IntroductionAbout 1000 PJ (1 PJ = 1015 J) peryear are consumed for heat treat-ment processes with a maximumtemperature above 1000 °C in Ger-many [1]. This corresponds to 7 % ofthe total primary energy demand ofGermany. It is assumed that otherdeveloped countries spend a similar-ly high contribution of primary ener-gy for heating processes. The ceramic industry is one of themost energy intensive branches andconsumes more than one percent ofprimary energy in Germany. Energycost amounts 7,3 % of gross valueadded in the glass, ceramics andindustrial minerals branch [2]. Theimprovement of energy efficiency iscurrently driven by cost reduction.But other reasons for a careful mini-mization of energy consumptiongain more importance: the CO2footprint of products influences thedecision of end consumers to pur-chase. Moreover, the sustainabilityof companies affects their image. Recently the joint project ENITECwas finished after three years. Resultswere presented in a workshop inBayreuth [3] and a final report [4]. Itwas demonstrated that energy canbe saved during ceramic productionby improving composition andforming processes of green com-pacts, drying parameters, heat treat-ment and finishing processes. A very important contributioncomes from the optimization of thedebinding and sintering process. Itwas shown, that significant energysavings can be achieved without lossof product quality. In situ measure-ment methods and computer simu-lation, based on the in situ data, playan important role in well aimedprocess optimization. These meth-ods were already presented in a pre-vious article in the current journal[5]. Since the ceramic production chainis rather long, many process para-meters have to be carefully opti-mized in order to achieve the bestproduct quality with lowest energyconsumption. A significant reduc-tion of complexity can be achievedwhen the quality of the green com-pacts is evaluated before firing

(Fig. 1). If this quality is insufficientthe previous process steps of rawmaterial selection, preparation andforming have to be improved. Green compacts need excellenthomogeneity on the micro-, meso-and macro scale. Reasons are:• Micro scale: the ceramic particles

have to be arranged homogenous-ly on a scale smaller than 20 µm.Otherwise preferential sintering ofparticles in a locally denser config-uration will occur. Agglomeratesare formed, reaching final densitymuch earlier than the residualstructure. Grain growth sets inwithin the agglomerates, whichoften deteriorates the mechanicalproperties of the ceramics. In addi-tion the large pores which formbetween the agglomerates needvery high thermal energy to beeliminated by further sintering.

• Meso scale: microstructure varia-tions on a scale of 20–100 µm haveto be eliminated as well. Frequent-ly, this is the size range of defectswhich control the strength of theceramic parts. Already few defectsin the size of 20–100 µm per cubiccentimeter can significantly deteri-orate strength and reliability ofceramics. Elimination of thesedefects enables material-savingdesigns and reduces waste. Ineither case energy savings areobtained with ceramic production.

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• Macro scale: the green density hasto be in a very small range withinthe entire component. Otherwisean uneven shrinkage occurs duringsintering causing deformation andloss of near net shape perfor-mance. This leads to theexceedance of dimensional toler-ances or increasing finishing cost.Since the final density is nearlyconstant after sintering, the devia-tion is roughly proportional to thevariation of green density. E.g., a1 % increase of green density leadsto an increase of shrinkage of100 µm in a component of 10 mmdiameter. So green density varia-tions within a component orbetween different componentshave to be carefully avoided.

In this article methods are discussedwhich enable a careful evaluation ofgreen compacts on the micro-,meso- and macro scale. These methods have been devel-oped and tested within the projectENITEC. Experimental effort, accura-

Process Engineering

Energy Efficiency during Production of Ceramics

Friedrich RaetherFraunhofer-Institut für Silicatforschung95448 Bayreuth, Germany

E-mail:friedrich.raether@isc.fraunhofer.dewww.htl.fraunhofer.de

Fig. 1 Reduction of complexity in ceramic process development by green compact evaluation

microstructure. Planar cuts throughthe structure are required for a quan-titative evaluation of homogeneity.With green compacts, the prepara-tion of planar surfaces by traditionalgrinding and polishing is not applic-able because they are too weak.After a careful thermal treatment attemperatures below the onset of sin-tering, strength is improved by sur-face diffusion without affecting par-ticle arrangement. Then grinding and polishing can besuccessful but particle break off dur-ing machining can cause prepara-tion artifacts. In contrast, ion beamtechniques are universally applicableto prepare planar cuts. Especially suitable is the so calledCross Section Polishing (CSP). Anargon ion beam hits the sample sur-face parallel to the planar cut andproduces a planar area of about 100 x 100 µm². Since the sample isnot subjected to any forces, particlebreak out is completely avoided. Yet,the preparation of a planar cut byCSP technique requires about 5 h.Afterwards a SEM imaging tech-nique, which provides a very highcontrast between pores and parti-cles, has been used (Fig. 2). In-house software for processing andanalysis of these SEM images hasbeen developed. In a first stepbrightness gradients are removed ona scale much larger than the particlesize and then the images are bina-rised (Fig. 2). The binary images are divided insubsections and the pore fraction ineach subsection is automaticallydetermined. The variance of thispore fraction is used as measure formicro scale homogeneity. The vari-ance depends on the size of the sub-

sections. Therefore, it is scaled by thevariance obtained from a randomdistribution of pores. Fig. 3 shows a variance analysis ofan alumina green compact.Although the green density was sim-ilar, a clear difference between dif-ferent forming parameters can beseen. Fig. 4 demonstrates the higher sin-tering activity of the more homoge-nous green sample emphasizing thebenefits of evenly distributed parti-cles on the micro scale. The experimental effort for CSPpreparation, SEM imaging and vari-ance analysis is comparatively highin spite of automatic image analysis.Alternatively, micro-scale homo-geneity can be evaluated indirectlyby in situ measuring sintering shrink-age. The experimental effort of thein situ measurement is smaller but aunique interpretation of the mea-surements requires a narrow processwindow (identical raw materials,compositions etc.).

Meso scaleComputed tomography (CT) is aversatile tool to detect microstruc-ture defects in a size range of20–100 µm. It provides a 3D imageof the green compact with a resolu-tion of some microns. The timerequired for measuring a CT image isbetween 30 min and some hours.Usually the manufactures of ceram-ics cannot dispose of CT devices dueto their high cost. Therefore an alternative method wastested to detect defects on the mesoscale which needs small investment.It is based on an immersion liquidwhich has the same index of refrac-tion as the ceramics. The immersionmethod can only be used if theceramics consists of a single inor-ganic phase and if this phase is opti-cally isotropic. The index of refrac-tion of the immersion liquid has tobe matched accurately to the indexof refraction of the ceramics. Forthat, organic liquids with differentindices of refraction are mixed in theappropriate ratio to obtain the tar-get index. After infiltration of the immersionliquid the compacts become optical-ly transparent because light in thevisible wave length range is onlyweakly absorbed in most ceramicsand the immersion liquid eliminatesscattering effects. Defects which are located within theinfiltrated volume can be detectedby their different optical density.

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cy and field of application of themethods will be presented.

Evaluation methods forgreen compact quality

Micro scaleParticle size of most technical ceram-ics is in the range of 0,1–10 µm. Inthis size range, Scanning ElectronMicroscopy (SEM) is the mostappropriate method for imaging of

Process Engineering

Fig. 2 SEM image of a green sample after CSP preparation (l.) andcorresponding binary image (r.)

Fig. 3 Variance in binary SEM images using different forming parameters

Fig. 4 Sintering shrinkage L/L0 of the green samples shown in Fig. 3

Fig. 5 Defect in a green compact whichwas infiltrated with immersion liquid(top) and SEM image of the defect prepared by CSP technique (bottom)

Small defects are identified using alight microscope (Fig. 5). Defectdensity can be evaluated statistically.Moreover, the area of a defect canbe selected for target preparation,e.g. by CSP technique (compare pre-vious section) and SEM analysis(Fig. 5). This allows conclusionsabout possible sources of impurities. The immersion method can be mod-ified to detect large pores or cracksin the compacts. For that, a small part of the immer-sion liquid is removed after infiltra-tion. This causes a redistribution ofimmersion liquid within the com-pact depleting the larger pores andcracks whereas small pores are stillfilled completely with the liquid. Solight is scattered at the depletedstructures and they become visiblein the light microscope [4].The effort for using the immersionmethod is small – once an appropri-ate immersion liquid has been iden-tified. It has to be applied in a fumecupboard because harmful vaporsfrom the organic solvents occur.

Macro scaleIn principle various methods areavailable to measure the distributionof porosity on the component sizescale: • Small samples can be extracted

from the green parts and the bulkdensity of these samples can bemeasured either by theArchimedes method or by volumeand weight measurement. Fromthe bulk density, porosity can beeasily calculated if the true densityof the ceramics is known. Other-wise it can be measured usingArchimedes method or He pyc-nometry.

• From X-ray absorption local densi-ty can be measured and porosity

can be calculated. Either radiogra-phy or CT is available to obtain pla-nar or 3d distribution of porosity.

• The shape distortion after sinteringis related to the porosity distribu-tion within the green parts. Soindirectly sintering distortion canbe used to detect uneven porosity.

In the ENITEC project these methodshave been compared with regard toeffort and accuracy. Cylindrical zir-conia compacts with a diameter of20 mm or 70 mm formed by coldisostatic pressing were selected forthe tests. The density measurementof small samples extracted from thecompacts by Archimedes or volumemeasurement required no expensiveequipment. Yet, the accuracy of the measure-ments was limited to ±0,5 % even ifthey were repeated several times(Fig. 6). X-ray methods are signifi-cantly more expensive. Moreover, itturned out, that the accuracy of theX-ray methods was rather low –especially close to the surface of thegreen parts. This was attributed to scatteringeffects. It limits the use of X-raymethods because large density gra-dients are expected in the surfacezone of green compacts. For mea-suring sintering distortion, test sam-ples, either disks or cylindrical rods,were extracted from the green parts. Disks were cut with their symmetryaxis parallel to the symmetry axis ofthe cylindrical compact. Rods, with 20 mm diameter, wereextracted using hollow drills withtheir symmetry axis perpendicular tothe green part (Fig. 7a). With thesintered disks sintering distortionwas measured at its faces using con-focal microscopy whereas sinteringshrinkage of the rods was measuredat the lateral surface using thermo

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optical methods [6]. For the later 5to 10 measuring windows weredefined – evenly distributed overthe length of the sample (compareFig. 7a). The change of the sample width wasrecorded in situ within the measur-ing windows during sintering(Fig. 7b). Window position was adjusted tofollow exactly the shrinkage of thesample. Total shrinkage was extract-ed from the width of the individualwindows at 900 °C during heatingand cooling of the samples beforeand after sintering. The resolution ofthe thermo-optical method wasabout 10 µm leading to an accuracyof ±0,3 % in porosity measurement.The resolution of the confocalmicroscopy was even better, withinfew microns, but interpretation wasmore difficult because deformationat the opposing faces was not welldefined. It was considered that measuringsintering distortion is the most accu-rate method to determine porositydistribution in green compacts. Thesamples used have to be extractedfrom the green compact in a simplegeometry which reflects possibledensity gradients in the part.

Conclusions and outlookThe careful evaluation of green com-pact homogeneity contributes to theidentification of process parameterswhich lead to lower firing cost, lesswaste and better near net shape per-formance of ceramics. Finishing could be completely elimi-nated for small zirconia parts afterthe macro scale homogeneity of thegreen compacts had been ensured(Fig. 8). The energy saving was 24 %for a dental implant and 42 % for an

Process Engineering

Fig. 6 Porosity derived by Archimedes method in small samples extracted from a cylindrical green compact (sampleposition is indicated in the insert)

Fig. 7 a–b Extraction of small samples from a larger cylindrical green part and measuring win-dows used during sintering(a), change of sample width L/L0 within measuring windows (b)(the magnification shows the spread between individual windows)

a b

improved in a holistic approach con-sidering product quality, heat treat-ment parameters, high temperaturematerials and furnace equipment.HTL will closely cooperate withmaterial manufacturers and furnacecompanies in this project and pre-sent results in a further workshop in2015.

AcknowledgementThe project ENITEC was funded bythe German Federal Ministry of Educa-tion and Research (BMBF) with theassistance of the Project ManagementAgency Karlsruhe (PTKA). The authorgratefully acknowledges the help ofR. Herborn, J. Baber, M. Römer, H.J.Seel and N. Henning with the mea-surements and the fruitful coopera-tion with the project partners Cer-amTec, BCE, Lapp, Eisenmann, FCTand Fraunhofer IWM.

References[1] Pfeifer, H. et al.: Praxishandbuch Ther-

moprozesstechnik I, Grundlagen,Prozesse Verfahren, 2. Auflage, VulkanVerlag, Essen, 2010

[2] AG Energiebilanzen e.V. (AGEB),Berlin, 2011, www.ag-energiebi-lanzen.de

[3] Project „Effiziente Niederenergie Ent-binderungs- und Sintertechnik in derKeramikherstellung“ (ENITEC),www.effizienzfabrik.de/projekte/enitec and www.enitec.org

[4] Energieeffizienz bei der Keramikher-stellung. Abschlussbericht ENITEC.Hrsg.: F. Raether, ISBN 978-3-8163-0644-3, VDMA-Verlag, Frankfurt(2013)

[5] Raether, F.: Energy and cost reductionwith heat treatment of ceramics,cfi/Ber. DKG 88 (2011) [6/7] E43-46

[6] Raether, F.: Current state of in-situmeasuring methods for the control offiring processes. J. Am. Ceram. Soc. 92(2009) 146-152

[7] Project “Energieeffiziente Thermo-prozesse” (ENERTHERM), project peri-od: September 2012 – February 2018,www.htl.fraunhofer.de

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extruder screw, respectively [4].Other important energy savings areobtained by a careful optimization offiring conditions including thedesign of furnaces and firing stacks[4]. Recently a large project calledENERTHERM was started at Fraun-hofer HTL [7]. In ENERTHERM theenergy efficiency of industrial hightemperature processes shall be

Process Engineering

Fig. 8 a–b Dental implant (a) and extruder screw made of zirconiaceramics (b) (Source: BCE Special ceramics GmbH)

a b