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Available online at www.sciencedirect.com Journal of the European Ceramic Society 32 (2012) 3567–3573 Influence of platinum foil impurities on sintering of functional ceramics Thomas Graule a,, Andrea Ulrich b , Markus Wegmann a a Laboratory for High Performance Ceramics, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129, CH-8600 Dübendorf, Switzerland b Analytical Chemistry, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129, CH-8600 Dübendorf, Switzerland Received 30 November 2011; received in revised form 2 May 2012; accepted 10 May 2012 Available online 30 May 2012 Abstract Impurities in ceramic powders or in the sintering atmosphere can have a strong influence on the densification and microstructure development during sintering. Our sintering studies have shown that the presence of bulk platinum and adsorbed minor amounts of adsorbed impurities during the sintering process can affect the microstructural and property development of materials via the gas phase. Four different oxide powders were shaped into green bodies and sintered in presence and absence of bulk platinum. Analyses of the materials after sintering show clear differences in the microstructure and the electronic properties between samples sintered in a furnace environment and those sintered in contact or in the close vicinity of platinum foil. When Pt foil had been present in the sintering set-up, trace chemical analysis detected accumulations of the platinum metal and other trace impurities at crucible surfaces which had not been in direct physical contact with Pt foil. © 2012 Elsevier Ltd. All rights reserved. Keywords: Sintering; Impurities; Platinum; Microstructure; Oxides 1. Introduction Impurities in the starting ceramic powders as well as con- taminants in the sintering atmosphere play a significant role in densification of the ceramic part as well as in the microstruc- ture and properties of the received ceramic material. 1 To avoid any contact between crucibles and green parts during binder removal and sintering platinum foil is typically used as a pro- tective material. Due to its high melting point of 1772 C and its low reactivity, components finished from, or containing consid- erable fractions of platinum metal are frequently encountered in the hot zones of processing and analysis equipment used in the field of materials science and engineering. These include substrates and support structures in sintering furnaces, crucibles and support assemblies in thermal analysis equipment, the heat- ing elements in Pt-wire furnaces, and thermocouple types B, R and S for temperature measurement. In general, the metal is considered to be highly inert with respect to its surroundings, especially when no obvious visible reaction occurs between the platinum and the material being processed, or where there is no Corresponding author. Tel.: +41 58 765 4123; fax: +41 58 765 4150. E-mail address: [email protected] (T. Graule). direct physical contact between the two. Practically no consider- ation is generally given to possible contamination of the charge material by platinum foil or contaminants on the platinum foil surface originating from cross contamination and the effect on the properties of the resulting product. An exception to this generalization is in the area of glass melt processing where Pt contamination has been treated on occasion in the literature. In this industry the topic is of prime interest because platinum constructs are used extensively to con- tain, agitate and form highly corrosive glass melts (e.g. crucibles, stirrers, channels, bushings) and its presence in concentrations as low as 0.001 wt% is known to promote the nucleation of crys- talline phases in the glass product. 2,3 In certain cases platinum contamination has been confirmed and effects on the glass prod- uct have been recorded, 4 while in others no obvious changes in the glass properties have been observed. 5 Due to the direct contact between the melts and the platinum metal in glass melt processing, the contamination of the glass with the metal is a logical consequence. In light of the refractory nature of platinum, it is more difficult to believe that contami- nation may also occur by means of transport in the gas phase. However, research has shown that when oxygen is present at temperatures between 900 C and 1500 C, platinum metal can be oxidized to PtO 2 and transported in the gas phase. 6,7 This 0955-2219/$ see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.05.018

Influence of platinum foil impurities on sintering of functional ceramics

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Page 1: Influence of platinum foil impurities on sintering of functional ceramics

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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 32 (2012) 3567–3573

Influence of platinum foil impurities on sintering of functional ceramics

Thomas Graule a,∗, Andrea Ulrich b, Markus Wegmann a

a Laboratory for High Performance Ceramics, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstr.129, CH-8600 Dübendorf, Switzerland

b Analytical Chemistry, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129, CH-8600 Dübendorf, Switzerland

Received 30 November 2011; received in revised form 2 May 2012; accepted 10 May 2012Available online 30 May 2012

bstract

mpurities in ceramic powders or in the sintering atmosphere can have a strong influence on the densification and microstructure developmenturing sintering. Our sintering studies have shown that the presence of bulk platinum and adsorbed minor amounts of adsorbed impurities duringhe sintering process can affect the microstructural and property development of materials via the gas phase. Four different oxide powders werehaped into green bodies and sintered in presence and absence of bulk platinum. Analyses of the materials after sintering show clear differences

n the microstructure and the electronic properties between samples sintered in a furnace environment and those sintered in contact or in the closeicinity of platinum foil. When Pt foil had been present in the sintering set-up, trace chemical analysis detected accumulations of the platinumetal and other trace impurities at crucible surfaces which had not been in direct physical contact with Pt foil.

2012 Elsevier Ltd. All rights reserved.

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eywords: Sintering; Impurities; Platinum; Microstructure; Oxides

. Introduction

Impurities in the starting ceramic powders as well as con-aminants in the sintering atmosphere play a significant role inensification of the ceramic part as well as in the microstruc-ure and properties of the received ceramic material.1 To avoidny contact between crucibles and green parts during binderemoval and sintering platinum foil is typically used as a pro-ective material. Due to its high melting point of 1772 ◦C and itsow reactivity, components finished from, or containing consid-rable fractions of platinum metal are frequently encounteredn the hot zones of processing and analysis equipment used inhe field of materials science and engineering. These includeubstrates and support structures in sintering furnaces, cruciblesnd support assemblies in thermal analysis equipment, the heat-ng elements in Pt-wire furnaces, and thermocouple types B,

and S for temperature measurement. In general, the metal is

onsidered to be highly inert with respect to its surroundings,specially when no obvious visible reaction occurs between thelatinum and the material being processed, or where there is no

∗ Corresponding author. Tel.: +41 58 765 4123; fax: +41 58 765 4150.E-mail address: [email protected] (T. Graule).

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955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2012.05.018

irect physical contact between the two. Practically no consider-tion is generally given to possible contamination of the chargeaterial by platinum foil or contaminants on the platinum foil

urface originating from cross contamination and the effect onhe properties of the resulting product.

An exception to this generalization is in the area of glasselt processing where Pt contamination has been treated on

ccasion in the literature. In this industry the topic is of primenterest because platinum constructs are used extensively to con-ain, agitate and form highly corrosive glass melts (e.g. crucibles,tirrers, channels, bushings) and its presence in concentrationss low as 0.001 wt% is known to promote the nucleation of crys-alline phases in the glass product.2,3 In certain cases platinumontamination has been confirmed and effects on the glass prod-ct have been recorded,4 while in others no obvious changes inhe glass properties have been observed.5

Due to the direct contact between the melts and the platinumetal in glass melt processing, the contamination of the glassith the metal is a logical consequence. In light of the refractoryature of platinum, it is more difficult to believe that contami-ation may also occur by means of transport in the gas phase.

owever, research has shown that when oxygen is present at

emperatures between 900 ◦C and 1500 ◦C, platinum metal cane oxidized to PtO2 and transported in the gas phase.6,7 This

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3568 T. Graule et al. / Journal of the European Ceramic Society 32 (2012) 3567–3573

Table 1Materials systems.

Material/sintering aids Grade/source D50 (�m) Binder Solids loading (vol%) Ref.

SiO2/– Aerosil OX50, Evonik 0.11 Polyethylene/wax 52 8,9BaTiO3/4Al2O3–9SiO2–3TiO2 Ticon F, Ferro Corp. 0.8 Wax 60 10Ba0.7985Sr0.2La0.0015TiO3/4Al2O3–9SiO2–3TiO2

a 0.8 Wax 60 10B

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2

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a0.7985Pb0.2La0.0015TiO3/4Al2O3–9SiO2–3TiO2a

a Positive temperature coefficient (PTC) compositions based on Ticon F, Ferr

lso applies for possible ceramic like contaminants in the plat-num surface originating from the reused platinum foil. Sincehis temperature range is also typical for many sintering pro-esses in ceramic engineering, it must therefore be consideredhat platinum contamination can occur in cases where there is nobvious reaction between the sample and platinum, and even inituations where there is no direct physical contact between thewo. In all, compact green bodies of four different oxide pow-ers, namely SiO2 and three BaTiO3-based compositions haveeen sintered in both the presence and absence of bulk plat-num. The sintered materials have been characterized in termsf microstructure and properties and trace chemical analysesave been performed to check for the presence of the elementt in both the samples and the crucibles used during sintering.

. Experimental

.1. Shaping

All materials under consideration here were prepared byay of a thermoplastic-binder-based extrusion method. Detailsf the materials are summarized in Table 1 together withhe associated literature reference where the process detailsave previously been described in detail. In general, thexide powder/thermoplastic binder feedstocks were blendedn an electrically-heated twin-blade mixer (Haake Rheomix000/Rheocord 9000 combination, Thermo Electron Corp.,peyer, Germany) and subsequently extruded at temperaturesbove the respective binder melting points as fibers with vari-us cross-sectional geometries (circular, rectangular, triangular)nd minor dimensions between 150 and 500 �m using a capillaryheometer (Rosand RH7-2, Malvern Instruments Ltd., Malvern,K).

.2. Sintering

Samples were sintered in an electric tube furnace (HST5/610 1500 ◦C Split Tube Furnace, Carbolite Furnaces Ltd.)quipped with an alumina furnace tube (99.7% Al2O3, Halden-anger). Generally two sintering runs with the same scheduleere performed for each material, one featuring metallic plat-

num in the immediate vicinity of the samples, and one without.The SiO2 fibers were sintered in closed-top alumina crucibles

99.7% Al2O3, Haldenwanger) lying at the base of the crucible.n one case no platinum was present anywhere within the closedrucible, in the second case a curved piece of 99.9% Pt foil0.1 mm thick, Sigma Aldrich) was placed over the samples.

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his foil was carefully positioned in order to avoid direct con-act with the fibers, the minimum separation between the twoeing approx. 2 mm. The organic binder was removed from theamples by binder removal in air (15 ◦C/min ramp to 500 ◦C, 2 hwell; 150 ◦C/min ramp to 750 ◦C, 2 h dwell) and sintering waserformed immediately thereafter (15 ◦C/min ramp to 1100 ◦C,

h dwell).Fibers of the non doped BaTiO3 composition in Table 1 were

intered in the same closed-top alumina crucibles and with theame substrates as described above, but draped over a 99.7%l2O3 rod fixed horizontally across the top of the crucible. In

his case there was no direct contact between the substrate at thease of the crucible and the hanging samples, the minimum sepa-ation between the two being approx. 5 mm. The extrudates werehermally debound under static air (1.5 ◦C/min ramp to 400 ◦C,◦C/min ramp to 700 ◦C, 1 h dwell) and sintered immediately

hereafter (10 ◦C/min ramp to 1360 ◦C, 1 h dwell).Fibers of the La/Sr- and La/Pb-doped BaTiO3 compositions

isted in Table 1 were debound and sintered lying either on Ptubstrates, or on substrates prepared from the respective titanateompositions. These substrates were affixed to a 99.7% puritylumina rod which could be slid parallel to the furnace tube axisnto the hot zone. For the debinding step, the samples on theirubstrates were positioned in the hot zone and subjected to theebinding schedule described in the previous paragraph. Fol-owing debinding and after sliding the rod with the samples outf the furnace, the furnace was heated up to 1360 ◦C. The sam-les were subsequently slid rapidly into the hot zone at 1360 ◦C,enerating a heating rate of ≈1000 ◦C/min. After sintering for0 min in static air, the fibers were removed from the furnacend cooled rapidly to RT.

.3. Analyses

Microstructural analyses of the samples after sintering wereerformed with a scanning electron microscope (SEM; JSM-300F, Jeol Corp.). Samples were mounted on aluminum sampletubs on adhesive carbon pads and sputtered with an Au–Pdlloy to prevent electrical charging during the SEM analy-is (V = 20 kV, WD = 39 mm) Powder X-ray diffraction (XRD;’Pert Pro MPD, PANalytical B.V.; Cu-K� radiation, I = 40 mA,

= 40 kV, step-size = 0.017◦ 2θ) for phase analysis was con-ucted with pulverized material mounted on silicon single

rystal holders with vaseline.

The ferroelectric behavior of non doped sintered BaTiO3bers (circular cross-section, ∅ = 260 �m, L = 2.5 mm) was

nvestigated using a recently developed characterization method

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T. Graule et al. / Journal of the European Ceramic Society 32 (2012) 3567–3573 3569

Fig. 1. Data for the LA-ICP-MS analyses of the interior surfaces of the alumina crucibles used in this study: (a) crucible used for sintering materials only in thepresence of Al2O3 and MgO and (b) crucible used for sintering material in the presence of Pt. The samples for these measurements were taken from the crucibleroofs, i.e. the sample in (b) had no direct physical contact with Pt foil at any time. While operation of both the laser and the ICP-MS are initiated at t = 0 s, a timeds ately

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hutter prevents the laser light from reaching the sample until t = 20 s. Approximo the ICP-MS, hence the delay of approx. 30 s before a signal is measured. The

ased on a modified dynamic mechanical analyzer (DMA).12

fter polarizing an individual fiber for 5 min at RT in an electriceld of 3.5 kV/mm, its free strain evolution was measured as

function of an applied electric field which was cycled in theange ±3 kV/mm at a frequency of 2.777 mHz. In this fashion,o-called butterfly curves were generated for each fiber.

The semiconducting behavior of the La/Pb- and La/Sr-dopedaTiO3 fibers (L = 2 mm, ∅ = 400 �m and 420 �m, respec-

ively), was measured using the two-point zero-power DCethod.10 The sample resistance was measured every 10 ◦C-

ncrement while the temperature of the air surrounding theample was heated at a rate 300 ◦C/h, thus generating the equilib-ium resistivity–temperature characteristic for each individualber.

Trace elemental analysis in the samples and at the crucibleurfaces was performed using a modified Nd:YAG LASER (LA)erkinElmer/Sciex 32013–16 attached to a PerkinElmer/SciexLAN 6000 quadrupole inductively coupled plasma mass spec-

rometer (ICP-MS). The material mounted in the samplingell was ablated for 60 s by the laser (λ = 266 nm, Q-switchelay = 300 �s, pulse repetition rate = 10 Hz). The ablated par-icles were transported from the cell by a 5.0-grade argon gasow to the plasma mass spectrometer. The ICP-MS was oper-ted under standard hot plasma conditions (RF power = 1250 W,.3 l/min sample gas, 15 l/min cool gas and 0.8 l/min auxiliaryas).

. Results and discussion

.1. LA-ICP-MS analysis

Visual comparison of the inner surfaces of the crucibleshowed that those which had contained Pt substrates had beeniscolored a faint grey-yellow during sintering, whereas the oth-rs retained the bright white color observed for the crucibles in

tts

a further 10 s are required to transport the ablated material in flowing argon gaser remains open 60 s, hence the rapid reduction in the signal after approx. 90 s.

heir as-delivered condition. Microanalytical ultra-trace chemi-al analysis using LASER ablation inductively coupled plasmaass spectroscopy was subsequently performed to investigate

he chemical nature of the discolored material (Fig. 1). In thel2O3 crucible used to sinter the various materials without plat-

num present, the measurement only generated a strong Al-signaltemming from the base crucible material (Fig. 1a). In additiono the bulk Al signal, the sample from the crucible used to sin-er material in the presence of platinum also generated a cleart signal well above the background. Here it is interesting toote, that the contamination was present despite the fact that thenalyzed material was taken from a location inside the cruciblehich had never been in direct contact with platinum (Fig. 1b).LA-ICP-MS as a local micro-analytical method allows

patially-resolved multi-element analysis of localized areas asell as the acquisition of depth profiles. The form of the tran-

ient signals in Fig. 1b seems to indicate that Pt is concentratedt every surface of the sample. Determinations in different loca-ions yielded varying signal intensities, and at some locationso platinum could be detected. This leads to the conclusion thathe platinum deposit is not homogeneously distributed at theurface. A quantification of the Pt concentration has not beenossible to date due to the unavailability of suitable calibrationtandards.

Parallel analyses for the other constituent elements in theaterials sintered in the crucibles, i.e. Si, Ba and Ti, showed

hat these had not contaminated the crucibles. The elements La,b, Sr, Y and Cu were not considered because the La/Sr- anda-Pb-doped BaTiO3 fibers were sintered without crucibles (seeection 2).

The fact that traces of the element Pt were detected at theurface of the crucible at locations which were never in con-act with the Pt foil leads to the conclusion that platinum is

ransported in the vapor phase in the furnace atmosphere atemperatures on the order of 1000 ◦C and above. This conclu-ion is in agreement with previous research which concludes
Page 4: Influence of platinum foil impurities on sintering of functional ceramics

3570 T. Graule et al. / Journal of the European Ceramic Society 32 (2012) 3567–3573

F ered 4F he out

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ifully cleaned, but yet contaminated used platinum foil (e.g. usedfor the protection of superconducting ceramics e.g. containingBa, Sr and copper) was detectable at a few sample locations. This

ig. 2. SEM micrographs of extruded triangular cross-section SiO2 fibers sintrame (c) shows a higher magnification image taken at the fracture surface of t

hat in the presence of oxygen, the metal is oxidized to PtO2hich can then migrate through the gas phase by chemical vapor

ransport.6,7 Furthermore refractory oxides as contaminants maye easily transported as suboxides via gas phase from the foil tohe not completely densified sintered parts. With this in mind,he results of the characterizations performed on the sintered

aterials presented below can be discussed.

.2. SiO2

SEM analysis revealed that SiO2 fibers sintered under a hoodf Pt foil exhibit a totally different microstructure from thoseintered in the Pt-free Al2O3 crucible (Fig. 2). While the latteraterial exhibits a homogeneous fracture surface typical of an

morphous glass (Fig. 2a), the fibers sintered in the presencef platinum foil contaminants are characterized by a curiousore–shell morphology at the fracture surface (Fig. 2b) and aighly porous microstructure on the submicron scale (Fig. 2c).onsiderable differences are also evident in the XRD data for

he two cases (Fig. 3). While the material sintered in the Pt-ree environment is totally amorphous, the silica sintered in theresence of Pt exhibits considerable crystallinity in the form ofristobalite and tridymite.

Analysis of the sintered SiO2 with LA-ICP-MS showed a

trong signal of the Si primary constituent (Fig. 4). The tracelemental analysis was difficult because of the small sampleize and the brittleness of the material, nevertheless a verymall Pt signal accompanied by a significant amount of other

Fccw

h at 1100 ◦C in air: (a) on an Al2O3 substrate and (b) on a Pt foil substrate.ermost shell in (b).

mpurities originating from the cross contamination of the care-

ig. 3. XRD patterns of SiO2 fibers sintered 8 h at 1100 ◦C in air in either alosed Pt-free Al2O3 crucible (black curve), or under a hood of Pt foil (greyurve) in a closed Al2O3 crucible. The silica in the latter case was not in contactith the platinum metal at any time.

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T. Graule et al. / Journal of the European Ceramic Society 32 (2012) 3567–3573 3571

100

1000

10000

100 000

1000 000

150100500

Inte

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Time in s

Al

Si

Ti

Ba

Pt

Cu

Sr

Co

Fig. 4. Trace elemental analysis by laser ablation inductively coupled plasmamp

inicn

aflcfoosithtificDttApfim

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Fig. 5. SEM micrographs of extruded rectangular cross-section BaTiO3 fiberssintered 10 min at 1360 ◦C in air hanging over (a) Al O and (b) a Pt foil sub-sd

mtpocm(

tcdWs1nforces. As the BaTiO3 grains grow by liquid phase sintering,the platinum may either be swept ahead of the crystallizationfront and become concentrated at the grain boundaries, or it

ass spectrometry of silica glass fiber indicating contamination by platinum andlatinum foil surface impurities.

ndicates that ultra-traces of platinum and additional contami-ants from the surface of the reused platinum foil have migratednto the bulk of the sintered material, but as noted above in thease of the crucible surfaces, the distribution of the element doesot appear to be homogeneous.

By combining the hypothesis that PtO2 is present in thetmosphere surrounding the samples during sintering with theact that platinum is a known nucleating agent for crystal-ization in glasses, the mechanism which leads to the unusualore–shell morphology observed for the fibers sintered under Ptoil (Fig. 2b) can be speculated upon. After decomposition of therganic binder (below 500 ◦C), the material contains 48 vol%pen porosity (Table 1). As the temperature rises toward theintering temperature of 1100 ◦C, PtO2 vapor as well as othernorganic matter containing vapor begins to be generated,6,7 andhis will diffuse from the surface of the debound fiber into theighly porous microstructure. Upon deposition on the SiO2 par-icle surfaces, the PtO2 induces crystallization of the particlesn the near-surface region, and subsequently this zone densi-es more slowly (Fig. 2c) than still Pt-free material nearer theenter of the fiber which continues to sinter by viscous flow.ue to the different shrinkage rates of the surface and core, a

ensile stress perpendicular to the fiber surface is developed inhe microstructure, leading to delamination of the surface layer.s PtO2 vapor and vapor originating from the contaminatedlatinum foil surface diffuses further and further from the sur-ace into previously platinum-free material, this process repeatstself several times, leading to the observed fracture surface

orphology.

.3. BaTiO3

For the BaTiO3 samples, differences also manifested them-elves between the fibers sintered in the Pt-containing and Pt-free

et-ups. While the BaTiO3 fibers in the latter case were col-red dark brown after sintering and exhibited a relatively broadistribution of grain sizes (Fig. 5a), the fibers processed in theresence of Pt were a light cream color and a homogeneous t

2 3

trate. The fiber in (b) had no direct physical contact with the Pt foil at any timeuring processing.

icrostructure with a clearly more narrow grain size distribu-ion (Fig. 5b). The sintering set-up also had an effect on theiezoelectric properties of the fibers (Fig. 6). Fibers sinteredver platinum exhibited stronger hysteresis in their butterflyurves and a higher coercive fieldc (Ec ≈ 0.75 kV/mm) than theaterial sintered in an environment containing only alumina

Ec ≈ 0.5 kV/mm).Assuming that PtO2 vapor is present above approx. 900 ◦C in

he air surrounding the fibers sintered over Pt foil, it can be con-luded that platinum oxide and vapor from volatile suboxides iseposited on the fibers at the beginning of the sintering process.hen the eutectic reaction between the 4Al2O3–9SiO2–3TiO2

intering aid generates a liquid phase as the temperature reaches240 ◦C,18 the deposited PtO2 will be rapidly and homoge-eously distributed throughout the microstructure by capillary

c The coercive field Ec is the electric field which needs to be applied to returnhe material to its original length, i.e. back to the zero-strain condition (ε = 0).

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3572 T. Graule et al. / Journal of the European Ceramic Society 32 (2012) 3567–3573

Fig. 6. Field-induced strain characteristics of extruded circular cross-sectionBaTiO3 fibers sintered 10 min at 1360 ◦C (260 �m sintered diameter): blackcurve, Al2O3 substrate; grey curve, Pt foil. Prior to measurement the fibers werepolarized in an applied electric field of 3.5 kV/mm, and during the measurementthe applied field was cycled at 2.777 mHz. Such “butterfly” curves are character-iu

mtisfiphaBiipcm

3

BdotmotoscPh

Fig. 7. Electrical resistivity as a function of ambient temperatureof circular cross-section extruded Ba0.7985Sr0.2La0.0015TiO3 (BST) andBa0.7985Pb0.2La0.0015TiO3 (BPT) fibers sintered 10 min at 1360 ◦C in air(420 �m and 400 �m sintered diameters, respectively); black curves, on sub-strates of the respective titanate composition; grey curves, on Pt foil. The curveshape is characteristic of rare-earth doped semiconducting positive temperaturecoefficient (PTC) thermistors based on BaTiO3, the transition temperature wherethe switch from negative to positive temperature coefficient behavior occursb

psaibr

4csHfootitaoobo

stic of piezoelectric materials which exhibit considerable dimensional changesnder the influence of an applied electric field.

ay be incorporated into the bulk, or both. In the current casehe experimental data lead to the suggestion that the elements present both at the grain boundaries and in the grains them-elves. The refined microstructure (Fig. 5b) relative to that of theber sintered under Pt-free conditions (Fig. 5a) indicates that thelatinum may act as a grain-growth inhibitor, and the strongerysteresis of the fiber sintered over Pt suggests that the elementlso inhibits switching of ferroelectric domains present in theaTiO3 grains when the electric field is reversed (Fig. 6). The

ncorporation of dopant into the barium titanate lattice is alsondicated by the difference in color observed between the sam-les from the two sintering set-ups, with color changes beingharacteristic of a change in the electronic band structure of theaterial.

.4. La/Sr- and La/Pb-doped BaTiO3 compositions

Whereas in the SEM the microstructures of thea0.7985Sr0.2La0.0015TiO3 and Ba0.7985Pb0.2La0.0015TiO3id not exhibit any visually obvious differences as a functionf substrate material, a variation in sample color was visibleo the naked eye. In the case of the La/Sr-doped composition,

aterial sintered on Pt foil possesses a clearly lighter shadef blue than the analogous fibers sintered in the absence ofhe foil. For the La/Pb-doped composition, the reverse trend isbserved. A definite difference between samples from the twointering set-ups is also evident in the resistivity–temperature

haracteristics (Fig. 7). The La/Sr-doped material fired ont foil exhibits a resistivity approx. one order of magnitudeigher across the measured temperature range than the material

PpL

eing fixed by the concentration of Pb or Sr in the respective compostion.17

rocessed on a substrate of the same titanate composition. Ahift in the characteristics as a function of substrate material islso evident for the lead-doped samples, however, here the trends in the opposite direction, with the samples fired on Pt foileing slightly less resistive across the measured temperatureange.

Since these two BaTiO3-based compositions also contain theAl2O3–9SiO2–3TiO2 sintering aid, PtO2 and additional crossontaminants could be introduced into the microstructure by theame mechanism as proposed above for the nondoped BaTiO3.ere the presence of Pt in the grain bulk is suggested by the dif-

erences in room temperature resistivity observed as a functionf the sintering set-up (Fig. 7). In semiconducting compositionsf BaTiO3, the resistivity below the ferroelectric – paraelectricransition temperature (here nominally at 50 ◦C and 200 ◦C)10

s governed by the resistivity of the grain bulk,19 and changesherein are associated with the presence of point defects (dopanttoms or vacancies) in the lattice. This correlates well withbserved color differences. While semiconducting compositionsf BaTiO3 with low room temperature resistivity exhibit intenselue colorations (here La/Sr-doped fibers sintered on titanatef the same composition, and La/Pb-doped fibers sintered ont), more highly resistive compositions are characterized by

aler shades of blue11 (La/Sr-doped fibers sintered on Pt, anda/Pb-doped material sintered on same).
Page 7: Influence of platinum foil impurities on sintering of functional ceramics

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. Conclusion

This study has shown that the presence of platinum and tracempurities adsorbed on the platinum foil in sintering set-upshich are used at temperatures at and above 1000 ◦C can influ-

nce the microstructural evolution and properties of sinteredxide materials, especially during sintering of microparts likehin fibers. Trace chemical analysis using LA-ICP-MS at thenterior surfaces of the crucibles which contained Pt foils duringhe sintering process confirmed the migration of platinum anddsorbed impurities through the gas phase to these surfaces. Thisesult, combined with published data on the oxidation behaviorf platinum and the localized detection of ultra-traces of Pt inome samples, leads to the conclusion that volatilization, chemi-al vapor transport, and subsequent deposition of PtO2 and othermpurities from the contaminated platinum foil on the materi-ls being sintered are responsible for the observed differencesetween these samples and samples sintered in a Pt-free envi-onment. Considering the widespread use of platinum foils in aariety of applications in the high-temperature processing andnalysis of materials, this remarkable result deserves attentiono avoid platinum foil induced cross contamination.

cknowledgement

The authors thank Dr. Juliane Heiber (Empa) for conductinghe piezoelectric measurements on the BaTiO3 fibers.

eferences

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