and silver ions impeded in these crystalline phases play the role of antibacterial species. The formation of different types of B-O-Al, Al-O-Al bonds in the network plays role in the enhancement of material properties. The bond types and their concentration in different phases can be documented utilizing both XRD and NMR spectroscopy.

XRD spectra (figure 1) showed a structural change when Al₂O₃ is added to the Ag₂O-B₂O₃ network. The amorphous structure is dominant in the composition of up to 20 mol% Al₂O₃. The formation of crystalline Ag₂Al₂B₂O7 phases is confirmed upon more additions of Al₂O₃. Increasing crystallinity of the Al₂O₃ rich glasses is accompanied by decreasing the rate of transformation of BO₃ units to BO₄ groups that lead to a decrease in the fraction of tetrahedral boron (N4)[19]. Besides, most Ag₂O are consumed firstly to modify Al₂O₃ forming AlO₄ groups and the rest are used to modify B₂O₃ to form BO₄ tetrahedral units. The shortage in Ag₂O concentration upon Al₂O₃ replacement forces some of Al₂O₃ to enter as AlO₆ to compensate for the electronegativity around the well-formed structural units. Thus the introduction of Al₂O₃ into the borate matrix produces not only Al-O-Al-O-B bonds but also gives rise to an increase in the tendency to crystallization as is shown in Figure (1).

This result of XRD is agreed to some extent with the TEM results presented by Figures (2 and 3). As is shown from the figures, different species of different morphologies are simply shown. The electron diffraction pattern of this composition confirms the formation of the crystalline structure of the distributed species. On the other hand, the homogenous morphology with amorphous EDP is seen for a glass of lower Al₂O₃ concentration.

3.2. Electrical Conductivity

It is also important in this work to study the electrical properties of the investigated glasses. This is because to apply such material in the field of bio applications, its ability to conduct or resist current should be determined. The trial will be done to correlate the changes of both conductivity and activation energy for conduction with the structural changes detected by NMR spectroscopy upon replacing Ag₂O with Al₂O₃. At first, it can be assumed that Ag⁺ ions related to the borate matrix are the predominant contributor to the conduction mechanism [20]

The studied glasses show a linear dependence of the logarithm of conductivity (log σ₂₀₀) with reciprocal of absolute temperature (1/T), Figure 4. This behavior is a feature of the ionic conduction process that can be described by the Arrhenius relation. Figure (5) shows that there is a fast change in both log σ₂₀₀ and E with increasing Al₂O₃ contents. The result reveals that conductivity decreases by about six orders of magnitude upon replacing 30 mol% Ag₂O with Al₂O₃. The remarkable change in conductivity is accompanied by an increase of E from 1.2 eV to 0.51 eV.

The change of the Vickers hardness (H_v) with the Al₂O₃ content is shown in Figure 6. It can be seen that the hardness number increases with increasing Al₂O₃ content and there is a large increase in hardness value of the investigated system from about 2.3 to 4 GP as aluminum oxide content increases from 0 to 30 mo%. It is known that the increase in hardness is related to the increase in the rigidity of glass [21, 22]. These results are consistent with the increase in activation energy for conduction (figure 5), the increase of both Hv and E_a indicated that the addition of Al₂O₃ created some units from five and six coordinated aluminum species. In addition, the relative concentration of three coordinated boron is also increased at expense of the four coordinated one. The presence of such units plays the role of strengthening the glass which may be reflected from the increasing behavior between the hardness number and Al₂O₃ content.

3.3. ²⁷Al NMR investigation

To understand the changes in the NMR spectra of the aluminate structural species, the role of Al₂O₃ must be firstly determined. For instance, the addition of Al₂O₃ to silicate or borosilicate glasses requires charge compensation which can be achieved by either depolymerization of the silicate network or formation of AlO₆. At a high Al/Si ratio (greater than 1), the tetrahedral (AlO_4/2) are avoided to linked together [23, 24] and as a result AlO₆ octahedral must be present in the form (AlO_6/2)³⁻. The situation is different to a great extend in Al₂O₃ borate glasses. since most of Al₂O₃ would suppress the continued formation of BO₄ [25]. This is because Al₂O₃ enters as a glass former (AlO₄) which is compensated with a modifier or with octahedral aluminum. Then not all the modifiers can consume in boron transformation but the main portion from Ag₂O can be used to form AlO₄ units as network former species.

Figure (7) represents ²⁷Al NMR spectra of three different compositions of the studied glasses. An intense ²⁷Al NMR resonance of chemical shift at about 55, 51, and 44 ppm arising from Al(4) was seen for glasses containing 10, 20, and 30 mol% Al₂O₃ respectively. This figure demonstrates that in every composition the main portion of the aluminum is acting principally as a network former (AlO₄) species. Indeed there was a broad spectral peak which evidenced that a little of Al₂O₃ enters as Al(6) (small peak at about 16 ppm) in glasses of 20 and 30 mol%Al₂O₃. Similarly, the appearance of the Al(5) resonance only as a shoulder indicates a rather low concentration of this structural type within the glass. The distortions of the peak that represents AlO₄ resonance may be considered due to the effect of second-order quadrupolar interactions due to the presence of even low concentrations from AlO₅ structural units within the glass [26]. This consideration can be more evident from the analysis of the experimental spectra by deconvolution process Figures (8-10). The resonance was found to have a contribution as the observed shoulder which can represent AlO₅ at about 35 and 40 ppm, Figures (9 and 10). Accurate integration of the NMR spectrum to give precise information concerning the relative amount of these structural elements was possible owing to the separation of the spectra into its component bands. The analyzed broad peak around 16 ppm (figure 8 and 9) represents AlO₆ octahedral units where such species (AlO6) is expected to be formed to avoid two Al(4) bonding [26, 27 ].

The NMR data therefore strongly suggest that adding Al₂O₃ changes the microstructure of the glass network. For instance, in the high Al/Ag ratio (>1), both Al(4) and Al(6) are significantly formed. But at lower Al/Ag ratios, only AlO₄ units are considered the main structural species representing the aluminate structure. Given that the excess aluminum can be consumed in constructing a regular silver borate phase containing aluminate species. This would imply that the glass at high alumina content is not homogenous. This result is agreed to some extent with the TEM results presented in Figure 3. As is shown from this figure different species of different morphology are seen (The electron diffraction pattern of this composition confirms the formation of the crystalline structure of the distributed species. On the other hand, the homogenous morphology with amorphous EDP is seen for a glass of lower Al₂O₃ concentration Figure (2).

The structure of silver aluminoborate (NAB) glasses containing a high concentration of Al₂O₃ (30 mol%) were studied by X-Ray diffraction, TEM spectroscopy, and ²⁷Al nuclear magnetic resonance (NMR) spectroscopy. The effects of replacing Ag₂O with AlO₃ content on the structure of the NAB glasses were evaluated. Al³⁺ ion enters into the glass structure mainly in fourfold coordination, forming (AlO_4/2)⁻ tetrahedral. A small amount of Al³⁺ is found in fivefold and sixfold coordination. Al₂O₃ acts as network formers producing more bridging oxygen atoms with BO₃ units. The exchange of Ag₂O with Al₂O₃ decreases the conductivity and increases the activation energy for ionic conduction. The hardness of the studied glasses is improved with increasing Al₂O₃ content.

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Structure and Properties of Mixed Former Aluminum Borate Glasses Modified With Silver Oxide

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Abstract

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1. Introduction

2. Experimental Work

2.1. Preparation of Glasses

2.2. Measurements

3. Results And Discussion

4. Conclusion

Declarations

References

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