Hyperfine Interact (2010) 198:219–228DOI 10.1007/s10751-010-0178-3

A short and long range study of mullite–zirconia–zirconcomposites

Nicolás M. Rendtorff · Maria S. Conconi · Esteban F. Aglietti ·Cecilia Y. Chain · Alberto F. Pasquevich · Patricia C. Rivas ·Jorge A. Martínez · María C. Caracoche

Published online: 21 July 2010© Springer Science+Business Media B.V. 2010

Abstract In the field of refractory materials, ceramics containing mullite–zirconiaare the basis of those most used in the industry of glass and steel. It is knownthat the addition of zircon improves the behavior of the refractory used in service.Knowing that some mullite–zirconia composites properties as fracture strength andthe elastic modulus E are associated with the material microstructure integrity, theeventual thermal decomposition of zircon into zirconia and silica could seriouslyalter the material elastic properties. In this paper the phase content of a seriesof mullite–zirconia–zircon (3Al2O3.2SiO2–ZrO2–ZrSiO4) composites is determinedat atomic level via perturbed angular correlations (PAC) and compared with thatderived from the long range X-ray diffraction technique. PAC results on the as-prepared materials indicate that all nominal zircon is present and that it involvestwo types of nanoconfigurations, one of them describing aperiodic regions. The

N. M. Rendtorff · M. S. Conconi · E. F. AgliettiCentro de Tecnología de Recursos Minerales y Cerámica (CETMIC: CONICET-CIC),Camino Centenario y 506, C. C. 49 (B1897ZCA), M. B. Gonnet, Buenos Aires, Argentina

N. M. Rendtorff · E. F. Aglietti · P. C. RivasCONICET, Buenos Aires, Argentina

C. Y. Chain · A. F. Pasquevich · J. A. Martínez (B) · M. C. CaracocheDepartamento de Física, IFLP, Facultad de Ciencias Exactas,Universidad Nacional de La Plata, 1900 La Plata, Argentinae-mail: toto@fisica.unlp.edu.ar

C. Y. Chain · A. F. Pasquevich · J. A. Martínez · M. C. CaracocheCIC-BA, Buenos Aires, Argentina

P. C. RivasIFLP, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata,1900 La Plata, Argentina

220 N.M. Rendtorff et al.

thermomechanical properties already reported for these materials could be relatedto the crystalline to aperiodic zircon concentrations ratio they exhibit.

Keywords Ceramics · XRD techniques · Gamma ray · PAC

1 Introduction

The relation among atomic structure, macroscopic properties and bulk behavior con-stitutes a relevant subject in the field of materials science, particularly for the designand development of ceramic materials. Crystalline and glassy phases, pores, grainboundaries, etc., affect mechanical, chemical and electrical properties as fracturestrength, chemical resistance, electrical and thermal conductivity and thermal shockresistance. These aspects also depend on factors as the chemical and crystallographicnature of the raw materials and on their processing to achieve high performanceproducts. Within this frame it is always interesting to perform investigations in orderto determine the phase content in the starting materials as well as in the resultingproducts. X-ray diffraction (XRD) is ordinarily used to characterize crystallinematerials and it has been widely applied for qualitative and semiquantitative phaseanalyses and for crystalline structure determinations. The Rietveld method [1, 2],in turn, which consists in the iterative fitting of structural and/or lattice parametersto the powder diffractogram, has allowed a precise quantitative determination ofcrystalline and amorphous phases content.

The existence of regions of aperiodic domains has been reported in some ceramicmaterials only when investigated by such short range techniques as perturbed angularcorrelations (PAC) [3, 4] or extended X-ray absorption fine structure (EXAFS)[5]. The PAC method [6], in particular, has proved to be an efficient tool inthe investigation of zirconia-based ceramics because it allows the determinationof different atomic configurations around zirconium sites, hardly resolvable byother techniques. Natural hafnium impurities in zirconium, randomly distributed atsubstitutional zirconium sites, become highly efficient hyperfine radioactive probes.In fact, the thermal neutron irradiation of the sample activates some of the 180Hfisotopes by transforming them in the 181Hf probes. The method briefly consists in aninspection of the angular correlation of the 133–482 keV γ –γ cascade emitted duringthe 181Hf to 181Ta β− decay. The comparison of the measured angular correlationagainst that of the isolated probe, known from the nuclear physics, allows gatheringinformation about the electric field gradients (EFGs) existing in the lattice wherethe nuclear probes are immersed. The information is drawn from the determinationof the so-called quadrupole parameters that describe the EFG at the zirconium site,i.e., its intensity (through the quadrupole frequency ωQ) and its departure from axialsymmetry (through the asymmetry parameter η). In addition, the degree of localdisorder due to the presence of impurities or defects in the lattice can be measuredthrough δ, the distribution width of ωQ. On account of the r−3 dependence ofthe quadrupole interaction, the technique is extremely localized and nonequivalentprobe surroundings can be determined. All PAC hyperfine quantities just defined

Mullite–zirconia–zircon composites 221

are obtained by fitting the experimental A2G2(t) spin rotation curve determined atthe laboratory.

Among zirconia based ceramics, mullite–zirconia–zircon (3Al2O3.2SiO2–ZrO2/ZrSiO4) multicomposites (MZz) are of great technological interest due totheir applications in glass and steel industries [7–12]. The MZz multicompositesexhibit excellent corrosion and thermal shock resistances and enhanced mechanicalproperties [13, 14]. These materials can be obtained by different processes [15–19].The content and structure of phases present in these materials may be relatedto the phase content of the raw materials and to the eventual zircon dissociationand monoclinic to tetragonal martensitic zirconia transformation [20]. It has beensuggested [14] that during MZz sintering total or partial zircon decomposition intozirconia and silica occurs depending on zircon concentration. Nevertheless, by usingXRD and SEM micrography, no evidence of the resulting silica was found.

The PAC technique has been recently applied to the study of mullite–zirconia andzircon raw materials [4]. It was found that both of them exhibited a novel hyperfineinteraction describing non diffracting regions of zircon quite distinguishable fromthose depicting crystalline zircon [21], monoclinic zirconia and a distorted monocliniczirconia already found in milled zirconias [22].

In this paper mullite–zirconia/zircon (3Al2O3.2SiO2–ZrO2/ZrSiO4) compositeswith increasing zircon amounts, sintered at 1,600◦C for 2 h, are studied via the PACmethod. Results on the as-prepared samples and also on two of them after annealedat 1,500◦C for 15 h samples are compared with those derived from the long rangeXRD technique and quantitative Rietveld analysis. The aim is to supply informationconcerning the zircon decomposition during the processing of these multicompositesand the eventual relation with the reported mechanical properties.

2 Experimental

2.1 Composites

Mullite–zirconia–zircon multicomposites were prepared following reference [14] andusing the commercial raw materials characterized by PAC in reference [4]. Mixturesof electrofused mullite–zirconia, named MZ, (MUZR, Elfusa LT, Brazil) powdermilled to D50 = 5 μm and 15, 25, 35 and 45 wt.% of zircon powder, named z,(Mahlwerke Kreuts, Mikron, Germany, D50 = 2 μm) were consolidated by slipcasting and dried at 110◦C. The resulting compacts, hereinafter labeled MZz15,MZz25, MZz35 and MZz45, were fired at 1,600◦C for 2 h at heating and coolingrates of 10◦C/min. To perform the XRD and the PAC experiments, powder sampleswere obtained from the compacts.

2.2 XRD analyses

XRD patterns were recorded at room temperature using the Cu-Kα radiation ina Philips 3020 diffractometer on the milled composites. Data were collected in therange of 10 ≤ 2θ ≤ 70◦ in steps of 0.04◦ with a counting time of 2 s per step.

222 N.M. Rendtorff et al.

Fig. 1 Diffractograms of thedifferent multicomposites.M: mullite, B: baddeleyite(m-ZrO2) and Z : zircon(ZrSiO4)

20 30 40 50

BB

BBBB

BB

B

B

M

M

MM

MM

M

MZz25

MZz35

Z

ZZ

Z

Z

Z

MZz45

MZz15

Inte

nsit

y (a

.u.)

Quantitative Rietveld analysis was carried out to characterize the crystallinephases present in all the composites using the FullProf program [23, 24].

2.3 PAC analysis

For the PAC experiments, the samples were irradiated in a ∼1013 neutrons/s cm2 fluxto activate 181Hf probes. The spin rotation curves A2G2(t) were obtained at roomtemperature using a four BaF2 detectors setup with a time resolution of about 1 ns athafnium energies.

3 Results and discussion

The Rietveld analyses of the diffractograms of mullite–zirconia–zircon composites,plotted in Fig. 1, revealed the presence of monoclinic zirconia (m-ZrO2 or bad-deleyite) and mullite in all samples. In MZz15 and MZz25 no zircon content wasdetected while in MZz35 and MZz45, 20 and 29 mol% of zircon were respec-tively determined. Figure 2 shows the relative fractions of mullite, zirconia and

Mullite–zirconia–zircon composites 223

Fig. 2 Nominal (full lines)and Rietveld molar fractionsfor zirconia (full squares),zircon (full circles) andmullite (open triangles)

MZz

15

MZz

25

MZz

35

MZz

45

0

25

50

75

100

Mol

ar f

ract

ion

(%)

Zirconia

Zircon

Mullite

Fig. 3 Spin rotation curvesA2G2(t) of the differentmulticomposites. Full linesrepresent the fitting curves MZz15

MZz25

-A2G

2(t)

MZz35

MZz45

time (ns)

224 N.M. Rendtorff et al.

Fig. 4 PAC quadrupoleparameters and relativefractions found in themulticomposites: m-ZrO2(full squares), m’-ZrO2(open squares), crystallinezircon cz (full circles)and aperiodic zirconxz (open circles)

0

25

50

75

100

50

100

150

0.00

0.25

0.50

0.75

1.00

MZz15

MZz25

MZz35

MZz45

0

5

10

f(%

)Q

(Mra

d/s)

η(%

)

zircon for MZz15, MZz25, MZz35 and MZz45 determined by the Rietveld analysis(symbols) as compared with their respective nominal molar fractions (full lines).An important mismatch can be observed for zirconium containing-compounds,which was attributed in a previous investigation [14] to the partially reversible zircondecomposition during firing into zirconia and non distinguishable by XRD silica.

Figure 3 shows the PAC spin rotation curves obtained for the activated compos-ites. The fitting procedure reveals the existence of several non equivalent sites forthe PAC probes, whose quadrupole parameters have already been reported for theraw materials [4]. As it can be seen in Fig. 4, the interaction of m-ZrO2 (full squares)predominates in all samples. It is accompanied by those describing both, the distortedmonoclinic nanoconfigurations (m’-ZrO2, open squares) and the aperiodic zirconform (xz interaction, open circles). Crystalline zircon (cz interaction, full circles) isrevealed only in MZz35 and MZz45.

A further comparison, now between nominal and PAC fractions, required theconsideration that both the m-ZrO2 and m’-ZrO2 interactions contribute to zirconia

Mullite–zirconia–zircon composites 225

Fig. 5 Nominal molarfractions (full lines) forzirconia and zircon and PACfractions for (m + m’)-ZrO2(full squares) and (xz +cz)-ZrSiO4 (full circles)

MZz15

MZz25

MZz35

MZz45

0

25

50

75

100

Mol

ar f

ract

ion

(%)

Zirconia

Zircon

MZz25

MZz45

time (ns) 2θ

Intensity (a.u)-A

2G2(t

)

Fig. 6 XRD diffractograms and PAC spin rotation curves of samples MZz25 and MZz45 beforeand after annealed at 1,500◦C for 15 h. The arrow points out the angular position of the most intensediffraction line of zircon

226 N.M. Rendtorff et al.

Table 1 Molar fractions ofzirconia and zircon onceannealed at 1,500◦C for 15 h

aCrystalline zircon

% mole phase Sample MZz25 Sample MZz45content m-ZrO2 ZrSiO4 m-ZrO2 ZrSiO4

Nominal 63 37 41 59Rietveld 64 ± 5 36 ± 3 43 ± 4 57 ± 4PAC 70 ± 6 30 ± 2a 49 ± 4 51 ± 4a

concentration and that zircon is present in its two forms, crystalline and aperiodic.In addition, nominal amounts had to be renormalized by discounting the mullitecontribution since PAC can not probe it.

As shown in Fig. 5, the amounts determined by PAC fit well the nominal quantitiesused for the synthesis. This prime agreement allows an explanation of the apparentdiscrepancy with Rietveld derived quantities evidenced in Fig. 2. Results clearlyindicate that all zircon used for the preparation is present, though involving notonly its crystalline network but also non diffractant aperiodic regions. The reportedhypothesis that zircon irreversibly decomposed (totally in MZz15 and MZz25 andpartially in MZz35 and MZz45) during preparation [14] must be reinterpreted. ThePAC technique clearly indicates that during the cooling step of the sintering process,decomposed zircon totally re-associates, a fraction of it becoming aperiodic. Thus,the improved thermal and mechanical behavior reported for the samples havingmore zircon could be related to the increasing amount of crystalline zircon relative toaperiodic zircon instead of to an excess of ZrO2 coming from zircon decomposition.

Knowing that the amount of crystalline zircon in the raw materials increases byheating at 1,500◦C during 15 h [4], samples MZz25 (with no crystalline zircon) andMZz45 (with both forms of zircon) were investigated again after being annealedin the same way. Figure 6 shows the corresponding PAC spectra and partialdiffractograms. For comparison, the diffractograms and the spin rotation curvesof the as-prepared samples have been included. Results indicate that the thermaltreatment was enough to cause the complete crystallization of zircon and the healingof the distorted monoclinic zirconia. Rietveld and PAC results are listed in Table 1together with the nominal molar fractions. The fractions drawn from both the longrange and the short range experimental techniques match fairly well the nominalamounts of the materials used to prepare the multicomposites.

4 Conclusions

A short range PAC and long range XRD study on mullite–zirconia–zircon com-posites with different amounts of zircon has been carried out on the as-preparedmaterials and after an annealing treatment.

The multicomposites involve two types of zircon nanoconfigurations, one of whichis not sensed by XRD, and two forms of monoclinic zirconia, regular and distorted,both diffracting at the same angular positions.

Zircon does not irreversibly decompose during the sintering process of these com-posites. Instead, it does it in a reversible way but giving rise to different zircon forms:the ordinary (crystalline zircon) and an aperiodic form. From the present study itseems that the improved thermal and mechanical behavior of these composites couldbe related to the amount of crystalline zircon.

Mullite–zirconia–zircon composites 227

The mismatch evidenced between nominal, XRD and PAC concentrations ofzirconium containing compounds, is overcome by a long term annealing at 1,500◦C.

The present results clearly indicate how the complementary use of short and longrange techniques can contribute to a thorough knowledge of the phase content ofcomplex materials.

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