Structure, properties and application of oxynitride glasses

Structure, properties and application of oxynitride glasses

J O U R N A L OF ELSEVIER Journal of Non-Crystalline Solids 181 (1995) 215-224 Structure, properties and application of oxynitride glasses Sumio Sa...

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Journal of Non-Crystalline Solids 181 (1995) 215-224

Structure, properties and application of oxynitride glasses Sumio Sakka * Fukui Universityof Technology, Fukui 910, Japan Received 5 August 1994

Abstract

The structure and properties of nitrided silicate glasses have been reviewed mainly on the basis of the works made by the author's group. Neutron diffraction analysis and 29Si and 2~d nuclear magnetic resonance spectroscopy have shown that in N a - S i - O - N and Y - A I - S i - O - N oxynitride glasses there exist N atoms bonded to two Si atoms as well as N atoms bonded to three Si atoms. It is also shown that N atoms hardly make bonds with A1 atoms in glasses containing both Si and A1 atoms and that N atoms do not enter into the glass network but form clusters consisting of AIN4 units in aluminate glasses. The elastic moduli and hardness markedly increase with increasing N content. The increase in elastic moduli with increasing nitrogen content is attributed to the presence of N atoms bonded to three Si atoms. This has been confirmed by an ab initio molecular orbital calculation, which shows that the bending force constant for model molecule N[Si(OH)3] 3 is very large compared with that for O[Si(OH)3] 2. It is shown that a drastic improvement in chemical durability by nitriding is seen in some glasses but not in others. The ionic conductivity of alkali silicate glasses increases with increasing nitrogen content. Finally, a brief explanation is made on the attempts in Japan for applying oxynitride glasses as industrial products.

I. Introduction

Oxynitride glass [1,2] is prepared by replacing a part of the oxygen in an oxide glass by nitrogen. In the 1960s Mulfinger's group [3] prepared a sodalime-silica glass that contained about 3.2 wt% N by melting the glass batch in an N 2 atmosphere, and found that the incorporation of N pronouncedly reduced the water content of the glass. Elmer and Nordberg [4] made nitrided silica glass by heating porous silica glass in an NH 3 atmosphere, showing

Presented at the 12th University Conference on Glass Science, Alfred University, Alfred, NY, USA, 25-29 July 1993. * Corresponding author. Tel: +81-766 22 8111. Telefax: +81766 22 8117.

that the nitrided glass exhibits higher viscosities and electrical resistivities than non-nitrided silica glass. In the 1970s an intensive research effort was made on the sintering and high-temperature strengths of Si3N4-based ceramics called nitrogen ceramics. In the course of these studies, it was found [5] that the silicate oxynitride glasses formed in the grain boundaries of the nitrogen ceramics deteriorate the hightemperature mechanical strength of the nitrogen ceramics and that, from the standpoint of glass, A I S i - O - N , M g - S i - O - N and A 1 - Y - S i - O - N oxynitride glasses formed at grain boundaries are more resistant to high temperatures than the corresponding non-nitrided glasses. These findings triggered many studies on oxynitride glasses and, since the late 1970s, the preparation and properties of various oxynitride glasses have been investigated [6-12].

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved

SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 5 1 4 - I

216

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224

Generally, the physical properties of oxide glasses such as hardness, elastic modulus, fracture toughness and glass transition temperature are improved. This means that the replacement of O by N strengthens the glass structure. It should be mentioned here that among the replacements of oxygen in oxide glasses by anions only nitrogen and carbon (C 4- ) strengthen the glass: the replacement of oxygen by other anions, i.e., S, Se, Te, F, C1, Br, and I, weakens the glass. The strengthening of the glass structure on incorporation of N atoms has been explained on the basis of the structural model proposed by Mulfinger [3], in which a nitrogen is coordinated by three Si atoms in silicate oxynitride glasses. It has been believed that all N atoms in a silicate oxynitride glass are coordinated by three Si atoms, but recently it has been suspected [13-15] that some of the N atoms may be bonded to two Si atoms or one instead of three. This bonding question and other structural problems are dealt with in this work [16]. The elastic moduli and hardness increase with increasing N content. This increase is attributed to the presence of N atoms bonded with three Si atoms. This bonding will be given a theoretical explanation by an ab initio molecular orbital calculation of the bending force constant for model molecules [17]. The change of ionic conductivity of alkali silicate glasses with nitrogen content is discussed [18,19]. The chemical durability of an oxide glass is much improved by the incorporation of nitrogen in some glasses, but not in others [20]. The improvement of chemical durability is discussed in terms of the composition of the original oxide glass in more detail. Finally, the possibility of industrial application of oxynitride glasses is mentioned.

2. Structure of oxynitride glasses 2.1. Coordination of N atoms

The incorporation of nitrogen into silicate glasses modifies their physical and chemical properties to a considerable extent [1,2]. Most of the properties change in the direction which is expected from the strengthening of chemical bonding: the elastic moduli and Viekers hardness increase, the glass transition and softening temperatures increase, the thermal ex-

pansion coefficient decreases and the chemical durability is improved. These changes in properties have been explained based on the Mulfinger model [3], in which the glass structure becomes dense due to the presence of three Si(O,N) 4 structural units in a local region around an N atom and is strengthened on average due to the presence of three bonds per N atom instead of two bonds in the case of an O atom. This model has been supported by measurements of density [11] and elastic moduli [12], and infrared spectroscopy (IR) [21], X-ray photoelectron spectroscopy (XPS) [22] and 295i magic angle spinning nuclear magnetic resonance (MAS-NMR) [13]. Recent studies of the coordination state of nitrogen in oxynitride glasses based on 15N MAS-NMR [14] and molecular dynamics (MD) calculation [15] indicated, however, that nitrogen atoms bonded to two or one Si atoms may coexist with those bonded to three Si atoms in oxynitride glasses. We have studied the structure of N a - S i - O - N [16] and Y A I - S i - O - N [23] glasses by neutron diffraction. Neutron diffraction is useful for investigating the structure of glasses composed of light elements, because the cross-section of elements for neutron scattering does not decrease with decreasing atomic number of the element and does not change with the interatomic distance. With pulsed neutron sources, it was possible to extend the maximum scattering vector, Qmax, up to 300 cm -1. Therefore, one can expect a marked improvement in the resolution of the pair-distribution functions. Figs. 1 and 2 show the magnified first peak in the radial distribution function (RDF) curves of 20 Na20.80SiO 2 glass containing 0 and 4.4 at.% N and that of 17Y203 • 25A1203 • 58SIO 2 glass containing 5 at.% N, respectively. In Fig. 1, the peak at 0.1627 nm, which corresponds to the non-nitrided glass (Fig. l(a)), shifts to a longer distance at 0.1650 nm (Fig. l(b)) when nitrogen is incorporated. This shift suggests the existence of an Si-N bond as well as the Si-O bond in the oxynitride glass, because the Si-N bond is longer than Si-O bond. This peak is deconvoluted into Gaussian curves by the least-squares method. The bonding data obtained from Fig. 1 are listed in Table 1. The bonding data of 17Y203.25A1203 • 58SIO 2 glass containing 5.0 at.% are listed in Table 2.

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224 b

N:4.4 atomic %

8

-/~

6

I

1.4

1.6

217 RDF • Si-O

[ /

....... S i - N ÷ A I - O ( 4 )

\

...... A i - O ( 5 )

1.8

2.0 r /

2.2

/

/

2.4

2.6

2.8

10-' nm

Fig. 2. Radial distribution function of 17YzO3.25AI203 .58SIO 2 glass containing 5,0 at.% N [23]. Gaussian approximations for various ion pair distributions are shown. 1.4 1.5 1.6 1.7 1.8 r / 10-' nm

1.9 2.0

Fig. 1. Radial distribution function of 20Na20.80SiO 2 glasses containing 0 and 4.4 at.% N [16]. Dotted and dashed curves indicate the Gaussian approximations for Si-O and S i - N pair distribution functions, respectively.

dinated nitrogens are present in the Y - A I - S i - O - N glass, it is estimated that 86% of the N atoms are threefold-coordinated and 14% of them are twofoldcoordinated.

2.2. Effect of nitriding on the coordination of cations in glasses

Table 1 shows that the average coordination number of nitrogen in the N a - S i - O - N glass (NN_si in the table) is 2.42, indicating that not all N atoms are bonded to three Si atoms but some are bonded to two or one Si atoms. A simple calculation based on the assumption that only N atoms bonded to three or two Si atoms are present indicates that 58% of nitrogen atoms are bonded to two Si atoms and only 42% of nitrogen atoms are bonded to three Si atoms. Unuma et al. [15] obtained the value of about 2.3 as the average coordination number of nitrogen in the N a S i - O - N oxynitride glasses by means of a MD calculation. Table 2 shows that, in the Y - A 1 - S i - O - N glass, the average coordination number of N atoms is 2.86. If it is assumed that only threefold and twofold-coor-

The effect of nitriding on the coordination state of AI and Y atoms in Y - A 1 - S i - O oxide and Y - A 1 S i - O - N oxynitride glasses is discussed on the basis of the solid state MAS-NMR and neutron RDF analysis (Table 2). Fig. 3 shows the. 27A1 MAS-NMR spectra of the oxide and oxynitride glasses and the reference compounds containing various A I - O units. The 27A1 MAS-NMR spectra of both oxide and oxynitride glasses have a broad peak at about 60 ppm. The tailing on the upfield side is attributed to the distribution of aluminum sites in the glass. In addition, weak peaks appear at about 30 and 10 ppm, respectively. This indicates that the incorporation of

Table 1 Atomic distance, d, and coordination number, N, for S i - O and S i - N pairs in 20Na20 • 80SiO 2 glasses without and with 4.4 at.% N [16] Composition

Si-O

of glass

d (nm)

Nsi -

Si-N

20Na20.80SiO 2 20Na20 • 80SIO2 (4.4 at.% N)

0.1627 0.1626

3.91 3.43

O

NO-

1.74 1.71

Si

d (rim)

Nsi - N

NN - si

0.1709

0.39

2.42

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224

218

Table 2 Interatomic distance, d, and coordination number, N, for Si-O, S i - N A1-O and Y - O pairs in 17Y203 • 25A1203 - 58SIO 2 glasses without and with 5.0 at.% N [23]

Composition

Si-O

of glass

d (nm)

Nsi-o

d (nm)

Si-N Nsi_ N

NN-si

AI-O (%) mlO4

A105

mlO6

Y-O d (nm)

Nv_ 0

17Y203 • 25A1203 • 58SIO2 17Y203 - 25A1203 • 585iO 2

0.164 0.163

4.0 2.9

0.173

1.09

2.86

56 57

35 31

9 12

0.232 0.234

5.9 6.0

(5.0 at.% N)

nitrogen does not affect the coordination of Al atoms; AI atoms are coordinated only by oxygens. The NMR spectra of the reference crystals tell that the peaks at 0, 35 and 60 ppm correspond to AlO 6 (octahedral site of AI), AlO 5 (distorted trigonal bipyramid) and AlO 4 (tetrahedral site of Al) sites. Comparison of the spectra of glasses with those of reference crystals indicates that both oxide and oxynitride glasses contain AlO6, AlO 5 and AlO 4 groups. Table 2 shows that in the oxynitride glass the coordination number of Si atoms is less than 4, indicating that the N atoms enter the glass network, replacing oxygen atoms in the Y - A i - S i - O glass [23]. The presence of an RDF peak at about 0.172 nm due to the Si-N bond indicates that N atoms enter the network by combining with Si atoms. The formation of Si-N bonds is also supported by the downfield shift of the main peak of the 29Si NMR

MR

~AI o

Y-AI-SI-O-N

200

100

0

-100

-200

6 / ppm

Fig. 3. 27A1 MAS-NMR spectra of 17Y203.25AI203.58SiO2 glasses without and with 5.0 at.% N [23]. *, spinning side band.

spectra. The value of 2.9 (Table 2) for the average coordination number of Si atoms dearly indicates that three oxygens and one nitrogen are bonded to one silicon to form an SiO3N structural unit. This unit was found in L n - S i - O - N compounds by Dupree et al. [24]. Aujla et al. [25], based on the 298i NMR spectra, reported that in addition to SiO3N groups, SiO2N 2 groups may be present in the Y - S i O - N oxynitride glass. It can be stated that in the present Y - S i - A I - O - N glass, the SiO3N groups are predominant over SiO2N 2 groups. It is seen from Table 2 that the coordination number of Y atoms is about six in both oxide and oxynitride glasses,

2.3. Formation of AI-N bonds in oxynitride glasses Aluminum oxide is an important component of silicate glasses. A l 2 0 3 is classified as intermediate from the point of view of glass-forming ability of the oxide. It does not form glass by itself, but Al atoms enter the glass network by replacing Si atoms in silicate glasses. Therefore, the presence or absence of an A l - N bond within the network is a very interesting and important problem. Aujla et al. [25], based on the 295i NMR study, reported that a nitrogen atom is connected to two Si atoms and one Al atom in Y 1 . 0 3 A l l . 0 3 S i l . 0 3 N x O 6 _ x g l a s s , implying a preference for Si-N bonding compared with Al-N bonding. Hater et al. [13] found a similar tendency in the M g - A i - S i - O - N glass. An IR study by Loehman [7] and a Raman spectroscopic study by Rouxel et al. [26] showed that there is no A l - N bonding in glass. No unambiguous evidence for the existence of A l - N bonding in oxynitride glasses, however, has yet been found. We have taken up this problem in Y - A i - S i - O - N glass. As shown in Fig. 3, the 27A1NMR spectrum of

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224

crystalline AIN consisting of A I N 4 units shows a distinct peak at 120 ppm. If the A I - N bonds are present as A I ( O , N ) 4 structural units, it is expected that there should be a downfield shift of this peak or the peak at 60 ppm. In the 27A1NMR spectrum of the oxynitride glass, however, neither a new peak nor a shift of those peaks is observed in the region of 60-120 ppm, implying a lack of AI-N bonding. Here, the broad peak appearing at 120 ppm in both oxide and oxynitride glasses is attributed to a spinning side band (SSB). The IR spectra of oxide and oxynitride glasses support this result; no peak appears at about 700 cm-1 due to the stretching vibration of A1-N bond. This leads to the conclusion that a N atom has a much stronger tendency to form chemical bonds with Si than with A1 in Y - A 1 - S i O - N glass. The preferential bond formation of N atoms with Si atoms than with AI atoms leads to the question as to what happens when the glass does not contain Si atoms but only AI atoms as possible network-constructing cations. We have selected the compositions 61CaO. (39 - x)Al20 3 • 2xA1N (x = 0,1,2,3) and 15MgO • 61CaO • (39 - x)A1203 • 2xAIN (x = 0, 3, 5, 8) for this study. Doradowx et al. [27] determined the glass-formation region of C a - A 1 - O - N glasses and Durham and Risbud [28] studied the formation and properties of B a - M g - A I - O - N glasses, but they did not discuss the effect of incorporation of N on the glass structure. The 27A1 NMR spectra of alkalineearth aluminate glasses have shown that there are no A I ( O , N ) 4 units in the present glasses and N atoms are present as A 1 N 4 groups which are assumed to form clusters or very fine particles [29]. This result together with the above information can be summarized as follows: N atoms make bonds with Si atoms preferentially in glasses containing Si and A1 atoms. No bonding is possible between AI and N atoms. In aluminate glasses where no Si atoms are present, N atoms do not make bonds with A1 atoms forming networks.

3. Properties of oxynitride glasses 3.1. Elastic modulus

It is known that the elastic moduli and Vickers hardness of oxynitride glasses increase with increas-

219

150 I

AYS





~100 E

_/SiO=

c

g

-~NCS 0

1

2

3

4

5

Nitrogen content / wt% Fig. 4. Young's modulus as a function of nitrogen content of 36CaO.24AI203.40SiO 2 glasses with N content [11]; AYS: AI203-Y203-SiO 2 glass; NCS: 16Na20.10CaO.74SiO 2 glass.

ing N content [1]. So far, no exception has been found. An example is shown for Young's modulus of the 36CaO.24A1203 • 40SiO 2 glass in Fig. 4 [11]. In order to explain the higher elastic modulus of an oxynitride glass than that of the corresponding nonnitrided silicate glass, we have applied an ab initio molecular orbital calculation [30] to model molecules cut from the glass networks. The ab initio molecular orbital calculation using the Hartree-Fock wave function, a minimal basis set STO-3G and Gaussian 80 computer program system has been carried out on model molecules for the silicon oxynitrides, such as N[Si(OH)3] 3 (a), (HO) 3SiNHSi(OH) 3 (b) and (HN 2)3 SiOSi(NH 2)3 (c), and silica glass (HO)3SiOSi(OH) 3 (d), in order to estimate the interatomic potentials and bending force constants [17]. Fig. 5 shows two of the model molecules after optimization in terms of potential energy. Fig. 6 shows the interatomic potential as a function of the bridging bond angle of S i - X - S i (X = O or N). It is seen from Fig. 6(d) that the dependence of the potential for the S i - O - S i bonds on the bridging angle is very small, indicating that the angle can be changed freely. On the other hand, the steep potential curve shown in Fig. 6(a) suggests that the Si(O,N) 4 tetrahedra are fixed tightly at the part which contains the nitrogen atom in the silicon oxynitride glass. The S i - X - S i bending force constants, Ko, and Si-X stretching force constants, Kr, calculated from the potential data are shown in Table 3. The elastic modulus of the oxynitride glass is discussed on the basis of K o and K r values shown

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224

220

Table 3 Stretching and bending force constants for the bridging Si-X-Si groups in model molecules (X = O or N) [17]

0 " ~ ~ 1 0 ~ "

d

o ~

1.501A

t

~

'0"

Fig. 5. Perspective of the model molecules [17]: (a) N[Si(OH)3]3 model molecule for silicon oxynitride glass; (d) (HO)3SiOSi(OH) 3 model molecule for silica glass.

in Table 3. The bending force constant, Ko, in the (HO)3SiOSi(OH) 3 model molecule representing silica glass is 9.6 N / m , while K o in N[Si(OH)3] 3 is higher at 97.3 N/re. This difference indicates that the molecule containing the bridging N atom is not flexible at the bridging N atom, compared with the corresponding O atom, leading to a high elastic modulus of oxynitride glasses. This inflexible nature of the molecule at the bridging N atom may be attributed to the Si-N-Si bonding and the additional effect of the coordination of the N by three Si atoms. The calculation of the overlap population of the Si-N bond based on Mul-

~

'1 7 / 2

4

!, 8.

2

~

0

0

t

100

,

,~

120

Stretching force constant, K r ( N m -1)

Bending force constant, K0(Nm -1)

(HO) 2SiOSi(OH) 3 (HO)3SiNHSi(OH) 3 (H2N)3SiOSi(NH2) 3 N[Si(OH)3]3

743 610 663 507

9.60 28.9 12.4 97.3

1~.5"

0.11Q~

,"

Model molecules

--~

140"-

160

180

BrldglngangleSI-X-SII deg. Fig. 6. The potential energy curve for model molecules for silicon oxynitride glass and silica glass plotted as a function of the bridging angle [17].

liken's population analysis [31] has shown that the Si-N bond is more covalent than the Si-O bond [17]. It is assumed that the higher resistance of the Si-N-Si bond to bending than the Si-O-Si bond has its origin in the more covalent nature of the former bond. 3.2. Electrical conductivity of glass

Electrical properties of oxynitride glasses have been studied by only a few workers. Elmer and Nordberg [4] reported that incorporation of nitrogen lowers the electrical conductivity of silica glass. Leedecke and I~ehman [32] found that the dc conductivity of Y - A I - S i - O glasses increases on incorporation of N atoms. Thorp and Kenmuir [33] showed that the dielectric constant and ac conductivity of Mg-A1-Si-O-N and Ca-A1-Si-O-N oxynitride glasses increase with increasing nitrogen content. We have shown [18,19] with alkali silicate glasses, in which the conduction mechanism is well established, i.e., electrical conduction is attributed to alkali ion transport, that the conductivity increases with increasing N content. Fig. 7 shows the electrical conductivity at 40°C as a function of nitrogen content for 30Na2070SiO 2:N and 30Li20.70SiO 2 :N glasses. It is dearly seen that the ionic conductivity of oxide glasses increases with increasing nitrogen content. It should be noted that the activation energy for conductivity decreases with increasing N content. The effects of nitrogen on the electrical conductivity can be explained by the equation proposed by Anderson and Stuart [34]: AH = (2.1 - r ) Z Z o e 2 / 3 . 5 y ( r + ro) + 4~rGro( r - rD) 2,

(1)

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224

where y is the covalency parameter, G is the shear modulus, r is the ionic radius of the network-modifying cation and r D is the doorway radius (0.6 .~). Since Si-N bonds are more covalent than Si-O bonds as shown in the preceding section, the covalency parameter, y, would increase in oxynitride glasses. Hence, the electrostatic energy expressed by the first term on the right-hand side of Eq. (1) would be reduced on average by the incorporation of nitrogen, and this would result in a decrease in the activation energy for conduction and an increase in the conductivity. On the other hand, the shear modulus would increase on nitridation, which would cause an increase in the network strain energy, giving rise to an increase in the activation energy. Eq. (1) indicates that the contribution of both terms are dependent on the ionic radius, r. That is, the decreasing effect of the electrostatic energy is more significant than the increase in the network strain energy for smaller ions. This suggests that the incorporation of nitrogen decreases the electrostatic energy to a larger extent than the increase in the network strain energy for Li and Na ions. This also explains the experimental result showing that the effect of nitrogen on the electrical conductivity appears more pronouncedly for the Li system than for the Na system.

3.3. Chemical durability Chemical durability of a material is important when it is used in practice. Day's group [35,36] have -7.2

-7.6

u~

-8.C

\ NOA

Nn-SI-O-N

~ -7.2

,Li2~ ,36Li20"64SiOz

_~ -7.8

-8.q ~" LON LOAI 2

I

LI-SI-O-N I I

4 6 N/(O+N) / %

8

Fig. 7. The dc conductivities of the N a - S i - O - N and L i - S i - O - N glasses at 40°C as a function of the nitrogen content [19]: NON and LON: melted in N 2 atmosphere; NOA and LOA: melted in air.

221

~, 10 E o d~ E o

• Na-Si-O-N o Ca-AI°SI-O-N

5

,

o 0

1

2

3

4 Nitrogen content / wt%

Fig. 8. Weight loss of glasses as a function of N content for the immersion in 0.1N NaOH for 72 h at 40°C [20]: O, 30Na2070SiO 2 : N glass; O, 36CAO- 24AlzO 3 •40SiO 2 : N glass.

made many studies on the improvement of chemical durability of phosphate glasses by nitridation, but little work has been done on silicate oxynitride glasses. Our systematic studies of the chemical durability of N a - S i - O - N [37,38], L i - S i - O - N [19], K - S i O - N [20] and C a - A I - S i - O - N [20] glasses show that the effect of nitriding on the chemical durability varies with the glass systems; the durability is improved by incorporation of nitrogen in alkali silicate glasses, while almost no improvement is seen in alkaline-earth aluminosilicate glasses such as CaA I - S i - O - N [39] and M g - A 1 - S i - O - N glasses. An example showing this situation [20] is illustrated in Fig. 8. It is seen that 30Na20 • 70SiO 2 glass is much less durable to the alkaline solution than 36CAO. 2 4 A 1 2 0 3 " 4 0 S i O 2 glass when no nitrogen is incorporated. Addition of nitrogen improves the durability of the alkali silicate glass, while the durability of the aluminosilicate glass remains the same. It is remarkable that at nitrogen contents higher than 1 wt% the originally much less durable alkali silicate glass becomes much more durable than the aluminosilicate glass. It is interesting to see that both acid- and alkaliresistance are improved when the alkali silicate glass is nitrided [37], as seen from Fig. 9. The mechanism of corrosion in an acid solution is different from that in an alkaline solution. The dissolution of sodium in a hydrochloric acid solution and the formation of a reaction surface layer observed in the durability measurements [37] indicate that in acids ion exchange between Na + in glass and H 3 O + in the solution occurs [39,40]. The increase in durability to acids by

S. Saldca/Journal of Non-Crystalline Solids 181 (1995) 215-224

222

the incorporation of N atoms is assumed to be a result of suppression of the rate of ion-exchange H30+~,~--Na ÷ by the densification of glass structure and the increase in bulk modulus of glass. The corrosion of alkali-silicate glasses in the alkaline solution is described by the attack of hydroxyl ion (OH-) as shown below [39-41]: -=Si-O-Si= + OH- ~ --Si-O- + HO-Si=-.

/

r~ 10]~

0 Water • Acid

'S

~ AIke.

I\

81-~

I\

o[

0

,

1

"-- ~:==:'~ 2

0

O 0wt%N • 3.4 wt% N 40"C, 5 days

30 E 0

E _o

251

10

(2)

The improvement of the chemical durability in alkaline solution may be interpreted in terms of the formation of covalent Si-N bonding [17] in the glass structure which would reduce the rate of attack of the glass network by OH- anions. Fig. 10 shows the effect of the pH of the test solution on the weight loss for 24Ca0.16A1203 • 40SiO 2 glasses without N and with 3.4 wt% N [20]. It is seen that the glasses have a high resistance in the solution around pH = 7. It is also seen that the incorporation of N does not affect the weight loss in the solution throughout the pH range tested. This effect is explained by the presence of A1203 in the glass. Paul [41,42] discussed that A1203 is not stable in either acidic or basic solutions because its dissolved forms A13+ and AlOE have activity higher than the non-dissolved form AI(OH)3, respectively, in the acidic and basic solutions. If it is assumed that the nitrogen incorporated in the glass does not make bonds with AI and does not have a large effect on the bonding character, A1203 components would be

12

35

3

Nitrogen content I wt % Fig. 9. Weight loss of N a - S i - O - N glasses as a function of N content for immersion in water (40°C, 24 h), 0.1N HC1 (40°C, 24 h) and 0.1N NaOH (40°C, 72 h) [38]. Composition of glass: 30Na20 •70SiO 2 : N.

5 0 0

2

4

6

8

10

12

14

pH Fig. 10. Weight loss versus pH of the test solution for C a - A 1 - S i O - N glasses with and without N for immersion for 5 days at 40°C. The values for pH = 1 were taken for 3 days' immersion. The values for pH 5 and pH 9 were 0.0 m g / c m 2 even after 14 days' immersion [20].

dissolved in the acid and alkali solutions even if N were incorporated. This dissolution would cause the destruction of the glass framework structure, because the AI203 content is fairly high in the present calcium aluminosilicate glasses.

4. Application of oxynitride glasses It is easily presumed that the application of oxynitride glasses as materials may be based on (1) high mechanical strength combined with high thermal stability, and/or (2) excellent chemical durability. An attempt to produce oxynitride glass fibers has been made in Shimadzu Corporation, Kyoto, Japan [43,44]. Multifilaments of M g - C a - A 1 - S i - O - N glass are made by melting the glass in a molybdenum container in an N 2 atmosphere and drawing fibers through orifices on the bottom of a molybdenum bushing. Transparent, colorless fibers thus prepared contain 12 wt% nitrogen and are characterized by very high hardness of more than 1000 k g / m m 2, high Young's moduli of 180 GPa and high tensile strengths of 4 GPa. The oxynitride glass fibers may be useful for preparing composites such as translucent fiber-reinforced glasses [45] and fiber-reinforced aluminum metal [46]. Reinforced aluminum metal will provide lightweight, high-strength materials.

S. Sakka /Journal of Non-Crystalline Solids 181 (1995) 215-224 It is to be noted [43,46] that fabrication of transparent oxynitride fibers needs strict control of the starting materials and production conditions. The starting materials should not be c o n t a m i n a t e d with impurities. The m e l t i n g temperature, heating and cooling schedule and m e l t i n g time have to be strictly controlled. Otherwise one w o u l d obtain colored fibers which include fine metallic particles. The colored fibers show a wider strength distribution and a lower average strength than transparent h o m o g e n e o u s fibers. Oxynitride glass coatings appear suitable for protecting substrates of high-melting metals o w i n g to their high heat resistance. It has been s h o w n that the oxynitride glass adheres well to c h r o m i u m [47]. The preceding sections have s h o w n that nitriding increases the chemical durability of originally chemically weak oxide glasses to water and acid and alkaline solutions. This m e a n s that one can obtain relatively l o w - m e l t i n g but highly durable glasses that can be applied to coating and sealing of metals. At present, however, no attempt to apply such oxynitride glasses in practice in Japan is yet k n o w n .

5. Conclusions In nitrided silicate glasses, some N atoms are b o n d e d to three Si atoms and others to two. The mechanical and thermal properties are i m p r o v e d by N atoms b o n d e d to three Si atoms. O n nitriding, the elastic m o d u l i and hardness increase in all oxide glasses, while the i m p r o v e m e n t in chemical durability depends on the glass compositions. With high strength and high m e l t i n g temperature, oxynitride glass fibers m a y be applied to reinforcement of glasses and metals that melt at low temperature, such as a l u m i n u m .

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