Evaluation of Non Crystalline Phase in AZS Refractories by XRD Methods

doi:10.4236/njgc.2011.12005

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New Journal of Glass and Ceramics, 2011, 1, 28-33

doi:10.4236/njgc.2011.12005 Published Online July 2011 (http://www.SciRP.org/journal/njgc)

Evaluation of Non Crystalline Phase in AZS

Refractories b y XRD Methods

M. S. Conconi1,2,3, N. M. Rendtorff1,3,4, E. F. Agli e tti1,4,5

1CETMIC (Centro de Tecnología de Recursos Minerales y Cerámica, (CIC-CONICET-CCT La Plata)), M. B. Gonnet, Argentina;

2Facultad de Ingeniería de la Universidad Nacional de La Plata. Argentina; 3CIC-PBA, Buenos Aires Argentina; 4Facultad de

Ciencias Exactas de la Universidad N acional de La Plata. Argentin a; 5CONICET La Plata, Argent ina.

Email: [email protected]

Received Ma y 20th, 2011; Revised J une 17th, 2011; Accepted June 24th, 2011.

ABSTRACT

The relation between the atomic structure and the macroscopic properties and behaviors of a material constitute one of

the obje c tiv e s of the materials sc ie nce, particularly in the design and development of ceramic materials. Crystalline and

non crystalline phases together with pores, grain boundaries, etc. affect mechanical and fracture prop-erties as we ll a s

chemical resistance and electric properties. These aspects will be bonded to the raw materials chosen and the whole

processing route. In glass industry, although there are other electrofused refractories such as the alumina ones used in

the feeding of the fusion kilns, probably the most used refractories in contact with the melted glass are electrofused ma-

terials that belong to the Al2O3-SiO2-ZrO2 system commonly named AZS. Exceptionally for refractory materials the

amount of the glassy phase in a AZS material is important and appreciable; and makes them particularly adequate for

containing fussed glass. The glass proportion will define much of their prop-erties and behaviors. In the present work

the results of the non crystalline phase quantification of two samples of commercial AZS materials are presented and

compared. These were obtained by three different methods using in the X ray powder diffraction (XRD) techniques. The

first method consists in the linear interpolation of the base lines of the diffractograms compared to the amorphous silica

and th e fully crystalline quartz. The other two methods are based in the application of the Rietveld method. One is the

internal standard method with quartz as fully crystalline standard and the other one consist in the inclusion of the glas-

sy phase to the refinement with a structural model that can be understood as the widening of the peaks consequence of

an extreme decrease in the crystallite size of a quartz phase. The three methods showed equivalent results (with d iffer-

ences less than 3%) for the two samples and demonstrated that are adequate for the quantification of the non crystalline

phase in this kind of materials.

Keywords: XRD, Rietveld Method, Non Crystalline Pha se, Refractories

1. Introduction

In glass industry, although there are other electrofused

refractories such as the alumina ones used in the feeding

of the fusion kilns, probably the most used refractories in

contact with the melted glass are electrofused materials

that belong to the Al2O3-SiO2-Zr O 2 system commonly

named AZ S .

In gla ss ind ustr y, altho ugh t here are o ther electrofused

refractories such as the alumina ones used in [1,2]. T hese

materials have influenced drastically the quality levels

and productivity of the glass fabrication processes [3].

The first advance achieves by these materials was the

improvement in the corrosion resistance, increasing the

life of the kilns, together with the quality of the

processed glass. Other important progresses accomplish

were the improvement in the mou nting time and d ecrease

in the devitrification cords and lower amount of bubbles

produced by the “blistering” process [4-6].

Exceptionally for refractory materials the amount of

the glassy phase in a ASZ material is important and ap-

preciable; The weight proportions is around 20% and

makes them particularly adequate for containing fussed

glass. The glass proportion will define much of their

properties and behaviors.

To validate and compare three quantification methods

for the non crystalline phase of an electrofused refractory

material is the principal objective of the present wo rk. In

the materials field the use of the X ray diffraction tech-

nique fo r the non c r yst al li ne or a morp hous fra ct io n q ua n-

Eva lua t ion of Non Crystalline Phase in AZ S Refr actories by XRD Methods

tification is a permanent challenge that has been studied

from diverse forms. Three of them are compared in the

present wor k.

The typical composition of an AZS material of the

normal filling cast type, from the supplier information is

sho wn in Table 1 .

Ohlberg [7] developed a method for determining the

crystallinity percentage (C%) in partially devitrified

glasses, by the interpolation of the base line of the dif-

fractogram between the corresponding to amorphous

silica and fully crystalline quartz.

( )

% 100g

CII

−

= ×−

(1)

Where Ig, I m y Ic are the diffractogram intensities at 2θ

= 22.5℃ corresponding to a sample 100% glass

(amorphous silica), the partiall y crystalline phase (prob-

lem sample) and the 100% crystalline standard (quartz)

respectively.

This equation commonly utilized for determining

crystallinity i n vitro-ceramic materials in a wide range of

proportions [8,9], it could be valid to assume that the

amorphous or non crystalline (NCO%) proportion can be

obtained from the following e quation defining NCOh% as

a compleme nt of the crystallinity (NC% + C% = 100).

( )

% 100% 100100

gc gc

II II

NC CII II

−− −

=−=× =×

−−

(2)

The Rietveld method [10,11] has demonstrated to be

an effective tool for quantitative phase analysis in di-

verse materials [12,13]. The quantitative analysis is car-

ried out from the scale factors refined for each phase (Si)

according to the following equation:

( )/

i ii

ip pp

S ZMV

WS ZMV

∑

(3)

Table 1. Typical compositions of an electrofused AZS re-

fract ory .

Chemical

compositiona (% wt.) Mi neralo g ical

composition (% wt.)

ZrO2 + HfO2 35.4 m-ZrO2 33.0

Al2O3 48.0 Al 2O3 47.0

SiO2 15.2 Amorphous phase 20.0

Na2O 1.5

TiO2 0.04

Fe2O3 0.04

a. http://www.sefpro.com/fused-cast -azs.a spx

Where Wi is the weight fraction of the i-phase over all

the present phases. Si, Zi, Mi, Vi and τi are the scale

factor, the number of molecules per unit cell, the mo

lecular weight, the unit cell volume and the mass-absor-

ption correction factor of the particles for the i-phase,

respect tively.

In the Rietveld analysis, the crystalline structure of

each phase in the sample should be known. Hence this

method does not allow including the amorphous or

non-crystalline phases. However several authors had put

into practice the quantification of these phases using the

Rietveld refi nement i n efficie nt way. De la T orre applied

the method for samples with the aggregate of a fully

crystalline (100%) internal standard in a known propor-

tion, and determined the experimental conditions which

affect the uncertainty of the amorphous phase determina-

tion using different internal s tandards [14].

Le Bail demonstrated that it is possible to include the

silica glass in the Rietveld refinement through a struc-

tural model with crystalline defects [15]. Lutterotti [16]

applied Le Bail method for the silica glass introducing

defects from the crystal size for reproducing the peak

widening; verifying this method for standard samples of

quartz and amorphous silica, after he applied it to sani-

tary ceramic and to a AZS refractory. Finally Ward [17]

compared two Rietveld methods in flaying ashes. The

first one with the internal standard aggregate in known

proportion and other one introducing the amorphous

phase in the refinement program through the incorpora-

tion of experimental standards of non crystalline phases

like meta-caolin or tr idimite.

Mechanical mixtures of crystalline and non crystalline

had been commonly used as standards for studying the

efficiency of non c ryst all ine quantifi cation methods.

In electrofused or sintered materials, these mixtures

are not the most adequate, due to the fact that the phase

distribution in the standards differs from the actual stu-

died materials.

In the first case crystalline and non crystalline par-

ticles are clearly differentiated and produce different

diffractions compared to the produced in samples with

particles where both type of phases are together in the

same particle. In consequence the comparison of diverse

metho ds with sa mples with u nknown a morp hous cont ent

will allow validating them.

In the present work the results of the non crystalline

phase quantification of two samples of commercial AZS

materials are presented and compared. These were ob-

tained by three different methods based in the X ray

powder diffraction (XRD).

The redefinition of the Ohlberg equation (Equation 2)

was used for the first method. Milled quartz (SiO2) was

Eva lua t ion of Non Crystalline Phase in AZS Refracto r ies by XRD M ethods

used as fully crystalline standard for the first Rietveld

refinement method. Finally the Le Bail model based me-

thod was applied with the amorphous phase incorporated

as a nanocrystalline material with a ß-Carnegieite struc-

ture.

2. Experimental Procedures

The analyzed material consisted in a monolithic electro-

fuse d commercial AZS refractory normal filling type

(AZS ER 1681 RN, Saint-Gobain SEFPRO, Italy). Par-

ticularly two samples (AZS1 y AZS2) of the material

were studied coming from different blocks. For the anal-

ysis samples were milled in Agatha mortar up to mesh

100.

The chemical analysis of the samples was carried out

by Atomic Emission Spectroscopy by inductive coupled

plasma (Varian Vista AX CCD Simultaneous ICP-AES)

with the exception of the zirconium which was done by

X ray Fl uorescence (Shimadzu EDX800HS).

For the amorpho us phase characterization Silicon dio-

xide (SiO2) powder was used as standard (Carlo Erba

RPE) for obtaining Ig in the Ohlberg method and for

refining the pure glassy phase in the Rietveld Method.

Also 15%wt. of milled quartz was used as internal stan-

dard aggregate before the Rietveld quantification. This

was chosen because it presents a similar absorption coef-

ficient to the sample [14]. For obtaining the Ic T he same

crystalline quartz was used.

Materials were analyzed by XRD (Philips 3020

equipment with Cu Kα radiation in Ni filter at 40 kV to

20 mA). D ifractograms were carried out between 10 and

70 in 2θ with 0.04 steps of 3 seconds.

The powder XRD patterns were analyzed with the

program FullProf [18], which is a multipurpose pro-

file-fitting program, including Rietveld refinement. The

starting crystallographic data for each phase were ex-

tracted from the literature.if any non crystalline phase is

present in the sample when the Rietveld refinement done,

the internal standard content would be overestimated.

The percentage of amorphous phase in the sample with-

out the aggregated standard can be calculated using the

following equation [14]:

(1/ )

%10 %

100

IS s

NC W

−

= ×

−

(4)

Where NCIS% is the non cr ystalline content by the inter-

nal standard method, WS is the internal standard propor-

tion aggregated (%) and RS is the internal standard eva-

luated by the Rietveld method.

For obtaining the actual phase content of each present

phase they should be corrected by the amorphous phase

evaluated.

The only refined parameter in the Le Bail model re-

finement was the scale parameter. For the other phases

scale factor was accompanied by the cell parameters, and

the rest of the parameters which describe the profile. The

background was calculated from the interpolation of

several 2θ: intensity pairs. Moreover the background was

not refined between 5˚ and 45˚, while in the rest of the

diffractogram they were refined with the other of the

parameters.

Befo re intro ducing the no n crystal line p hase i n the re-

finement of the studied samples, the pure amorphous

silica was analyzed for determining the crystalline and

profile parameters.

3. Results and Discussion

The results of the chemical analysis of the samples are

presented in Table 2. The ZrO2 content was calculated

from the ele mental Zr content obtained by XRF.

In the XRD test Al2O3 together with monoclinic and te-

tragonal zirconia were detected. There were not detected

any Silicon (Si) containing crystalline phase, evidencing

an important silica rich (≥ 70%) non crystalline phase.

This fact supports the assumption of approximating the

non crystalline phase of these materials wi th silica glass.

3.1. Ohlberg method

In order to apply Ohlberg equation (Equation 2) the cor-

responding intensities of the diffractograms at 2θ = 22.5

for both samples AZS1 and AZS2. Both samples pre-

sented almost ide ntical intensi ties, in Figure 1 a detail of

the superposed diffractograms for AZS1 sample, the

amorphous silica and crystalline quartz between 15˚ and

35˚ is shown and i n Table 3 the results of the amorphous

quantification of both samples are revealed.

3.2. Internal Standard Method

In Figure 2 the diffractogram with its corresponding

refinement curve is shown for sample AZS2 with the

15%wt. quartz aggregate. There it can be observed the

experimental profile (dots) and the theoretical profile

(continuous), the corresponding positions of the diffrac-

tion lines of each phase (alumina, monoclinic zirconia,

quartz and tetragonal zirconia respectively) are expose in

vertical bars, finally the difference between the observed

profile and the theoretical pr ofile is shown in the base of

the graph.

After the Rietveld refinement the quartz evaluated

content was 19.1%wt. in AZS1 and 19.2%wt in AZS2.

The actual contents of non crystalline and crystalline

phases by this method are shown in Table 4. T he evalu-

ated crystalline and non crystalline content for the two

different samples are equivalent.

Table 2. Chemical composition of the studied materials.

Eva lua t ion of Non Crystalline Phase in AZ S Refr actories by XRD Methods

Chemical

Compositiona

Samp le

AZS1 AZS2

SiO2 15.4 14.7

Al2O3 45.9 45.9

Fe2O3 0.33 0.42

CaO 0.10 0.12

MgO 0.02 0.02

Na2O 1.29 1.28

K2O 0.03 0.03

TiO2 0.08 0.04

ZrO2 + HfO2 32.4 3 2.3

a:ICP and XRF results

Table 3. Non crystalline content evaluated by the Ohlberg

me thod (Equa ti o n 2).

Sample NCOh%

AZS1 23.3

AZS2 23.2

Table 4. Quantitative analysis results from the Rietveld

Me thod, with inte rnal Standard.

Phase

AZS1

(%wt.)

AZS2

(%wt.)

Al2O3 46.5 47.3

m-ZrO2 27.8 26.8

t-ZrO2 ≈ 1 ≈ 1

NCIS% 24.7 24.9

Table 5. Quantitative analysis results from the Rietveld

Me thod by Le Bail model.

Phase

AZS1

(%wt.)

AZS2

(%wt.)

Al2O3 48.1 42.9

m-ZrO2 28.0 30.3

t-ZrO2 ≈ 1 ≈ 1

NCLB% 22.9 25.8

Figure 3 presents de Rietveld refinement figure using

the Le Bail model in sample AZS1 made with the me-

thod described before. Diffraction lines correspond to:

Alumina, Monoclinic Zirconia, amorphous silica and

tetragonal zirconia respectively. Non crystalline and

crystalline phase contents are presented in Table 5. The

m-ZrO 2 and non crystalline content for the two different

samples are almost equivalent2. The alumina content in

AZS1 is higher than the one evaluated in the other sam-

ple.

Figure 1. Crystalline quartz, amorphous silica and AZS1

sample diff ract ogra m between 1 5˚ - 35 ˚.

Figure 2. Rietveld refine ment of sample AZS2 w ith crystal-

line quartz as int ernal standard.

Figure 3. Rietveld refinement of sample AZS1 with the Le

Bail model.

Eva lua t ion of Non Crystalline Phase in AZS Refracto r ies by XRD M ethods

Figure 4. Co mpari so n of t he non cry st alli ne c ont ent eval ua-

tion by the three appl ied methods.

4. Summary

The quantification results are compared in Figure 4 (bar

chart). Although the three methods are based in com-

pletely different principle, their re sults are e quivalent,

with differences below 3% for the studied material from

two di fferent samples , showi ng that t he three models are

adequate for the studied system, moreover the results

match with the results provide by the material supplier

(Table 1). In fact the results for the three applied me-

thods are slightl y hi g her .

Although the simplicity of Ohlberg method, it can be

applied only for materials that do not present diffraction

lines in 2θ = 22.5, and this method do not provides in-

formation ab out the other crystalline phases.

A complete phase quantification (crystalline and non

crystalline) can be carried out by both Rietveld refine-

ment based methods, but the Lebail model is recom-

mendable because it is not necessary to contaminate the

sample with the addition of the internal standard and it

could be easily incorporated to a routinely Rietveld

phase quantitative analysis without any increase in the

number of X ray diffractograms.

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