Solid State Ionics 180 (2009) 1121–1124

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Solid State Ionics

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Redox reactions and inward cationic diffusion in glasses caused by CO and H2 gases

Morten M. Smedskjaer, Yuanzheng Yue ⁎Section of Chemistry, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark

⁎ Corresponding author. Tel.: +45 99408522; fax: +4E-mail address: yy@bio.aau.dk (Y.Z. Yue).

0167-2738/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.ssi.2009.05.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 January 2009Received in revised form 8 April 2009Accepted 13 May 2009

Keywords:Ionic diffusionSilicate glassesReductionSurface modification

We find that inward diffusion of network-modifying cations can occur in an iron-containing silicate glasswhen it is heat-treated in CO/CO2 (98/2 v/v) or H2/N2 (1/99 v/v) gases at temperatures around the glasstransition temperature. The inward diffusion is caused by the reduction of ferric to ferrous ions and thisdiffusion leads to formation of a silica-rich surface layer with a thickness of 200–600 nm. The diffusioncoefficients of the network-modifying divalent cations are calculated and they are different for the glassestreated in the CO and H2 gases. At the applied partial pressures of CO and H2, the H2-bearing gas creates thesilica-rich layer more effectively than the CO-bearing gas. The layer increases the hardness and chemicaldurability of the glass due to the silica network structure in the surface layer.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Pure silica glass (SiO2) possesses a wide range of applications due toexcellent thermal [1], chemical [2,3], and mechanical properties [4].However, the production of this glass usually requires very hightemperatures for melting and forming (up to 2400 °C). Addition ofalkali or earth alkaline oxides to SiO2 significantly lowers its meltingtemperature, but properties such as hardness and thermal and chemicaldurabilities are at the same time deteriorated. Although considerableprogress has been made in developing the sol–gel technology forsynthesis of silica glass [5], there is still a long way to go for realizingeconomically profitable mass production of bulk silica glasses. Recently,we have developed a technique for recreating a silica-rich layer on thesurface of a traditional glass, i.e., a polyvalent-bearing alkali-alkalineearth silicate glass by means of an internal diffusion process [6,7]. Theglass is heat-treated under a H2/N2 (1/99 v/v) atmosphere at atemperature near the glass transition temperature (Tg). Tg is the onsettemperature at which a glass transforms into a liquid. The reduction ofthe polyvalent element (e.g., Fe3+ to Fe2+) occurs as a result of both H2

permeation (dissolution and diffusion) and outward flux (from interiortoward the surface) of electron holes (Fig.1). The latter process requiresan inward diffusion of mobile network-modifying cations (primarilyMg2+, Fe2+, and Ca2+) to maintain charge neutrality, which in turncreates a silica-rich nanolayer on the glass surface. This techniqueprovides a way to produce a bulk glass with surface propertiesapproaching those of pure SiO2 in a more efficient, cheaper mannercompared with the conventional technique. The thickness of the silica-rich layer can be controlled by tuning the conditions of the heat-treatment [6,7]. The inward diffusion process is a mirror-image of theoutward diffusionprocess that occurs in silicate glasses duringoxidationof ferrous (Fe2+) to ferric (Fe3+) iron in atmospheric air [8–13].

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Aprerequisite for creating the silica-rich layer is the outwardfluxofelectron holes, i.e., that H2 permeation cannot completely govern thepolyvalent reduction process since this will prevent the motion ofelectron holes, and consequently, the inward cationic diffusion [6,7]. Atlow pressure, the concentration of dissolved H2 molecules in the glassis proportional to theH2 partial pressure in theH2/N2 gas [14].Wehavepreviously shown that when using 10 vol.% H2, the silica-rich layercannot be created. This is because theH2 partial pressure is so high thatall the Fe3+ ions are reduced entirely by H2 molecules before themodifying cations start to diffuse. However, two reduction processesoccur simultaneouslywhen the glass is heat-treated inH2/N2 (1/99): afast process due toH2 permeation and a slower onedue to outwardfluxof electron holes [6,7]. Thismeans that a proper H2 partial pressure is acrucial factor for ensuring the inward diffusion, and hence, theformation of the silica-rich nanolayer. In addition, the rate ofpermeation depends on the size of the gaseous element or molecule[15]. To reveal whether a gaswith larger reducingmolecules thanH2 ata higher pressure in the atmosphere can be used to induce theformation of the silica-rich layer, we apply a CO/CO2 (98/2 v/v)atmosphere as a reducing agent for heat-treatments at Tg of iron-bearing silicate glasses. Afterwards, we compare the concentrationprofile of the surface layer of the CO/CO2 treated glass with that of theH2/N2 (1/99) treated glass. The information on the dependence of thecreation of the silica-rich surface layer on the gas type and compositionis important for clarifying the mechanism of the inward diffusionprocess and for the application of the surfacemodification technology.

2. Experimental

Two glasses named 6wtFe and 3wtFe were prepared by meltingmixtures of analytical reagent-grade raw materials at 1500 °C underatmospheric air. Composition of the 6wtFe glass (in wt.%) is 69.4 SiO2,10.8 CaO, 9.3 MgO, 4.4 Na2O, and 6.1 Fe2O3, whereas that of 3wtFe

Fig. 1. Schematic representation of mechanism for reduction of a polyvalent element(A). Both gaseous permeation (dissolution and diffusion) and outward (from interiortowards the surface) flux of electron holes contribute to the reduction process. Me is anetwork-modifying element, h• is an electron hole, and H2 is the reducing gas molecule.

Fig. 2.UV–VIS–NIR spectra of 0.20 mm thick 6wtFe glass samples: untreated and heatedin H2/N2 (1/99) and CO/CO2 (98/2) at Tg=926 K for 16 h. The band positioned near1050 nm is due to a d→d transition of Fe2+. The inset shows the change in Fe2+

concentration (Δ(Fe2+)) as a function of the square root of the heat-treatment duration(ta0.5) for treatment in H2/N2 (1/99) (■) and CO/CO2 (98/2) (▲). Fe2+ concentrationswere calculated using the absorption coefficient of Fe2+ equal to 3.90 L mol−1 mm−1.For the glass treated in H2/N2 (1/99) at Tg for 8 h, Δ(Fe2+) was determined by threeindependent measurements. Based on these results, the accuracy of the determinationof Δ(Fe2+) is estimated to be±2 µmol/g.

1122 M.M. Smedskjaer, Y.Z. Yue / Solid State Ionics 180 (2009) 1121–1124

glass is 71.0 SiO2, 11.1 CaO, 9.6 MgO, 4.5 Na2O, and 3.2 Fe2O3. Here, alliron (Fe2+ and Fe3+) is reported as Fe2O3. Na2O and CaO wereintroduced into the batch using their respective carbonates. SiO2 wasintroduced as quartz, Fe2O3 as Fe2O3, and MgO as Mg(OH)2·(MgCO3)4·(H2O)5.

Conventional transmission 57Fe Mössbauer spectroscopy measure-ments on powdered samples were used to determine the iron redoxstate of the untreated iron-containing glasses. A constant accelerationspectrometer with a source of 57Co in rhodium was used. Thespectrometer was calibrated using a foil of α-Fe at room temperature.The ratio [Fe3+]/[Fetot], where [Fetot]=[Fe2+]+[Fe3+], was found tobe approximately 0.7 for both glasses. The Tg values of 6wtFe and3wtFe were measured using differential scanning calorimetry (DSC)according to the procedure of Yue [16], and found to be 926 K and921 K, respectively.

The obtained glasses were cut in cylinders and then ground by asix-step procedure with SiC paper under ethanol, followed bypolishing with 1 µm diamond suspension. Heat-treatments in theH2/N2 (1/99) atmosphere were conducted at 1 atm in an electricalfurnace. The glass samples were inserted into the cold furnace and thegas-flow was turned on. Heating and cooling of the furnace wereconducted at 10 K/min. Treatments in CO/CO2 (98/2) were conductedsimilarly, but the heating and cooling rate was 5 K/min. The partialpressure of oxygen was kept at a known value in the H2/N2 (1/99)atmosphere by using a Fe3O4/Fe2O3 redox buffer. Fe2O3 and Fe3O4

powders were mixed in the molar ratio 3:2 and placed inside thefurnace together with the samples. In the CO/CO2 (98/2) atmosphere,the oxygen partial pressure was controlled by the CO–CO2–O2

equilibrium.Fourier transform infrared (FT-IR) and ultraviolet-visible-near

infrared (UV–VIS–NIR) absorption spectra were measured usingdoubly polished 0.2 mm thick glass slides with Bruker Vertex 70 FT-IR and Analytik Jena UV–VIS–NIR Specord 200 spectrophotometers,respectively. From FT-IR spectra, the permeation of H2 and CO into theglasses can be investigated as incorporated OH and CO3 groups aredetectable in IR spectra [17,18]. UV–VIS–NIR spectra were recorded todetermine the change in the iron redox state as a function of heat-treatment conditions. The Fe2+ ion has a maximum absorption peaknear 1050 nm [19,20] but the position and intensity of this peak varywith glass composition. The absorption coefficients for our glasseswere not known and therefore the absorption coefficient of theLambert-Beer equation was calculated: A= c×ε×t, where A isabsorbance, c the concentration, ε the absorption coefficient, and tthe sample thickness. By using the [Fe3+]/[Fetot] ratio and the totaliron content, the concentration of ferrous iron was calculated in theuntreated 6wtFe glass. Plotting the absorbance near 1050 nm versusthe sample thickness (0.12, 0.20, 0.40, and 0.80 mm) gave a linear

relation (R2=0.997). From the slope of this plot (c×ε), the absorptioncoefficient was calculated to be 3.90 L mol−1 mm−1.

To study the cationic diffusion processes, compositional analysis ofthe glass surfaces was carried out using electron-gas secondaryneutral mass spectroscopy (SNMS) with an INA 3 (Leybold AG)instrument. The analyzed area had a diameter of 5 mm and wassputtered using Kr plasma with an energy of ~500 eV. The timedependence of the sputter profiles was converted into depthdependence by measuring the depth of the crater at 10 differentlocations on the same sample with a Tencor P1 profilometer.

Two properties of the heat-treated glasses were tested. Vickershardness was measured 25 times for each sample using a StruersDuramin 5 microindentor at a load of 0.25 N and a hold time at themaximum load of 5 s. The lengths of the indentation diagonals weremeasured using an optical microscope (reflection method). Chemicaldurabilitywas tested bymeasuring leached amounts of Na+ andMg2+

ions after dissolution in 0.25 M HCl and KOH solutions. The sampleswere immersed in plastic containers with 20 cm3 test solution for each1 cm2 of the glass surface area. The containers were mounted on athermostatic shaking assembly at 90 °C (agitated at 100 ppm) and after12 h, the sampleswere removed from the solutions. Atomic absorptionspectroscopy (AAnalyst 100, Perkin Elmer) was employed to measurethe concentrations of Na+ and Mg2+ in the test solutions.

3. Results and discussion

Fig. 2 shows UV–VIS–NIR spectra of glasses heat-treated at Tg for16 h in H2/N2 (1/99) or CO/CO2 (98/2), respectively. A maximumabsorption peak is seen near 1050 nm, which is attributed to theexistence of the Fe2+ ions [19,20]. When the glass is heat-treated inH2/N2 (1/99) or CO/CO2 (98/2) for a given duration, the intensity ofthe Fe2+ band increases, indicating that Fe3+ is reduced to Fe2+. Thechange in Fe2+ concentration (Δ(Fe2+)) increases approximatelylinearly with the square root of the heat-treatment duration (ta0.5),implying that diffusion-limited kinetics occurs (see inset of Fig. 2).Fig. 3 illustrates the effect of heat-treatments on the IR spectra of theglasses. Bands at 3550 and 2850 cm−1 are caused by O–H stretchingvibrations of weakly and strongly hydrogen-bonded OH species,respectively [21]. Bands near 1860 and 1630 cm−1 can be assigned tocombination modes and overtones of the silica glass matrix [22]. The

Fig. 3. FT-IR absorption spectra of 0.20 mm thick 6wtFe glass samples: untreated andheated in H2/N2 (1/99) and CO/CO2 (98/2) at Tg=926 K for 16 h. The bandassignments are discussed in the text.

Fig. 4. SNMS depth profiles of the 6wtFe glass heat-treated in CO/CO2 (98/2) atTg=926 K for 16 h. The curves are plotted as concentration of the element at a givendepth divided by the concentration of the same element in the bulk of the glass (C/Cbulk).

1123M.M. Smedskjaer, Y.Z. Yue / Solid State Ionics 180 (2009) 1121–1124

bands positioned at 1510 and 1425 cm−1 are assigned to vibrations ofchemically dissolved carbonate species. One of the carbonate oxygensis attached to a tetrahedral site via a non-bridging oxygen (NBO). Thiscomplex is associated with Ca2+ [18]. The following reactions, writtenin the notation of Hess [23], account for the observed bands as a resultof treatments in H2/N2 (1/99) and CO/CO2 (98/2):

H2 þ 2NaFe3þO2 þ 4SiOSi→4SiOðFe2þÞ0:5 þ 2SiOHþ 2SiONa

CO þ 2NaFe3þO2 þ SiOCa0:5 þ 3SiOSi→4SiOðFe2þÞ0:5 þ SiCO3Ca0:5

þ 2SiONa:

In the notation, the formulas depict the bonding environment of theoxygen anions. NaFe3+O2 represents a Fe3+, which is tetrahedrallycoordinated with oxygen and charge-balanced by Na+. SiOSi corre-sponds to a bridging oxygen connecting two silica tetrahedra. SiOH is asilica tetrahedron containing a hydroxyl group. SiCO3Ca0.5 is a carbonatespecies connected to a NBO and Ca2+. SiO(Fe2+)0.5, SiONa, and SiOCa0.5represent that Fe2+ (octahedral coordination), Na+, and Ca2+ areconnected to aNBO, respectively. In summary, the results showthatbothH2 and CO are capable of permeating into the glass. Fe3+ is reduced to agreater extent in H2/N2 (1/99) than in CO/CO2 (98/2), even though theCO partial pressure is much higher than that of H2. This is explained bythe faster permeation rate of a H2molecule due to its smaller size. Basedon the covalent radii of H, C (sp), and O [24], we have calculated thelengths of H2 and CO molecules to be 1.2 and 2.7 Å, respectively.

The SNMS depth profile of the 6wtFe glass heat-treated in CO/CO2

(98/2) at its Tg for 16 h is shown in Fig. 4. A pronounced decrease ofthe concentration of Mg2+, Ca2+, and Fe2+ towards the surface isobserved (thickness: 300–350 nm). Na+ also diffuses towards theinterior. Even though alkali ions are normally found to be faster thanearth alkaline ions in glasses due to their lower charge [25,26], thediffusion depth of Na+ is smaller than that of Mg2+, Ca2+, and Fe2+,which is in agreement with our previous studies [6,7,27]. The inwarddiffusion occurs to charge-balance the outward flux of electron holes,and the charge might be most effectively transferred by the divalentcations.

It should be noted that an enrichment of Na+ is observed in thedepth interval from 100 to 150 nm. This is ascribed to the depletion ofMg2+, Ca2+, and Fe2+ ions in this range because their depletion causesa relatively high concentration of Na+ ions. It should also be notedthat the surface depletion of network-modifying cations is not due tothe polishing procedure for two reasons. First, the glass was groundusing SiC papers under ethanol and polished using a diamond paste,

i.e., no leaching of cations should occur. Second, a SNMS profile of theuntreated glass does not show any inward diffusion of cations [7].

The SNMS profile of the glass treated in CO/CO2 (Fig. 4) indicatesthat themechanism of Fe3+ reduction in CO/CO2 (98/2) is the same asthat in H2/N2 (1/99). This mechanism is shown in Fig. 1. The internalreduction of Fe3+ generates electron holes (h•). An outward flux of h•

occurs, which is driven by the gradient in oxygen activity across thereaction zone. h• are filled by electrons released by ionic oxygen at thesurface since oxygen is released into the reducing atmosphere as CO2.The outward flux of h• is accompanied by inward flux of network-modifying cations to maintain the charge-balance. Hence, the inwardcationic diffusion is driven by reduction of the high valence to the lowvalence state of the polyvalent cation. To explore whether or not thereaction is rate-limited by the diffusion of divalent cations, diffusioncoefficients for the divalent cations should be calculated andcompared to known values of diffusion coefficients for divalentcations in similarly polymerized glasses. The diffusion coefficient for adivalent cation (DM2+) can be calculated by using the followingequation [8,28]:

DM2+ =Δn2

XM2+ Δt lnpWO2p VO2

� � ð1Þ

where Δξ is the thickness of the modifier layer, XM2+ is the cation molefraction of the divalent cation M2+, Δt is the reaction time, p′O2

is thepartial pressure (i.e., activity) of oxygen at the free surface, and p″O2

ispartial pressure of oxygen at the internal reaction front. p′O2

is fixed bythe CO–CO2–O2 equilibrium and is equal to 5·10−27 bar at Tg=926 K.p″O2

depends on the initial iron redox ratio and is calculated to beapproximately 5·10−3 bar at Tg=926 K [27]. Inserting these valuesinto Eq. (1) gives an diffusion coefficient of Fe2+ cations in CO/CO2

(98/2) of ~1·10−18 m2/s. The value agrees well with diffusionmeasurements for divalent, network-modifying cations in glasses ofsimilar polymerization [9,29]. This provides clear evidence for themechanism illustrated in Fig. 1. Hence, both CO permeation andoutward flux of electron holes contribute to the reduction of Fe3+.

In summary, the inward cationic diffusion causes the creation of asilica-rich surface layer in the iron-containing glasses. For the 3wtFeglass treated in CO/CO2 (98/2) at its Tg for 16 h, the layer thickness is~200 nm. This implies that when lowering the concentration of Fe3+

ions, the extent of divalent ionic diffusion decreases, and therefore,the layer becomes thinner. The layer is also created when the 6wtFeglass is heat-treated in H2/N2 (1/99). However, in this case thethickness is ~600 nm at Tg for 16 h, which gives a value of DM2+ of

Table 1Effect of the atmosphere of the heat-treatment on the Vickers hardness (Hv) andchemical durability of the 6wtFe glasses.

Property Untreated 98 vol.% CO 1 vol.% H2

Hv (GPa) 8.9±0.2 9.3±0.2 9.9±0.3C(Na+)acid (mg/L) 8.7±0.3 5.1±0.2 1.9±0.1C(Mg2+)alkali (mg/L) 2.4±0.3 1.6±0.1 1.3±0.1

The treated samples have all been heated at Tg=926 K for 16 h. Chemical durability ofthe glasses is expressed by the leached amount of Na+ after 12 h in 0.25 M HCl solution(C(Na+)acid) and Mg2+ after 12 h in 0.25 M KOH solution (C(Mg2+)alkali).

1124 M.M. Smedskjaer, Y.Z. Yue / Solid State Ionics 180 (2009) 1121–1124

~5·10−18 m2/s [27]. This suggests that H2 be more effective increating the silica-rich surface than CO even though the oxidationpotential of CO is larger than that of H2 at 926 K (Tg) [30]. Thedifference in the layer thickness must then be due to the difference insize of the two gaseous molecules. To neutralize the electron holes atthe surface, H2 and CO molecules must first penetrate into theuppermost surface layer, subsequently be dissolved in the structure,and simultaneously contact and reduce the ferric ions in the glassstructure. The penetration, and hence, reduction process is easierwhen the molecule is small.

The hardness and chemical resistance of the untreated and heat-treated samples are displayed in Table 1. From the structural point ofview, the earth alkaline and alkali cations disrupt the continuous Si–Orandom network, and so introduce NBOs to the glasses. Their removalfrom the surface clearly increases the hardness and chemicalresistance of the glasses. The increase is most pronounced as a resultof the H2-treatment as treatment in this atmosphere creates thethickest silica-rich layer.

4. Conclusions

A silica-rich surface layer can be created by heat-treating an iron-bearing glass at its Tg in both CO- and H2-containing atmospheres. Thelayer is created due to the inward diffusion of network-modifyingcations. By calculating the diffusion coefficient for the divalent cations,we have clarified the mechanism of the inward diffusion. The glasssurface becomes structurally more polymerized due to the removal ofnetwork-modifying cations from the surface. Consequently, thehardness and chemical durability of the glasses are enhanced. Inaddition, it is found that the extent of the inward diffusion is larger as aresult of the H2-treatment than of the CO-treatment. This is attributedto the fact that H2 has a smaller size than CO, so that the former morereadily reduces the ferric ions in the surface structure than the latter.

Acknowledgements

We thank N. Bonanos for performing heat-treatments in CO/CO2,T. Peter for performing SNMS measurements, and J. Deubener forsupporting experiments and useful discussions. This work wassupported by the International Doctoral School of Technology andScience at Aalborg University under Ph.D. stipend No. 562/06-FS-28045.

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