Preparation of orthophosphate glasses in the MgO–CaO–SiO 2 –Nb 2 O 5

Abstract

Background:

Niobia/magnesia-containing orthophosphate invert glasses were successfully prepared in our earlier work. Orthophosphate groups in the glasses were cross-linked by tetrahedral niobia (NbO₄) and magnesia.

Objective:

The aim of this work is to prepare calcium orthophosphate invert glasses containing magnesia and niobia, incorporating silica, and to evaluate their structures and releasing behaviors.

Method:

The glasses were prepared by melt-quenching, and their structures and ion-releasing behaviors were evaluated.

Results:

³¹P solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies showed the glasses consist of orthophosphate (PO₄), orthosilicate (SiO₄), and NbO₄ tetrahedra. NbO₄ and MgO in the glasses act as network formers. By incorporating SiO₂ into the glasses, the chemical durability of the glasses was slightly improved. The glasses reheated at 800°C formed the orthophosphate crystalline phases, such as β-Ca₃(PO₄)₂, Mg₃(PO₄)₂ and Mg₃Ca₃(PO₄)₄ in the glasses. The chemical durability of the crystallized glasses was slightly improved.

Conclusions:

Orthosilicate groups and NbO₄ in the glasses coordinated with each other to form Si–O–Nb bonds. The chemical durability of the glasses was slightly improved by addition of SiO₂, since the field strength of Si is larger than that of Ca or Mg.

Keywords

Biomaterials phosphate invert glass orthophosphate magnesium silicate niobium

1. Introduction

Titania-containing calcium phosphate invert glasses (TiCa-PIGs) and niobium-containing calcium phosphate invert glasses (NbCa-PIGs) are composed of short phosphate groups, such as ortho- and pyrophosphates ( $Q_{p}^{0}$ and $Q_{p}^{1}$ ), which cross-link with titania/niobia tetrahedral TiO₄/NbO₄ forming a glass network [1–3]. TiCa-PIGs were successfully coated on a Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy by heat-treatment at 800°C for 1 h [4]. The resulting layer, including β-Ca₃(PO₄)₂, β-Ca₂P₂O₇ and TiO₂ (rutile) crystal phases and residual glassy phases, bonded strongly with TNTZ [5,6]. TiCa-PIGs-coated TNTZ showed good bioactivity in vivo [4]. NbCa-PIGs enhanced the differentiation of osteoblast-like cells (e.g. alkali phosphatase activity, ALP) with a trace amount of niobate ion dissolved from the glasses (0.04–0.06 mM) [7].

In our previous work, the nominal composition of 37.5MgO·37.5CaO·20P₂O₅·5Nb₂O₅ (mol%, denoted by 20P) glass and glass-ceramic were prepared, and their structures and release behaviors were discussed as a fundamental work [8]. When MgO was incorporated into the phosphate invert glasses, it acted as an intermediate oxide [9,10]. Mg²⁺ ions released from the glasses are expected to promote cell adhesion [11], proliferation [12] and differentiation [13]. The phosphate group of 20P was composed exclusively of $Q_{p}^{0}$ , cross-linked by tetrahedral niobate/niobia (NbO₄) and magnesia [8]. 20P showed three glass transition temperatures, which indicate the coexistence of several network structures [8]. The chemical durability of 20P was comparably lower than that of Ti/NbCa-PIGs containing a large amount of MgO in the glass [2,3,8]. The glass-ceramic of 20P included orthophosphate crystal phases and its chemical durability was improved compared with that of the mother glass [8].

A trace amount of silicate ion has been reported to stimulate osteoblast proliferation [14] and differentiation [15], hence, addition of SiO₂ to the glasses may enhance bone formation. With the exception of certain special compositions, the formation of bonds (such as a P–O–Si bond) between phosphate and silicate groups in glasses is difficult. In this work, we focused on the coexistence of several glass-networks in 20P. By incorporating SiO₂ into 20P, the silicate group is expected to enter the niobate/niobia or magnesia network. The aim of this work is, as a fundamental work for designing biomedical materials, to prepare SiO₂-containing orthophosphate invert glasses, and to evaluate their structures, crystallization behaviors and ions-releasabilities to Tris buffer solution.

2. Materials and methods

(37.5 − x/2)MgO·(37.5 − x/2)CaO·20P₂O₅·xSiO₂·5Nb₂O₅ (mol%, $x = 0, 2.5, 5$ , denoted by 20P-xSi) glasses were obtained by melt-quenching. Glass batches were prepared by manually mixing MgO (99.0%), CaCO₃ (99.5%), H₃PO₄ (85% liquid), SiO₂ (99.0%) and Nb₂O₅ (99.9%), which were subsequently dried under an infrared lamp overnight and then stored at 140°C. All the reagents were purchased from Kishida Chemical Co., Japan. The batches were melted in a platinum crucible at 1500°C for 30 min and the melts were quenched by rapid squashing between two stainless-steel plates. The glass transition ( $T_{g}$ ) and crystallization temperatures ( $T_{c}$ , defined as the onset of crystallization) of 20P-xSi were determined by differential thermal analysis (DTA; heating rate: 5 K/min, Thermoplus TG8120, Rigaku). The glasses structures were investigated by solid-state ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR, JNM-ECA600II, JEOL, 242.955 MHz), and laser Raman spectra with range from 220 to 1300 cm⁻¹ (NRS-3300, 532.08 nm, 6.4 mW, JASCO). ³¹P MAS-NMR was measured in a 3.2 mm rotor spinning at 15 kHz with single-pulse experiments using 0.1 μs pulses (256 in total) spaced by 5 s and using ammonium dihydrogen phosphate as a reference (1.0 ppm). The spectra were reconstructed with a Gaussian fit. To examine the crystallization behaviors of the glasses, the resulting 20P-xSi were reheated at 800°C (heating rate: 5 K/min) for 3 h (denoted by 20P-xSi-C). Crystalline phases were analyzed by powder X-ray diffractometry (XRD, X’pert-MPD, PANalytical).

Powders of glasses and crystallized glasses were obtained by grinding and sieving (125–250 μm). A releasing test was examined by immersing 15 mg of the samples in 15 mL of 50 mM Tris buffer solution (TBS) at 37°C and pH 7.4 over 7 days. The concentrations of Mg²⁺, Ca²⁺, P⁵⁺, Si⁴⁺ and Nb⁵⁺ ions in TBS were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPS-7510, Shimadzu). The molar releasing fraction of the different elements was calculated using the following equation [8,16]: $\begin{matrix} (1) & Release rate (%) = \frac{(a / M_{wa}) \times 10^{5}}{(Frac,_{a} \times M_{w, glass}) / (m_{glass} \times V_{solution})}, \end{matrix}$ where a is the concentration of the element of interest in mg·L⁻¹; $M_{wa}$ is the atomic weight of the ion; $Frac,_{a}$ is the nominal molar fraction of the element in the glass; and $M_{w, glass}$ , $m_{glass}$ and $V_{solution}$ are the molecular weight, the mass of the sample soaked, and the volume of TBS, respectively. Glass powders after being soaked in TBS for 7 days were observed by scanning electron microscopy (SEM, JSM-6301F, JEOL).

3. Results

The glasses exhibited two $T_{g}$ values (denoted by $T_{g - 1}$ and $T_{g - 2}$ hereafter), at approximately 604°C and 662°C, respectively. $T_{c}$ value of the glasses were approximately 710°C. $T_{g}$ , $T_{c}$ and glassification degree (GD), which was calculated using $(T_{c} - T_{g - 1}) / T_{g - 1}$ (K/K) [17], showed no significant difference between the glasses. GD of the glasses were calculated with $T_{g - 1}$ . The values were approximately 0.12.

³¹P MAS-NMR spectra, as presented in Fig. 1, showed a sole peak between 10 and −10 ppm, assigned to $Q_{p}^{0}$ group. The peak top position and full width of half maximum (FWHM) of peak for the glasses were approximately 1.6 ppm and 8 ppm respectively. No significant differences between the glasses were observed.

Fig. 1.

³¹P MAS-NMR spectra of 20P-xSi.

Laser Raman spectra of the glasses are shown in Fig. 2(A), and FWHM of 840 and 965 cm⁻¹ are shown in Fig. 2(B). The phosphate group in the glasses showed Raman bands corresponding solely to the $Q_{p}^{0}$ group [10]: the symmetric stretching of the O–P–O bending modes of the $Q_{p}^{0}$ at 430 cm⁻¹, the symmetric stretching of the P–O bonds of the $Q_{p}^{0}$ at 590 cm⁻¹ and the (PO₄)_sym stretching mode of the non-bridging oxygen in $Q_{p}^{0}$ at 965 cm⁻¹. The vibrations at 840 cm⁻¹ corresponded to the symmetric stretching of orthosilicate ( $Q_{Si}^{0}$ ) [18,19] and NbO₄ tetrahedral group [20]. There was almost no significant difference in the FWHM of $Q_{p}^{0}$ (965 cm⁻¹) between the glasses. On the other hand, the FWHM of the bands corresponding to $Q_{Si}^{0}$ and NbO₄ (840 cm⁻¹) increased from 68 to 82 cm⁻¹ with increasing SiO₂ content in the glasses.

Fig. 2.

(A) Raman spectra of 20P-xSi and (B) FWHM of 840 and 965 cm⁻¹ for 20P-xSi, as a function of SiO₂ content in the glasses.

When 20P-xSi were reheated at 800°C, they were crystallized (20P-xSi-C), as shown in Fig. 3. The 20P-0Si-C showed XRD peaks corresponding to β-Ca₃(PO₄)₂ (ICDD card: 70-2065), Mg₃(PO₄)₂ (ICDD card: 75-1491) and Mg₃Ca₃(PO₄)₄ (ICDD card: 73-1182). With increasing SiO₂ content in the glasses, Mg₃(PO₄)₂ peaks disappeared and the intensities of β-Ca₃(PO₄)₂ peaks decreased. Niobate and/or silicate crystalline phases were not observed.

Fig. 3.

XRD patterns of 20P-xSi-C.

Figure 4 shows the ions-released percentages from 20P-xSi or 20P-xSi-C into TBS, relative to the original amount in the glasses or crystallized glasses. The ions-release percentage of phosphorus ion from 20P-xSi at day 7 decreased significantly from 22% to 14% corresponding to the SiO₂ content in the glasses. SEM observation of the surface of 20P-xSi after 7 days of soaking suggested that a gel-like layer formed on the surface of the glasses. The percentages of phosphorus ion released from 20P-2.5/5Si-C were larger than that released from 20P-0Si-C; those of magnesium and calcium ions were at almost same levels, whereas silicate and niobate ions were detected to be almost 0%.

Fig. 4.

Percentages of ions in the TBS after soaking 20P-xSi or 20P-xSi-C, relative to the total amounts for Mg²⁺, Ca²⁺, P⁵⁺, Si⁴⁺ and Nb⁵⁺ ions as a function of SiO₂ content in the glasses or crystallized glasses. The error bar represents the standard deviation. Inset shows the SEM images of 20P-xSi surface after being soaked in TBS for 7 days.

4. Discussion

The glassification degree (GD) of 20P-xSi was around 0.12. The value is larger than that of TiCa-PIGs (0.08) [3] and NbCa-PIGs (0.09) [2]; this indicates better glass forming ability. 20P-xSi showed several glass transition temperatures, which indicate the coexistence of several glass-network formers, such as phosphate, silicate, niobate/niobia [20,21] and magnesia [9,10]. Phosphate and silicate are well-known glass-network formers. Raman spectra showed that niobate/niobia in the glasses takes the form of tetrahedral NbO₄ (Fig. 2(A)), which acts as a network former [20] to form P–O–Nb bonds [22,23]. Brow reported that invert glasses do not have a continuous random network and their structures consist of short phosphate groups (i.e. $Q_{p}^{0}$ and $Q_{p}^{1}$ ) and modifier ions linked through the non-bridging oxygen [24]. 20P-xSi consists of $Q_{p}^{0}$ (PO₄), $Q_{Si}^{0}$ (SiO₄) and NbO₄ tetrahedra and modifier ions, such as Mg²⁺ and Ca²⁺ ions. According to Dietzel’s rule, MgO is an intermediate oxide [25]. MgO acts as a network former in phosphate glasses with high MgO content, consisting of $Q_{p}^{0}$ and $Q_{p}^{1}$ groups [9,10]. Therefore, MgO in 20P-xSi acts as a network former to form P–O–Mg bonds [8,16]. The cumulative network forming ability of phosphate, silicate, niobate/niobia and magnesia at low content of phosphate (i.e. 20 mol%) might explain the presence of only orthophosphate group ( $Q_{p}^{0}$ ) and larger glassification degree than that of Ti/NbCa-PIGs.

There were no significant differences in the peak top positions and FWHMs of $Q_{p}^{0}$ in ³¹P MAS-NMR spectra (Fig. 1), irrespective of the SiO₂ content. This implies that the coordination status of the $Q_{p}^{0}$ group does not change with incorporation of SiO₂; silicate makes no bond or coordination with phosphate group. The Raman band between 840 and 880 cm⁻¹ can be considered the symmetric stretching of orthosilicate ( $Q_{Si}^{0}$ ) [18,19] in common silicate glasses. In case of present orthophosphate invert glasses, Raman band of NbO₄ also observed in 840 cm⁻¹ [20]. The FWHM 840 cm⁻¹ (Fig. 2(B)) increased significantly with increasing SiO₂ content, indicating the diversification of coordination status in $Q_{Si}^{0}$ and NbO₄. This may imply $Q_{Si}^{0}$ and NbO₄ coordinate with each other and form Si–O–Nb bonds [26]. Originating from this glass structure, 20P-xSi-C is considered to consist of $Q_{p}^{0}$ -based crystals with the residual glassy phases containing silicate and niobate.

Ions-releasing percentage of 20P-xSi at day 7 was approximately 13%. This value is comparably larger than that of Ti/NbCa-PIGs (around 4–7% at day 7 [3]). This indicates the chemical durability of 20P-xSi is slightly lower than that of Ti/NbCa-PIGs. Ti/NbCa-PIGs contain large amounts of P–O–Ti/Nb bonds [2,3], which were reported to improve the chemical durability of phosphate glasses [23,27]. Mg²⁺ ion in phosphate invert glasses containing intermediate oxides (i.e. TiO₂ or Nb₂O₅) preferentially coordinates with $Q_{p}^{0}$ group to form P–O–Mg bonds [16], and degrades the chemical durability of the glasses [16,28,29], since P–O–Mg bonds are similar to Si–O–Mg bonds and weaken the glass network structure [30] and the resistance to hydrolysis [31]. As the only phosphate group in 20P-xSi is $Q_{p}^{0}$ , P–O–Mg bonds are preferentially formed in the structure. Therefore, the chemical durability of 20P-xSi was comparably lower than that of Ti/NbCa-PIGs [2,3]. Gel-like layers on the glass surfaces were observed after soaking TBS. With increasing SiO₂ content, the chemical durability of 20P-xSi was slightly improved, since the field strength of Si (1.57 valence/Å²) is larger than that of Ca (0.33 valence/Å²) or Mg (0.53 or 0.45 valence/Å², 4-fold or 6-fold coordination, respectively) [25]. The amounts of magnesium and calcium ions released from 20P-xSi-C were smaller, compared with 20P-xSi, due to the small amounts of soluble crystals. However, the releasing amount of phosphorus ions increased after the heat-treatment. The residual glassy phase may induce phase separation; the phases might be phosphate and niobium-silicate network. Phosphate network contains P–O–Mg bonds, which are weak from the view point of the chemical durability of the glasses [16,28,29], whereas silicate network contains Nb–O–Si bonds which improve the chemical durability [26].

5. Concluding remarks

Orthophosphate MgO–CaO–P₂O₅–SiO₂–Nb₂O₅ invert glasses and crystallized glasses were successfully prepared, and their structures and releasing behaviors were examined. The glasses showed several $T_{g}$ values, indicating the coexistence of several glass-network structures, such as $Q_{p}^{0}$ , $Q_{Si}^{0}$ and NbO₄ tetrahedral groups. MgO in the glasses was also considered to act as a network former to cross-link short glass-network forming groups (i.e. $Q_{p}^{0}$ , $Q_{Si}^{0}$ and NbO₄). In particular, silicate and niobate/niobia coordinate with each other to form Si–O–Nb bonds. The crystallized glasses included orthophosphate crystal phases, such as β-Ca₃(PO₄)₂, Mg₃(PO₄)₂ and Mg₃Ca₃(PO₄)₄, without silicate and niobate crystals. The chemical durability of 20P-xSi was lower than that of Ti/NbCa-PIGs, since magnesium in phosphate invert glasses formed P–O–Mg bonds, which favor hydrolysis. The chemical durability of 20P-xSi was slightly improved by addition of SiO₂, since the field strength of Si is larger than that of Ca or Mg. 20P-xSi is expected to enhance bone formation by ions released from the glasses, such as magnesium, silicate and niobium ions. These results are expected to be useful for designing novel biomedical materials.

Footnotes

Acknowledgements

This work was supported in part by JSPS KAKENHI Grant Number 25249094 and 26289238.

Conflict of interest

The authors have no conflict of interest to report.

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Preparation of orthophosphate glasses in the MgO–CaO–SiO 2 –Nb 2 O 5 –P 2 O 5 system

Abstract

Background:

Objective:

Method:

Results:

Conclusions:

Keywords

1. Introduction

2. Materials and methods

3. Results

5. Concluding remarks

Footnotes

Acknowledgements

Conflict of interest

References