Effect of melting temperature and time on iron valence and crystallization of iron phosphate glasses

E�ect of melting temperature and time on iron valence andcrystallization of iron phosphate glasses

C.S. Ray *, X. Fang, M. Karabulut, G.K. Marasinghe, D.E. Day

Department of Ceramic Engineering, Department of Physics and Graduate Center for Materials Research, University of Missouri-Rolla,

Rolla, MO 65409, USA

Received 12 March 1998; received in revised form 29 March 1999

Abstract

The e�ect of melting temperature and time on iron valence, dissolution rate (DR) in deionized water, and crystal-

lization of iron phosphate glasses was investigated using a 40Fe2O3±60P2O5, mol%, batch composition. The concen-

tration of Fe2� ions in these glasses increased from 17% to 57% as melting temperature increased from 1150°C to

1450°C, but remained nearly constant at about 20% for melting times longer than 1 h at 1200°C. Measurements by

di�erential thermal analysis (DTA) combined with X-ray di�raction (XRD) and thermogravimetric analysis (TGA)

showed that these glasses crystallized to Fe3(P2O7)2 and Fe4(P2O7)3 when heated in nitrogen between 600°C and 820°C,

but with continued heating in air at 820°C the Fe3(P2O7)2 changed to Fe(PO4), which produced a weight gain in the

sample associated with the oxidation of Fe2� to Fe3� ions. The DR (in deionized water) of these glasses was generally

very low (�10ÿ9 g cmÿ2 minÿ1) and nearly independent of the relative concentration of Fe2� or Fe3� ions, but decreased

with total iron content. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

Iron phosphate glasses are candidates [1±5] forimmobilizing intermediate and high level nuclearwastes. Currently, the only glasses being used forsuch applications are the borosilicates [6±11].However, borosilicate glasses may not be suitable[4,5] or cost e�ective for vitrifying certain types ofhigh level nuclear wastes (HLW), particularlythose containing signi®cant amounts of phos-phates, iron oxide, ¯uorine, and heavy metal ox-ides such as Bi2O3 and UO2. These componentsgenerally have a low solubility in borosilicateglasses, which e�ectively limits waste loading to a

low percentage such that the radioactive wastevolume becomes too large [5].

There is also evidence [12] that P2O5 causesliquid±liquid immiscibility (phase separation) inborosilicate melts. Phase separation in a glass candrastically reduce its chemical durability which isone of the properties of prime importance towhether a glass is suitable for vitrifying nuclearwastes. Besides avoiding the problem of phaseseparation, the waste loading can be increasedconsiderably in iron phosphate glasses since theyuse the P2O5 and Fe2O3 contained in the waste.For a comparable waste loading, the volume of aniron phosphate wasteform would be only 25% ofthat for a borosilicate wasteform [5]. Furthermore,the amount of additional material required toproduce the borosilicate wasteform such asSiO2, B2O3, and alkalies, was 6 to 7 times more

Journal of Non-Crystalline Solids 249 (1999) 1±16

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Tel.: +1-573 341 6432; fax: +1-573

341 2071; e-mail: [email protected]

0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 0 4 - X

than what was required for the iron phosphatewasteform.

The lower chemical durability and usuallyhigher crystallization tendency of many phosphateglasses, compared to borosilicate glasses, are ofconcern to using iron phosphate glasses as an al-ternative host for vitrifying high level nuclearwastes (HLW). Unlike conventional phosphateglasses, however, the iron phosphate glasses[13,14] have a chemical durability (in water at90°C) that equals or exceeds that of the borosili-cate glasses now used for vitrifying HLW. Theexceptionally high chemical durability of the ironphosphate glasses is believed [14,15] to be due tothe more hydration resistant Fe±O±P bonds whichreplace the more easily hydrated P±O±P bondspresent in non-iron containing conventionalphosphate glasses. Chemical durability of devitri-®ed iron phosphate wasteforms is close to [4] thatof existing glassy wasteforms, which suggests thatthe usefulness of iron phosphate glasses in vitri-fying nuclear wastes will not diminish if thewasteform crystallizes.

Iron phosphate glasses melted in air contain amixture of Fe2� and Fe3� ions even when the batchmaterials contain only Fe2� or Fe3�. Equilibriumbetween Fe2� and Fe3� redox couple depends up-on melting temperature, time and atmosphere.Any variation in these melting conditions, canchange the structure and properties of iron phos-phate glasses.

It has been reported [14] that a 40Fe2O3±60P2O5, mol%, glass has the highest chemical du-rability of the binary iron phosphate glasses. It wasfound [16,17] that the Fe2�/(Fe2� + Fe3�) ratio forthe glasses prepared by melting the 40Fe2O3±60P2O5, mol%, batch at 1200°C for 2 h in air, ni-trogen or oxygen was nearly the same (between 0.2and 0.3), which suggests that the e�ect of thesemelting atmospheres on the relative concentrationsof the Fe2� and Fe3� ions is negligible for this ironphosphate glass. The e�ect of melting temperatureor time on iron valence in iron phosphate glasses isunknown.

The present work was undertaken to determinehow the Fe2� (or Fe3�) concentration in ironphosphate glasses changes with melting temperatureor time using 40Fe2O3±60P2O5, mol%, composition

which contained only Fe3� ions in the batch.Crystallization behavior and chemical durability inwater at 90°C were also measured for these glasses.The purpose of this work was to understand howthe concentration of iron valence ions, crystalliza-tion, and chemical durability of the iron phosphateglasses are a�ected by melting history.

2. Experimental procedure

2.1. Glass melting

A well mixed batch (about 50 g) of a 40Fe2O3±60P2O5, mol%, composition was prepared bymixing crystalline powders of reagent grade Fe2O3

and P2O5. This mixture was melted in air at tem-peratures between 1150°C and 1450°C for 1 h inhigh density alumina (Coors AD998) crucibles.The ¯uid melt (<10 poise) was cast onto a coldsteel plate to form bars approximately 1 cm ´ 1cm ´ 5 cm which were annealed by holding at450°C for 2 h then cooling slowly to room tem-perature at an approximate rate of 1°C/h. Glasseswere also melted at 1200°C in air for times between1 and 24 h. There was no noticeable change in theblack color of the glasses when melted either atdi�erent temperatures or for di�erent times at1200°C. The amorphous character of the glasseswas con®rmed by X-ray di�raction analysis(XRD). No visible corrosion or chemical attack ofthe alumina crucible by the melt was observed, buta small amount of alumina (<2 wt%) was detectedin these glasses when analyzed by energy dispersiveX-ray analysis (EDAX). Compositional analysisby EDAX at randomly selected locations on apolished cross section showed these glasses to behomogeneous when melted at 1150°C for 1 h.

Compositions of selected glasses were measuredby wet chemistry using inductively coupled plasmaemission spectroscopy (ICP) as given in Table 1.The measured compositions in Table 1 are inagreement with batch composition.

2.2. M�ossbauer measurements

Binary Fe2O3±P2O5 batch composition changesto a ternary FeO±Fe2O3±P2O5 composition during

2 C.S. Ray et al. / Journal of Non-Crystalline Solids 249 (1999) 1±16

melting as some of the Fe3� ions in the batch arereduced to Fe2� ions. The concentration of Fe2�

and Fe3� ions in the glasses was calculated fromthe M�ossbauer spectra obtained at 23°C on anASA600 spectrometer. The spectrometer, whichutilized a room temperature 50 mC rhodium ma-trix cobalt-57 source, was calibrated at 23°C withan a-iron foil and the line width of the a-ironspectrum was 0.27 mm/s. M�ossbauer absorbers ofapproximate thickness 120 mg/cm2 were preparedusing powders of <50 lm particles of each glass.The M�ossbauer spectra were ®tted with the mini-mum number of broadened paramagnetic doubletsnecessary (usually eight) to match the absorptionenvelope with a mis®t of less than 0.003 as dis-cussed elsewhere [17].

The glass composition was calculated using themeasured concentration of Fe2� and Fe3� fromM�ossbauer spectra assuming that the original Fe/Pratio (0.67) of the batch composition was retainedin the glass. For example, the composition of aglass, which contained 20% Fe2� was calculated asfollows. First, 20% or 16 of the original 80 Fe3�

ions were converted to Fe2� ions and considered toform FeO. The remaining 64 Fe3� ions are con-sidered to remain as Fe2O3. No change in P2O5

content is assumed thus the ®nal composition ofthe glass will be 16FeO±32Fe2O3±60P2O5 or14.82FeO±29.63Fe2O3±55.55P2O5, mol%.

2.3. Di�erential thermal analysis (DTA)

Crystallization temperature for each glass wasmeasured by DTA (DTA-7, Perkin-Elmer Cor-poration, Norwalk, CT) at a heating rate of 10°C/min in a ¯owing (30 cm3/min) nitrogen atmo-

sphere. A typical sample weight of 60 mg and thesame particle size (<50 lm) glass powder as wasused for the M�ossbauer measurements, were usedfor DTA. A high purity alumina powder suppliedby Perkin-Elmer Corporation was used as thereference material. The temperature of the instru-ment was calibrated periodically at the meltingpoint of aluminum (660.4°C) and gold (1064.4°C).

DTA measurements were also conducted atheating rates of 2, 4, 6, 10, and 15°C/min for someof the glass samples to calculate their activationenergy for crystallization, E. An equation given byKissinger [18]

ln�T 2p =/� � E=RTp � const: �1�

was used for data analysis and calculating E. InEq. (1), Tp is the temperature at the maximum ofthe DTA crystallization peak measured at a heat-ing rate of / and R is the gas constant. As can beseen from Eq. (1), a plot of ln(T2

p//) vs 1/Tp shouldbe a straight line, whose slope is E/R.

The use of Kissinger equation for determining Ehas been criticized [19,20] to have questionablevalidity and suspected to yield misleading results.However, it has been demonstrated [21,22] that thevalues of E determined using Kissinger equationcan be accurate within �5% if the DTA experi-ments are conducted on samples that are eithercomposed of very ®ne particles (<50 lm) or fullynucleated (annealed) prior to DTA measurements.Both of these conditions, namely, a prior nucle-ation and the use of very ®ne particles for the glasssamples were ful®lled here.

Thermogravimetric analysis (TGA) was per-formed on glasses melted at 1150°C and 1400°Cin ¯owing air or nitrogen at a heating rate of

Table 1

Chemical analysis of the glasses prepared by melting a 40Fe2O3±60P2O5 mol% composition at di�erent temperatures

Melting temp. (°C) Melting time (h) Batch composition (wt%) Analyzed composition by ICP a (wt%)

Fe2O3 P2O5 Fe2O3 P2O5 Al2O3 Others b

1200 1 42.86 57.14 41.78 57.81 0.23 0.18

1300 1 42.86 57.14 39.83 58.78 1.18 0.21

1450 2 42.86 57.14 39.42 56.50 3.85 0.23

a Chemical analysis by coupled plasma emission spectroscopy (ICP) was performed by ACME Analytical Laboratories Ltd., 852 East

Hastings St., Vancouver BC, Canada V6A 1R6.b Others: SiO2, CaO, MgO, Na2O and K2O.

C.S. Ray et al. / Journal of Non-Crystalline Solids 249 (1999) 1±16 3

10°C/min using a Mettler type TA-1 simultaneousDTA/TGA instrument. The purpose of the TGAmeasurements was to determine whether the oxi-dation/reduction reaction that occurs in ironphosphate melts, also occurs in glasses (solid)when heated in air or nitrogen.

2.4. Crystallization

Selected glass samples were crystallized byheating at temperatures corresponding to theirDTA crystallization peak(s) for 24 h in both airand nitrogen. The crystallization products wereidenti®ed by XRD using a Scintag XDS2000 X-raydi�ractometer.

2.5. Chemical durability

Chemical durability of the glasses was evaluatedfrom dissolution rate (DR) in deionized waterat 90°C. A high DR value indicates a poorchemical durability. Bulk samples approximately10 mm ´ 10 mm ´ 1 mm in size were cut fromannealed glass bars. The surfaces of these sampleswere ground with 240 grit SiC paper, with oil asthe cooling agent, washed ultrasonically in acetonefor several minutes, dried at 110°C for 2 h, andthen weighed to �0.01 mg.

The polished, bulk glass sample was suspendedby a plastic thread in about 100 ml of deionizedwater in a polythelene bottle for 4, 8, and 16 days at90°C. The bottle containing the glass sample wasgently agitated once a day. At each time interval,the water was removed and the samples were wa-shed with acetone, dried, and weighed. Assumingthe dissolution was linearly proportional with time,the DR for the bulk glasses was calculated from themeasured weight loss, DW (g), total sample surfacearea, S (cm2), and the immersion time, t (min),using the following equation [4]:

DR � DW =�S � t� �g=cm2=min�: �2�

The DR for the crystallized forms prepared byheating the polished bulk glass samples at tem-peratures indicated in their DTA crystallizationpeaks, was also measured using the proceduresdiscussed above. All measurements were made induplicate and the average DR is reported herein.

3. Results

M�ossbauer spectra for glasses made by meltingat 1150°C, 1350°C, and 1450°C for 1 h are shownin Fig. 1. The Fe2� and Fe3� components shown inthe top spectra (A) in Fig. 1 are the sum of thefour Lorentzian doublets used to ®t each of thetwo iron sites. The weighted average M�ossbauerhyper®ne parameters, namely, the quadrupolesplitting (QS) and isomer shift (IS) for the Fe2�

and Fe3� ions, and the fraction of Fe2� ionspresent in the glass melted at di�erent tempera-tures (for 1 h) are given in Table 2. Also shown inTable 2 is the calculated composition of each glassbased on their Fe2� and Fe3� concentration asmeasured by M�ossbauer spectroscopy.

Fig. 2 shows the M�ossbauer spectra for glassesmelted at 1200°C for 1 and 24 h. The weightedaverage M�ossbauer hyper®ne parameters forglasses melted at 1200°C for times from 1 to 24 hare given in Table 3 along with the calculated glasscompositions. The concentration of Fe2� ions, ascalculated from the M�ossbauer spectra, for theglasses melted between 1150°C and 1450°C for 1 hand at 1200°C for di�erent times are shown sepa-rately in Fig. 3.

The fraction of Fe2� ions increased from 17%for the glass melted at 1150°C to 57% for the glassmelted at 1450°C, see Table 2 and Fig. 3. Thisincrease is clearly noticeable in the larger absorp-tion lines for Fe2� in the M�ossbauer spectra(Fig. 1) for the glasses melted at higher tempera-tures. Melting time has little e�ect on theM�ossbauer spectra (Fig. 2) or in the relative con-centration of iron ions (Table 3, Fig. 3) in theseglasses. As a result, the composition of the glassesprepared by melting at di�erent temperatureschanges widely (Table 2) compared to that for theglasses prepared by melting for di�erent times at1200°C (Table 3). However, the values for QS orIS for Fe2� and Fe3� ions (Tables 2 and 3), whichindicate the near coordination environment of theFe2� or Fe3� ions did not change signi®cantly forthese iron phosphate glasses when melted either atdi�erent temperatures or for di�erent times.

The DTA patterns obtained at a heating rate of10°C/min for glasses melted at 1150°C, 1250°C,and 1400°C for 1 h and for the glasses melted at


Fig. 1. M�ossbauer spectra at 23°C for glasses prepared by melting a 40Fe2O3±60P2O5, mol%, composition at 1150°C, 1350°C, and

1450°C in air for 1 h. The sum of the four Lorentzian doublets used to ®t each of the Fe2� and Fe3� sites is shown for one of the spectra

(curve A).

Table 2

Weighted average M�ossbauer hyper®ne parameters for glasses made by melting a 40Fe2O3±60P2O5, mol%, composition for 1 h at the

temperatures shown. Also shown are the fraction of Fe2� (as measured by M�ossbauer spectroscopy) and the calculated composition of

each glass

Melting QS (mm/s) (�0.05) IS (mm/s) (�0.05) Fraction of Fe2� (�0.02) Calculated glass composition (mol%)temp. (°C)

Fe2� Fe3� Fe2� Fe3� Batch Glass FeO Fe2O3 P2O5

1150 2.07 0.90 1.24 0.38 0 0.17 12.7 31.1 56.2

1200 2.07 0.90 1.24 0.38 0 0.19 14.1 30.1 55.8

1250 2.09 0.91 1.24 0.39 0 0.29 20.8 25.4 53.8

1300 2.09 0.92 1.25 0.39 0 0.35 24.6 22.8 52.6

1350 2.20 0.87 1.21 0.43 0 0.42 28.8 19.8 51.4

1400 2.14 0.93 1.24 0.40 0 0.50 33.3 16.7 50.0

1450 a 2.14 0.94 1.24 0.41 0 0.57 37.1 14.0 48.9

a Melted for 2 h. QS: quadrupole splitting; IS: isomer shift.


Fig. 2. M�ossbauer spectra at 23°C for glasses prepared by melting a 40Fe2O3±60P2O5, mol%, composition at 1200°C in air for 1 and 24

h. The sum of the four Lorentzian doublets used to ®t each of the Fe2� and Fe3� sites is shown for the top spectra (curve A).

Table 3

Weighted average M�ossbauer hyper®ne parameters for glasses made by melting a 40Fe2O3±60P2O5, mol%, composition at 1200°C for

the times shown. Also shown are the fraction of Fe2� (as measured by M�ossbauer spectroscopy) and the calculated composition of each

glass

Melting

time (h)

QS (mm/s) (�0.05) IS (mm/s) (�0.05) Fraction of Fe2� (�0.02) Calculated glass composition (mol%)

Fe2� Fe3� Fe2� Fe3� Batch Glass FeO Fe2O3 P2O5

1 2.07 0.90 1.24 0.38 0 0.19 14.1 30.1 55.8

3 2.06 0.90 1.25 0.39 0 0.19 14.1 30.1 55.8

6 2.05 0.92 1.25 0.38 0 0.21 15.5 29.2 55.3

9 2.08 0.91 1.25 0.38 0 0.22 16.2 28.7 55.1

12 2.15 0.89 1.22 0.40 0 0.24 17.5 27.8 54.7

24 2.09 0.91 1.24 0.40 0 0.26 18.8 26.8 54.4

QS: quadrupole splitting; IS: isomer shift.


1200°C for 1, 9, and 24 h are shown in Figs. 4 and5. As was observed in the M�ossbauer spectra, thereis a considerable change in the DTA patterns forthe glasses melted at di�erent temperatures, but nosigni®cant change is apparent in the DTA patternsfor the glasses melted at 1200°C for di�erent times.The DTA curves for the glasses melted below

1300°C generally contained two crystallizationpeaks, but only one sharp crystallization peak wasobserved for the glasses melted P1300°C. Thelower (®rst) temperature peak for the glassesmelted at di�erent temperatures clearly shifted tolower temperatures with increasing melting tem-perature i.e., with increasing Fe2� concentration inthe glass, Fig. 4. A similar decrease in the tem-perature of the lower (®rst) crystallization peakwas also observed with increasing melting time at1200°C, but this decrease was small compared tothat for the glasses melted at di�erent tempera-tures, compare the ®rst crystallization peaks inFigs. 4 and 5. The (second) crystallization peak athigher temperature in these glasses, however, doesnot appear to depend strongly either on the melt-ing temperature or the melting time in any sys-tematic way, and occurs at 820 � 5°C for all theglasses.

The XRD patterns for the glasses crystallized innitrogen (same atmosphere as used for the DTAmeasurements) for 24 h at temperatures indicatedby their DTA exotherms (about 650 and 820°C forthe glass melted at 1150°C, for example) revealthat the low temperature DTA peak is due to thecrystallization of ferrous/ferric pyrophosphate,Fe3(P2O7)2, and the high temperature peak is dueto the crystallization of ferric pyrophosphate,

Fig. 3. Concentration of Fe2� ions, as measured from the

M�ossbauer spectra, as a function of the melting temperature for

1 h (top axis) and melting time at 1200°C (bottom axis) for the

glasses prepared by melting a 40Fe2O3±60P2O5, mol%, com-

position. Typical experimental error is shown by the error bar

on one data point.

Fig. 4. DTA at 10°C/min in nitrogen for glasses prepared by

melting a 40Fe2O3±60P2O5, mol%, composition at (A) 1150°C,

(B) 1250°C, and (C) 1400°C for 1 h. The low (®rst) and high

(second) temperature DTA peaks are due to the crystallization

of Fe3(P2O7)2 and Fe4(P2O7)3, respectively.

Fig. 5. DTA at 10°C/min in nitrogen for glasses prepared by

melting a 40Fe2O3±60P2O5, mol%, composition at 1200°C for

(A) 1 h, (B) 9 h and (C) 24 h. Notice that the temperature and

size of the DTA peaks do not change signi®cantly with in-

creasing melting time.


Fe4(P2O7)3. The original black color of the glassdid not change when it was heated at the lower(650°C) crystallization temperature in nitrogen,but became deep blue when it was heated at thehigher (820°C) crystallization temperature. Thissuggests that the color of the Fe3(P2O7)2 crystals isblack and that of the Fe4(P2O7)3 crystals is blue.

When the sample heated previously in nitrogenat the higher (820°C) crystallization temperaturefor 24 h was given an additional heat treatment atthe same temperature for 24 h in air, the color ofthe sample changed from black or bluish black toyellow. The XRD results indicated that the sampleheated in air contained crystalline ferric ortho-phosphate, Fe(PO4), along with Fe4(P2O7)3, butno Fe3(P2O7)2 on its surface. The interior of thesample was still black or bluish black and con-tained Fe3(P2O7)2 and Fe4(P2O7)3, but no Fe(PO4)as identi®ed by XRD.

Similar heat treatment in nitrogen for 24 h forthe glass melted at 1400°C which contained anFe2� of about 50%, produced only Fe3(P2O7)2

crystals when it was heated at either 600°C or810°C. This is consistent with the DTA results forthis glass, which show only one crystallizationpeak at about 600°C, Fig. 4, curve C. This glasssoftens when heated at 820°C. As was observedpreviously for the glass melted at 1150°C, thissample was also found to contain crystals ofFe4(P2O7)3 and Fe(PO4), but no Fe3(P2O7)2 on itssurface when the heat treatment was continued at810°C in air. It is believed that the Fe3(P2O7)2

crystals on the surface of the samples underwent aphase transformation through oxidation (in air),which is most likely represented by the reaction,

2Fe3�P2O7�2 � 1=2O2 ! 2Fe�PO4� � Fe4�P2O7�3:�3�

According to Eq. (3), TGA for this glass shouldshow weight gain when heated in air, but noweight gain should occur during heating in anoxygen-free (nitrogen) atmosphere, which wasobserved in TGA for these iron phosphate glasses,see Fig. 6.

Fig. 6 shows the TGA at 10°C/min in air for theglasses melted at 1150°C and 1400°C, which con-tained an Fe2� of 17% and 50%, respectively. Also,

shown in Fig. 6 is the TGA in nitrogen at 10°C/min for the glass melted at 1400°C. To avoidmelting the samples, the temperature was keptbelow 810°C for the glass melted at 1400°C and825°C for the glass melted at 1150°C during thedynamic TGA ramp. The temperature was thenheld constant for about 6 and 2 h for the glassesmelted at 1400°C and 1150°C, respectively.

Oxidation of the Fe2� ions to Fe3� ions isclearly indicated by the weight gain for bothsamples (Fig. 6) when they were heated in air. Theabsence of a weight gain for the glass melted at1400°C when heating in nitrogen is further evi-dence of oxidation. The larger weight gain for theglass melted at 1400°C compared to that for theglass melted at 1150°C is consistent with its higherinitial Fe2� content. Calculations show that glassesmelted at 1150°C and 1400°C should gain weightby 0.74% and 2.19%, respectively, if all of the Fe2�

ions are oxidized to Fe3�, which is consistent withmeasured (Fig. 6) weight gains of 0.62% and2.17%. An agreement between calculated (2.19%)and measured (2.17%) weight gain for the glassmelted at 1400°C indicates that all the Fe2� ions inthis glass were oxidized to Fe3� ions during theTGA measurement in air. However, the slightlylower value for the measured weight gain for the

Fig. 6. TGA in air for the glasses prepared by melting a

40Fe2O3±60P2O5, mol%, composition at (A) 1400°C and (B)

1150°C for 1 h, and (C) TGA in nitrogen for the glass melted at

1400°C. A weight increase, which is believed due to the oxi-

dation of Fe2� ions to Fe3� ions, is evident when the samples

are heated in air (curves A and B), but no weight gain is ob-

served when these glasses are heated in nitrogen (curve C).


glass melted at 1150°C possibly indicates that notall of the Fe2� ions in this glass were oxidizedduring the TGA measurement in air.

These TGA results are consistent with theM�ossbauer spectra for the post-TGA samples,see Fig. 7, which also con®rm that all the Fe2�

ions in the glass melted at 1400°C were oxidizedto Fe3� after the TGA experiment, whereas someof the Fe2� ions were retained in the post-TGAsample for the glass melted at 1150°C. In Fig. 7,curves A and B show the M�ossbauer spectra forthe samples obtained after TGA experiment inair for the glasses melted at 1150°C and 1400°C,respectively. While a small absorption band cor-responding to Fe2� ions in Fe3(P2O7)2 is stillpresent in curve A, a similar absorption band forFe2� is totally absent in curve B. The M�ossbauerspectra in curve A suggests that about 3% of theFe2� ions are present in this sample, whichcorresponds to the value calculated from the

di�erence between the calculated (0.74%) andmeasured (0.62%) weight gain for this glass. Ashorter isothermal hold time in TGA (�2 h) forthe glass melted at 1150°C compared to that (�6h) for the glass melted at 1400°C, see Fig. 6, maybe the reason that not all of the Fe2� ions in thisglass were converted to Fe3�.

To determine the dependence of the oxidationrate of Fe2� to Fe3� ions on the heat treatmenttemperature, powdered samples of the glass meltedat 1400°C, which had an initial Fe2� concentrationof 50%, were heated in air at di�erent temperaturesbelow its liquidus (�840°C) for 24 h. The heattreated samples were then analyzed by M�ossbauerspectroscopy to determine the Fe2� content. Fig. 8shows how the concentration of Fe2� ions in theheat treated samples decreases or the rate of oxi-dation of the Fe2� ions to Fe3� ions increases withincreasing heat treatment temperature. The sam-ples heated at temperatures below 600°C werefound to be amorphous by XRD and those heatedabove 600°C was fully crystalline. As shown inFig. 8, the rate of oxidation from Fe2� to Fe3�

increased considerably for crystalline samples.However, the oxidation of Fe2� to Fe3� is reversedwhen the heat treatment temperature exceeds theliquidus temperature (Fig. 3).

Fig. 7. M�ossbauer spectra at 23°C for the samples crystallized

during TGA experiments in air for the glasses prepared by

melting a 40Fe2O3±60P2O5, mol%, composition at (A) 1150°C

and (B) at 1400°C for 1 h. While a small absorption line for the

Fe2� ions is still present in curve A (glass melted at 1150°C),

such a line is clearly absent in curve B (glass melted at 1400°C).

Fig. 8. Concentration of Fe2� ions as measured from

M�ossbauer spectra of the samples after heating the glass in air

for 24 h at the temperatures shown. The glass prepared by

melting a 40Fe2O3±60P2O5, mol%, composition at 1400°C for

1 h was used for these measurements. The decrease in Fe2�

concentration with increasing heat treatment temperature sug-

gests an oxidation of Fe2� ions to Fe3� ions.


The glass transition temperature, Tg, (measuredat the minimum of the small DTA endotherm,Figs. 4 and 5), the temperatures for the ®rst andsecond crystallization peaks, Tp1 and Tp2, (mea-sured at the maximum of the DTA exotherms),and the maximum height of the peaks, (dT)p1 and(dT)p2, for the glasses melted at di�erent temper-atures are given in Table 4. The estimated errorsfor the DTA peak heights and peak temperaturesgiven in Table 4 have been determined from sev-eral identical DTA runs. The values of (dT)p areshown separately in Fig. 9 as a function of themelting temperature. Also shown in Fig. 9 is thearea, A, under the ®rst crystallization peak mea-sured in arbitrary units of °C ´ s.

These parameters (Tg, Tp, (dT)p, or A) changevery little for the glasses melted for di�erent timesat 1200°C, so they will not be discussed. Theseresults for the glasses melted for di�erent times at1200°C are reasonable since, there was no signi®-cant change in the Fe2� concentration for theseglasses (Table 3).

The decrease in Tg and Tp1 (Table 4) and in-crease in (dT)p and A for the ®rst DTA peak(Fig. 9) indicate that the general crystallizationtendency for the glass increases with increasingFe2� content. The increase in peak height ((dT)p)and area (A) in Fig. 9 with increasing meltingtemperature (or Fe2�) for the ®rst crystallizationpeak is consistent with this peak being due to thecrystallization of Fe3(P2O7)2 which contains oneFe2� and two Fe3� ions.

The height of the second crystallization peak,which is due to the crystallization of Fe4(P2O7)3,decreases with increasing melting temperature andbecomes essentially zero for the glasses meltedP1300°C. This is also expected since crystallineFe4(P2O7)3 contains only Fe3� ions and theamount of Fe4(P2O7)3 is expected to decrease incrystallized glasses as the fraction of Fe3� de-creases. The ratio of Fe2� to total iron ions incrystalline Fe3(P2O7)2 is 1:2, so a glass containing

Table 4

Thermal properties as measured by DTA at a heating rate of 10°C/min in a ¯owing (30 cm3/min) nitrogen atmosphere for the

crystallization peaks in glasses made by melting a 40Fe2O3±60P2O5 mol%, composition at di�erent temperatures for 1 h in air. The

concentration of Fe2� ions in each glass, as measured by M�ossbauer spectroscopy, is also given

Melting

temp. (°C)

Fraction of

Fe2� (�0.02)

Tg � 3

(°C)

Tp1 � 3

(°C)

Tp2 � 3

(°C)

(dT)p1 � 0.25

(°C)

(dT)p2 � 0.25

(°C)

E � 10

(kJ/mol)

1150 0.17 511 652 815 0.96 2.02 265

1200 0.19 502 646 825 1.36 1.47 264

1250 0.29 505 616 825 2.33 0.50 305

1300 0.35 502 616 a 2.46 a 310

1350 0.42 500 602 a 4.03 a NM

1400 0.50 500 591 a 4.65 a 316

Tg: glass transition temperature; Tp1, Tp2: temperature at the maximum of the ®rst and second crystallization peaks, respectively;

(dT)p1, (dT)p2: maximum height of the ®rst and second crystallization peaks, respectively; E: activation energy for the ®rst crystalli-

zation peak.a No second crystallization peak; NM: not measured.

Fig. 9. The maximum height, (dT)p, and total area, A, of the

DTA peaks as a function of melting temperature for the glasses

prepared from a 40Fe2O3±60P2O5, mol%, composition. The

concentration of Fe2� ions, as measured from the M�ossbauer

spectra, is also shown on the top axis (not to scale) for each

glass melted at di�erent temperatures. No second DTA peak

was observed for glasses melted P 1300°C, so (dT)p for these

glasses is essentially zero, open circles.


an Fe2� concentration of P33% will not have anyFe3� left to crystallize Fe4(P2O7)3 (second DTApeak), if all of the potential Fe3(P2O7)2 fullycrystallizes (®rst DTA peak). The absence of asecond DTA crystallization peak for the glassesmelted P1300°C (Fig. 4 curve C or Fig. 9 opencircles) is consistent with their Fe2� content ofP35% (Table 2 and Fig. 3).

The ln(T 2p //) vs 1/Tp plots (Eq. (1)) for the

glasses melted at di�erent temperatures yieldednearly perfect straight lines with a correlationfactor of 0.995 or better. Typical plots are shownin Fig. 10 for the glasses melted at 1150°C and1400°C. The activation energy for crystallization,E, determined from the slope of these straight linesis given in Table 4 and in Fig. 11. It should benoted that to make a better comparison of the Evalues for the glasses melted at di�erent tempera-tures, only the ®rst crystallization peak was eval-uated for those glasses which exhibited two DTAcrystallization peaks, namely, the glasses meltedbelow 1300°C. Although, E for these glasses variedfrom 265 kJ/mol, for the glass melted at 1150°C, to316 kJ/mol, for the glass melted at 1400°C, node®nite correlation was evident between E and theconcentration of Fe2� ions in the glass.

Glasses melted at or above 1250°C had nearlythe same value for E, between 305 and 316 kJ/mol.

The reason for the signi®cantly lower value of E(264±265 kJ/mol) for the glasses melted at 1150°Cand 1200°C is not clear. Note that a sudden in-crease in E from an average value of about 265 to310 kJ/mol occurred near Fe2� of 33%, Fig. 11. Asmentioned earlier, glasses containing an Fe2�

concentration less than 33% crystallize toFe3(P2O7)2 and Fe4(P2O7)3 and those containingFe2� P33% crystallize to only Fe3(P2O7)2. Therange in E values, namely, 265±316 kJ/mol, forthese iron phosphate glasses is comparable tovalues reported [23] for conventional silicateglasses, such as a lithium disilicate glass whose Evalue, by the present method (Eq. (1)), is 300 � 15kJ/mol.

The average DR, of the iron phosphate glassesin deionized water at 90°C for 4, 8, and 16 days isgiven in Table 5 for glasses melted at 1150°C,1200°C, 1350°C, and 1450°C. The DR of theseglasses, which only varied from about 1.5 ´ 10ÿ9

to 4.5 ´ 10ÿ9 g/cm2/min is lower (better) than thatof some borosilicate glasses (�10ÿ8 g/cm2/min)such as CVS-IS and LD6-54-12 [4]. The excep-tionally high chemical durability for iron phos-phate glasses in water has been previously [13,14]attributed to the presence of hydration resistantFe±O±P bonds in the glass structure. However, asopposed to the suggestion in Ref. [14] that DRdecreases with increasing Fe3� concentration, theDR for the iron phosphate glasses investigated in

Fig. 10. Typical Kissinger plots (Eq. (1)) for the glasses pre-

pared by melting a 40Fe2O3±60P2O5, mol%, composition at

1150° (open circles) and 1400°C (solid circles). The ®rst or low

temperature DTA peaks corresponding to the crystallization of

Fe3(P2O7)2, see Fig. 4, obtained at heating rates (/) of 2, 4, 6,

10, and 15°C/min were used for these analyses.

Fig. 11. Activation energy for crystallization, E, determined

from Kissinger plots (Fig. 10) as a function of Fe2� concen-

tration for the 40Fe2O3±60P2O5, mol%, glasses prepared by

melting at di�erent temperatures.


the present work is found to be independent of theFe2� or Fe3� concentration in the glass.

The values of log DR for the glass melted at1150°C for 2, 4, and 8 days are shown in Fig. 12along with the log DR values for di�erent crys-tallized forms for this glass, namely, Fe3(P2O7)2,Fe4(P2O7)3, and Fe(PO4). Although the chemicaldurability (opposite to DR) for the crystallizedforms is slightly worse than that of the glass, itremains comparable to or better than that of thecommercial window glasses.

In another set of experiments the DR forthe glasses prepared by melting xFe2O3±(100 ÿ x)P2O5 with x� 10, 20, and 30 mol% batchcompositions at 1200°C for 1 h in air was mea-sured in deionized water at 90°C for 4 days. Al-though these glasses have di�erent starting

compositions or Fe/P ratio, all have nearly thesame Fe2� concentration of 25 � 1.5% as mea-sured by M�ossbauer spectroscopy. Since all ofthese glasses were melted at the same 1200°C, theresults of a constant Fe2� concentration for theseglasses again demonstrate that equilibrium be-tween the Fe2� and Fe3� ions depends primarily onmelting temperature. Attempts were also made toprepare glasses for compositions with x� 45 and50. Melts for these compositions partially crystal-lized during cooling; hence they were not used forchemical durability measurements.

The measured values of log DR for thexFe2O3±(100 ÿ x)P2O5, mol%, (x� 10, 20, and30) glasses are shown in Fig. 13 as a function oftheir Fe/P ratio. For comparison, the log DRvalues for the glasses prepared by melting the40Fe2O3±60P2O5 mol% composition which had anFe/P ratio of 0.67, are also shown in this ®gure as afunction of Fe2� concentration (data in Column 3,Table 5). Fig. 13 clearly demonstrates that the DRis independent of the relative concentration ofFe2� or Fe3� ions, but depends strongly on the Fe/P ratio. The DR decreases (chemical durabilityimproves) with increasing Fe/P ratio or the totaliron content in the glass. However, glass formationbecomes di�cult for the binary iron phosphate

Table 5

Average dissolution rate, DR, in deionized water at 90°C after

4, 8, and 16 days for glasses made by melting a 40Fe2O3±

60P2O5, mol%, composition at di�erent temperatures for 1 h

Melting

temp. (°C)

Fraction of

Fe2� (�0.02)

DR � 1.50 (10ÿ9 g/cm2/min)

4 days 8 days 16 days

1150 0.17 3.50 2.95 2.19

1200 0.19 1.46 2.58 1.95

1350 0.42 3.09 4.00 2.76

1450 a 0.57 4.51 3.09 2.66

a Melted for 2 h.

Fig. 12. Average dissolution rate, DR, as a function of time for

the glass (�), prepared by melting a 40Fe2O3±60P2O5, mol%,

composition at 1150°C and its di�erent crystallized forms,

namely, Fe3(P2O7)2 (�), Fe4(P2O7)3 (4), and Fe(PO4) (m). The

typical error bar is shown one data point.

Fig. 13. Average dissolution rate, DR, for iron phosphate

glasses in deionized water at 90°C after 4 days as a function of

Fe/P ratio (open circles, top axis) and Fe2� concentration

(closed circles, bottom axis). The typical error for the mea-

surements is shown on one data point.


compositions with Fe/P ratio >0.75 or Fe2O3

content >45 mol%.

4. Discussion

The results in Figs. 1±3 and Tables 2 and 3show that the equilibrium between the Fe2� andFe3� ions depends strongly on the melting tem-perature and shifts toward higher concentration ofFe2� as the melting temperature increases. Equi-librium between the two iron ions in the ironphosphate melts at a particular temperature ap-pears to be quickly reached in less than one hourand does not change signi®cantly for times longerthan one hour. Melting temperature, appears to bemore important than melting time for controllingthe composition of iron phosphate glasses.

The M�ossbauer hyper®ne parameters, namely,the QS or IS for Fe2� and Fe3� in Tables 2 and 3are consistent with those reported previously[16,17,24] for iron phosphate glasses, which typi-cally represent an octahedral or a distorted octa-hedral coordination for both ions, although thepresence of some iron ions in tetrahedral coordi-nation cannot be totally ruled out. The nearlyidentical values for QS or IS (Tables 2 and 3) in-dicate that the near coordination environment ofFe2� and Fe3� in these glasses is independent ofthe concentration of Fe2� or Fe3� ions, i.e., inde-pendent of the temperature or time for meltingthese glasses. These results suggest that the struc-ture of the iron phosphate glasses do not changesigni®cantly with the concentration of Fe2� orFe3� ions, which was also veri®ed by measuringthe IR spectra for these glasses, see Fig. 14. Asshown in Fig. 14, the IR spectra for the glassesmelted at di�erent temperatures did not change,which suggests that the structure of all theseglasses are nearly identical.

The general feature of the IR spectra inFig. 14 is identical to that reported [1,25] previ-ously for iron phosphate glasses. Using the pre-vious [1,25] band assignment, the high frequencyabsorption band between 1135 and 1065 cmÿ1

are assigned to the symmetric and antisymmetricvibrations of the P@O groups in the PO4 tetra-hedra. The bands at about 923 and 760 cmÿ1 are

associated with the antisymmetric and symmetricstretching modes, respectively, of the P±O±Pbonds, and the low frequency band at about 515cmÿ1 may be attributed to the bending modes ofPO4 groups. As suggested from M�ossbauermeasurements, the Fe2� and Fe3� ions occupythe sites with octahedral or distorted octahedralcoordination and do not participate in the actualglass network, which may be the reason for theabsorption bands of Fe±O vibrations being ab-sent in the IR spectra. Another reason may bethat the absorption bands for the Fe±O vibra-tions occur mostly, except for the one between500 and 600 cmÿ1, below the lower limit of thewave number range measured in the presentwork, i.e., below 450 cmÿ1 in Fig. 14. The bandbetween 500 and 600 cmÿ1 for the Fe±O vibra-tions may have superimposed with the broadband around 515 cmÿ1 for di�erent vibrationalmodes of the PO4 groups.

The quenched-in Fe2� ions in the glass, whichformed during melting due to the reduction ofFe3� ions, are oxidized again to Fe3� ions when theglass is reheated in air at temperatures below theliquidus temperature, see Figs. 6±8. The liquidustemperature of these iron phosphate glasses ap-pears to act as a kind of limiting temperatureabove which the Fe3� ions in the starting batch arereduced to Fe2� ions and this reduction process isfavored with increasing temperature above the

Fig. 14. IR spectra for the glasses prepared by melting a

40Fe2O3±60P2O5, mol%, composition at (A) 1150°C, (B)

1200°C, (C) 1250°C and (D) 1400°C for 1 h.


liquidus, see Fig. 3. Below this limiting tempera-ture, Fe2� ions in the glass are oxidized to Fe3� asindicated by the weight gain of the glass in air(TGA, Fig. 6), absence or decrease of the ab-sorption line for Fe2� ions in the M�ossbauerspectra of the post-TGA samples in air (Fig. 7),and by the decrease of total Fe2� concentration inthe heat treated glasses compared to that for theas-made glass (Fig. 8). This oxidation process isaccelerated also with increasing temperature belowthe liquidus, see Fig. 8.

Based on the DTA (Figs. 4 and 5), TGA(Fig. 6), and XRD results, it is believed that theoxidation of Fe2� to Fe3� ions is preceded by thecrystallization of the glass to Fe3(P2O7)2 andFe4(P2O7)3 crystals. The Fe2� ions in Fe3(P2O7)2

crystals are then oxidized to Fe3�, and Fe3(P2O7)2

undergoes a phase transformation according toEq. (3) to form Fe(PO4) and Fe4(P2O7)3. Asmentioned before, the oxidation of Fe3(P2O7)2 toFe(PO4) occurs primarily in a layer of about 10 lmthick on the surface where its color turns yellow,the interior of the sample was still black or bluishblack. This is probably due to the reason that theoxygen required for this transformation could notdi�use more than 10 lm in the 24 h time scalechosen for crystallization of these glasses.

The transformation of Fe3(P2O7)2 to Fe(PO4)may take place also by a reaction di�erent fromEq. (3), namely, by

Fe3�P2O7�2 �1

2O2 � 6Fe�PO4� � P2O5: �4�

A part of the Fe4(P2O7)3 that crystallized duringheating in nitrogen or air, may also transform toFe(PO4) as

Fe4�P2O7�3 � 4Fe�PO4� � P2O5: �5�While Eq. (4) correctly accounts for the ob-

served weight gain in the TGA, Eq. (5) does not.Furthermore, both Eqs. (4) and (5) yield anamount of free P2O5 as the reaction product,which was not detected by XRD. The chemicaldurability of a crystallized sample in the aqueoussolution should be much lower than that of itsglassy counterpart, if P2O5 was a reaction product,but this was not observed in the present work.Since Eq. (5) does not require any oxygen, the

interior of a sample should contain Fe(PO4) andappear yellow like its surface when crystallized for24 h at 810°C, but this was not observed either.Based on these observations, it is believed that thereaction in Eq. (3) is the most likely reaction thatoccurs when these iron phosphate glasses arecrystallized in air.

The height, (dT)p, and area, A, for the ®rstDTA peak of the glasses, which is due to thecrystallization of Fe3(P2O7)2, increased with in-creasing concentration of the Fe2� ions, see Fig. 9.Depending upon the Fe/P ratio in the glass, theamount of Fe3(P2O7)2 that can crystallize reaches amaximum at an Fe2� concentration of 33%. Forexample, for the 40Fe2O3±60P2O5, mol% glasswith an Fe/P molar ratio of 0.67, the maximumamount of Fe3(P2O7)2 that can crystallize would beabout 80 mol% which would decrease as the Fe2�

concentration either increases or decreases from33%. This means that (dT)p and A for the ®rstcrystallization peak in Fig. 9 also should be max-imum at an Fe2� concentration of 33%. The in-crease in (dT)p or A for the ®rst crystallizationpeak with increasing Fe2� ions (Fig. 9) of up to33% is consistent with this argument. However,the continued increase in both of these parameterswith increasing Fe2� content above 33% is notclearly understood, but may occur for any of thefollowing two reasons. First, another ferrousphosphate phase which crystallizes at temperaturesclose to that of Fe3(P2O7)2 may have crystallizedsimultaneously with Fe3(P2O7)2 in the glassescontaining more than 33% Fe2� (i.e., glassesmelted P1300°C), which kept both (dT )p and Afor the ®rst DTA peak increasing. Secondly, only afraction of the total Fe2� ions present in the glasscrystallized to Fe3(P2O7)2, even for the glassescontaining less than 33% Fe2�, and the remainingFe2� ions were left in a glassy phase. For example,if 60% of the Fe2� ions originally present in theglass take part in crystallizing Fe3(P2O7)2, then(dT)p and A for the ®rst DTA peak are expected toincrease with increasing total Fe2� content of theglass up to 55%.

The XRD results for the glass melted at 1400°C(Fe2� 50%) and crystallized at 600°C or 810°C innitrogen does not show any extra peaks other thanthose for crystalline Fe3(P2O7)2. This tends to rule


out the ®rst previous suggestion that a ferrousphosphate phase crystallizes simultaneously withFe3(P2O7)2 in glasses having an Fe2� concentrationP33% and leaves with the second possibility thatnot all, but a portion of the total Fe2� ions in theglass are consumed to crystallize Fe3(P2O7)2.

The DR, for these iron phosphate glasses de-creases with increasing total iron content or Fe/Pratio, but is independent of the relative concen-tration of Fe2� or Fe3� ions for a constant Fe/Pratio (Table 5, Fig. 13). It has been shown previ-ously that the iron phosphate glasses crystal-lize generally to ferrous-ferric pyrophosphate,Fe3(P2O7)2, and ferric pyrophosphate, Fe4(P2O7)3.Even though the structure of a glass and its crys-talline counterpart need not necessarily be identi-cal, it is reasonable to expect some similaritiesbetween the glass and crystalline structures. Onemay expect that short range structure of the ironphosphate glasses is close to that of the Fe3(P2O7)2

and Fe4(P2O7)3 crystals. The relative concentra-tion of these two structural groups in the glassshould depend upon the relative concentration ofthe Fe2� and Fe3� ions.

Unlike the structure of conventional phos-phate glasses where all the bridging oxygenbonds are of the P±O±P type, the structures ofFe3(P2O7)2 and Fe4(P2O7)3 both contain only14% P±O±P bonds [16,17], the remaining bondsbeing of the Fe±O±P type. Since the Fe±O±Pbonds are considered more hydration resistant[13,14] than the P±O±P bonds, the chemical du-rability of the iron phosphate glasses in water isobserved to be considerably higher than that ofthe conventional (iron-free) phosphate glasses.For a constant Fe2� concentration, the fractionof structural groups resembling the structures ofboth Fe3(P2O7)2 and Fe4(P2O7)3 crystals increasesin the glass with increasing total iron content orFe/P ratio. This, in turn, increases the number ofFe±O±P bonds compared to the P±O±P bonds inthe glass, there by, increasing the chemical du-rability (decreasing the values for DR) of theglass, see Fig. 13. For a constant Fe/P ratio anincrease in the Fe2� concentration increases thefraction of structural group resembling that ofFe3(P2O7)2, but decreases the fraction of struc-tural group resembling that of Fe4(P2O7)3. Since

the number of Fe±O±P bonds in the structures ofFe3(P2O7)2 and Fe4(P2O7)3 crystals are same,�86%, the relative change in the fraction of thesetwo structural groups caused by a change in theFe2� or Fe3� concentration in the glass, makesno or little change in the total number of Fe±O±P bonds. The chemical durability for the glasseswith a constant Fe/P ratio, therefore, remainsnearly unchanged with increasing concentrationof Fe2� or Fe3�.

5. Conclusions

A 40Fe2O3±60P2O5 mol% composition whichinitially contained only Fe3� ions, resulted inglasses that contained both Fe2� and Fe3� ions atvarying amounts when melted either at di�erenttemperatures or for di�erent times at a particulartemperature. Melting temperature had a strongere�ect than melting time on the relative concen-tration of Fe2� and Fe3� ions and, hence, is con-sidered more important for controlling thecomposition of the iron phosphate glasses.

The structure of these iron phosphate glassesdid not change in spite of a change in the Fe2�

concentration from 17% to 57%.Chemical durability of these iron phosphate

glasses as well as of their crystallized forms indeionized water was high.

Chemical durability for these glasses was inde-pendent of the relative concentration of Fe2� andFe3� ions, but improved with increasing Fe/P ratioin the glass. The highest chemical durability wasobserved for a glass with an Fe/P ratio of 0.67 i.e.,for the 40Fe2O3±60P2O5 mol% glass. Melts forcompositions containing Fe2O3 > 40 mol% (Fe/P > 0.67) showed a deteriorating tendency forglass formation and partially crystallized duringcooling.

Acknowledgements

The authors thank Dr R.K. Brow for helpfuldiscussions. This work was supported by the De-partment of Energy (DoE) through contract DE-FG07-96ER45618.


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Documents

Effect of melting temperature and time on iron valence and crystallization of iron phosphate glasses