Reticulated Open-Celled Zinc Oxide Ceramic Foams : Manufacturing , Microstructure , Mechanical , and Thermal Properties

Open-celled zinc oxide ceramic foams were prepared by the polymer sponge replication (Schwartzwalder) technique from aqueous ZnO dispersions with Sb2O3 and Bi2O3 as sintering additives, and mechanically stable ZnO foams with an average porosity of 93.6% were obtained.+eir microstructure consists of ZnO grains with a Bi-containing grain boundary phase together with a Zn-Sb-O secondary phase with spinel structure. +e obtained ZnO ceramic foams were characterized with respect to their morphology by computed tomography; in addition, the compressive strength and the thermal conductivity were determined, and the data were applied for modelling of the mechanical and thermal properties of the bulk ZnO strut material.


Introduction
Open-celled ceramic foams are used within several technological fields; with respect to the quantity, the most prominent applications are filter materials for metal melts in casting or catalyst supports [1].Ceramic foams for the above-mentioned applications are manufactured on an industrial scale by the polymer sponge replication process established by Schwartzwalder and Somers in 1961 [2]. is results in a macrocellular foam-like structure similar to that of the polymeric sponge used as template in this process.A variety of cellular ceramic materials has been produced by this technique, an overview can be found in references [3,4].However, to our knowledge, no zinc oxide foams have been prepared by the sponge replication process yet.
Zinc oxide is a semiconductor material, which crystallizes in the wurtzite structure type in analogy to aluminum nitride or silicon carbide, for example.erefore, it belongs to the class of adamantine compounds with a basic all-tetrahedral coordination of cations and anions [5].As a consequence of this simple structure type, the phonon conductivity is high, which results in a good thermal conductivity for adamantine compounds [6].Consequently, pure zinc oxide has a thermal conductivity of 50 W•m −1 •K −1 at room temperature, which is high for an oxidic compound [7].
A typical application for ZnO-based ceramics is varistors, which have a distinct nonohmic electrical conductivity, i.e., a very low conductivity below a characteristic breakthrough voltage and can be used for overvoltage protection [8].e nonohmic properties of ZnO-based varistors are a complex function of the microstructure, for example, the amount and distribution of secondary phases [9,10].Common additives for sintering and microstructural control in these varistor ceramics are the antimony and bismuth oxides Sb 2 O 3 and Bi 2 O 3 [11].Bismuth oxide forms a low-melting eutectic together with ZnO and promotes an effective low-temperature liquid phase sintering mechanism resulting in a distinct ZnO grain growth.On the contrary, antimony oxide reacts with ZnO to a ternary Zn-Sb-O phase, which restricts the ZnO grain growth by a pinning mechanism [12].Consequently, the Sb/Bi ratio in the ceramic raw material is essential for the final microstructure and the electrical properties of ZnO ceramics.
In the present work, the established polymer sponge replication-Schwartzwalder-process has been adopted for the manufacturing of zinc oxide ceramic foams.As sintering aids, Sb 2 O 3 and Bi 2 O 3 were used in a fixed molar ratio of 2 : 1. e obtained foams were characterized with respect to their microstructure (SEM and phase composition by XRD) and their macroscopic properties mechanical strength and thermal conductivity as a function of the total porosity and morphology.Finally, the bulk properties of the ZnO strut material were extrapolated from the obtained data by applying established structure (porosity)-property relations.

Powder Preparation.
e ceramic raw powder was prepared by ball-milling a dispersion of 200 g zinc oxide (d 50 � 0.2 μm, Harzsiegel Standard, Norzinco GmbH, Goslar, Germany), 7.28 g Sb 2 O 3 (1.0 mol% w. r. t.ZnO), and 5.82 g Bi 2 O 3 (0.5 mol% w. r. t.ZnO) in 300 mL ethanol for 4 h.A planetary ball mill (PM 400, Retsch GmbH, Haan, Germany) operating at a milling speed of 300 rpm and equipped with two 250 mL alumina grinding bowls and 50 alumina balls each with d � 10 mm was used.Afterward, the ZnO-Sb 2 O 3 -Bi 2 O 3 powder was collected by vacuum filtration and dried at 100 °C.

ZnO Foam Manufacturing.
For the ZnO foam, manufacturing reticulated polyester polyurethane (PU) foams (SP30P20R, Koepp Schaum GmbH, Oestrich-Winkel, Germany) with 20 pores per linear inch (ppi) and a cubic geometry with 20 mm × 20 mm × 20 mm were used as template structure.e PU foams were completely immersed into the ZnO dispersion and subsequently freed from the excess amount by manual squeezing of the foam template until its weight reached approximately 2.4 g (corresponding to ≈ 93.5% porosity in the final foam piece after sintering).After drying under ambient conditions, the PU template was removed thermally in three steps (110 °C/2 h, 250 °C/3 h, 400 °C/3 h, heating/cooling rate 1 K•min −1 ) in a circulating air furnace (KU 40/04/A, THERMCONCEPT Dr. Fischer GmbH, Bremen, Germany).Afterward, the samples were densified at 1100 °C for 3 h in air (heating rate of 3 K•min −1 ) using a 30 L sintering furnace (LH 30/14, Nabertherm GmbH, Lilienthal, Germany).For thermal conductivity measurements, rectangular PU templates with a dimension of 50 mm × 50 mm × 20 mm were coated with the ZnO dispersion; the excess of slurry was extruded with a roller press to reach a total weight of 15 g for the coated PU foam.Template removal and sintering were performed as described above.

ZnO Foam Characterization.
e total porosity of the foams (V pores /V foam ) was calculated from the geometric foam density, which is the foam mass m f divided by the geometric foam volume V f , and the skeletal density of the strut material.A skeletal density of 5.65 g•cm −3 was calculated for the ZnO material by the rule of mixture.As an approximation, the density and weight fraction of ZnO, Sb 2 O 3 , and Bi 2 O 3 according to the initial starting powder were used.e strut porosity (V strut pores /V struts ) and the strut pore volume were calculated from the dry, buoyant, and water-filled weight of the foams as determined by the water immersion/Archimedes' method according to the DIN EN 623-2:1993-11 standard [13].
To separate the porosity being present in the ceramic phase the volume of the hollow strut cavity caused by the removal of the PU foam template was subtracted from the overall strut porosity results.e volume of the hollow strut cavities was estimated from the average PU template weight of 0.244 g for a 8 cm 3 foam piece and a PU skeletal density of 1.1 g•cm −3 according to He-pycnometry.A volumetric shrinkage of the ZnO foam samples of 41% was considered in this estimation.For the porosity characterization, 37 specimens were analyzed, and the results were averaged.
e quantitative phase composition of the ZnO strut material was determined by powder X-ray diffraction (PANalytical X'Pert Pro Bragg-Brentano diffractometer, Co-Kα 1 /α 2 radiation) after ball-milling the respective sample for 5 min at 300 rpm.
e grinded powder was filled into a backloading sample holder and measured in a θ/θ reflection geometry with a 2θ range from 10 °to 160 °. e obtained diffraction patterns were analyzed by the Rietveld technique using the Topas Academic 5 program package [14,15].e thermal conductivity of the ZnO foams was determined using the transient plane source (TPS) technique and a TPS 2500 S device (Hotdisk SE, Gothenburg, Sweden) by placing the sensor in between two 42 mm × 42 mm × 17 mm foam samples with previously sanded surfaces [16].A TPS sensor with 9.908 mm in diameter and a heating power of 200 mW for a 10 s measurement was used.e thermal conductivity was calculated from the sensor temperature change [17].
e compressive strength was determined using a TIRAtest 2825 universal testing machine and circular loading plates with 150 mm in diameter and a crosshead speed of 1 mm•min −1 (TIRA GmbH, Schalkau, Germany).To ensure a more homogeneous load on the samples, a cardboard piece with 1 mm thickness was placed between the foam and the loading plates.From the obtained data, the maximum force was extracted and used for the calculation of the compressive strength.e results of 35 specimens were evaluated using a two-parameter Weibull distribution and the Visual-XSel 14 program package [18,19].From this 2 Advances in Materials Science and Engineering distribution, the average compressive strength σ fc (Weibull scale parameter) together with the modulus m (Weibull shape parameter) as a measure of the Weibull distribution's width were deduced.e microstructure of selected specimens was characterized by scanning electron microscopy using a XL30 ESEM-FEG microscope (FEI/Philips, Hillsboro/OR, USA) equipped with a secondary electron (SE) and backscattered electron (BSE) detector.e grain size distribution in the ZnO strut material was determined from the BSE micrographs by manually measuring the dimension of 150 individual grains.e elemental composition of the ZnO strut material was analyzed by energy dispersive X-ray spectroscopy (EDAX-AMETEK GmbH, Weiterstadt, Germany).Beforehand, the grinded sample material was uniaxially pressed at 30 MPa for 2 min into slabs, which were used for the EDS characterization.Nine EDS spectra were recorded at different positions on the sample and were used for the subsequent elemental analysis.
Micro-computed tomography (μ-CT) was applied for the characterization of the foam macrostructure.For a measurement using a nanotom S tomograph (Phoenix/GE Sensing & Inspection, Wunstorf, Germany), a set of 1080 radiographs with a resolution of 2304 × 2304 pixels was collected using an exposure time of 750 ms per image.e distances between detector and X-ray source (FDD) and between object and X-ray source (FOD) were adjusted to result in a voxel size of (9 μm) 3 .Data acquisition and reconstruction were performed with the Phoenix Datos│X 2.0 software package (Phoenix/GE Sensing & Inspection).For the calculation of the strut and cell size distributions, the CTAnalyser 1.17 program package was used after applying a 2 × 2 voxel binning to an effective voxel size of (18 μm) 3  (CTAn, Skyscan/Bruker microCT, Kontich, Belgium).e import of the collected CT data into the CTAn software, and the differential thresholding-based binarization procedure preceding the actual calculations were performed as described elsewhere [20].
e cell size and strut thickness distributions were calculated after filling the hollow strut cavities by performing a morphological closing operation in CTAn using a round kernel with r � 10 μm as described in a previous study [21].e strut filament thickness distribution was calculated without closing of the hollow strut cavities.

Results and Discussion
e dispersion of the ZnO-Sb 2 O 3 -Bi 2 O 3 powder mixture in water using an ethanolammonium citrate-based deflocculant was successful with respect to the rheological behavior up to a solid content of 77.5 wt.%/37.9vol.%. e obtained dispersion possessed the desired shear-thinning flow behavior and a viscosity suitable for the successful manufacturing of cellular ZnO ceramics by the polymer sponge replication technique [22,23].
3.1.Microstructure of ZnO Ceramic Foams.After sintering, mechanically stable ZnO ceramic foams were obtained which showed an intense yellow color (Figure 1). is is characteristic for zinc oxide materials and can be attributed to the presence of structural defects like oxide vacancies and interstitial zinc atoms in the hexagonal wurtzite structure of ZnO [8,24].
In the SEM micrographs of the strut material, a dense microstructure of well-sintered ZnO grains was observed (Figures 2(a [25,26].An antimony containing phase is found in form of octahedral crystallites with 0.3 μm, on average, in diameter (Figure 2(e)), which can be assigned to the cubic polymorph of the Zn 7 Sb 2 O 12 phase with spinel structure [26,27].
Moderate ZnO grain growth from 0.2 ± 0.07 μm in the starting powder to an average of 2.6 ± 0.9 μm in the foam strut material was found after sintering at 1100 °C (Figure 2(d)). is is significantly lower than grain sizes of 6 μm to 8 μm as reported in studies of dense ZnO ceramics made from similar powder mixtures [11,28].Most likely, this is a consequence of the pressureless preparation of the respective foam green bodies and the pressureless sintering procedure.
Powder X-ray diffraction reveals 89.0 wt.% of hexagonal wurtzite-ZnO as the main phase being present in the strut material (Figure 3, Table 1).As secondary phases, 10.3 wt.% of a cubic α-Zn 7 Sb 2 O 12 spinel phase and 0.7 wt.% tetragonal β-Bi 2 O 3 were found.Interestingly, the thermodynamically stable orthorhombic β-polymorph of Zn 7 Sb 2 O 12 as well as a Zn(BiSb) 3 O 7 pyrochlore phase were not detected.is is in contrast to other studies of the phase evolution in the ZnO-Sb 2 O 3 -Bi 2 O 3 system.For compact ZnO ceramics prepared under comparable conditions (sintering at 1100 °C, Sb/ Bi � 2), a pyrochlore phase and/or β-Zn 7 Sb 2 O 12 were reported as secondary phases [11,26,29].
In this context, the polymorphism of Zn 7 Sb 2 O 12 is known, but not fully understood yet.In recent studies, the α-polymorph has been identified as thermodynamically e yellow color is a consequence of structural defects in the ZnO lattice (oxide vacancies and interstitial zinc atoms).
Advances in Materials Science and Engineering stable phase above 1225 °C, which, nevertheless, is formed preferentially as metastable phase below 900 °C [27,30].In the temperature range between 900 °C and 1225 °C, α-Zn 7 Sb 2 O 12 transforms into the orthorhombic β-polymorph.Consequently, the formation of β-Zn 7 Sb 2 O 12 is expected for the ZnO foams sintered at 1100 °C.
However, the temperature at which this phase transformation occurs can be drastically affected by doping of α-Zn 7 Sb 2 O 12 with di-and trivalent metal ions like Cr 3+ , Co 2+ , or Ni 2+ , whereas Cr 3+ is most effective [31].Complete suppression of the orthorhombic β-phase is found for α-Zn 7 Sb 2 O 12 doped with at least 3.3 mol% Cr 3+ according to the formula Zn 6.8 Sb 1.9 Cr 0.3 O 12 .Consequently, the α-polymorph with spinel structure is stabilized at room temperature by this doping effect.In the case of doping with trivalent metal ions, an unity of 1/3 Sb 5+ and 2/3 Zn 2+ in   ) spinel [32].
In the case of the ZnO ceramic foams, a possible impurity is Al 3+ originating from alumina abrasions during the ballmilling procedure of the initial ZnO-Sb 2 O 3 -Bi 2 O 3 powder mixture.As the Al 3+ and Cr 3+ ions are similar in size and preferred coordination environment, a similar stabilization mechanism as described for the doping with Cr 3+ may be expected.An indication for this hypothesis can be found in the lattice constant of α-Zn 7 Sb 2 O 12 which is 848.7 pm for the spinel secondary phase in the ZnO strut material and therefore, 11 pm smaller compared to the literature values for the chemically pure α-Zn 7 Sb 2 O 12 phase [27,33].e lattice parameter of 848.7 pm, as measured, is very similar to the value of 846.4 pm reported for a spinel with the composition Zn 6 Sb 1.5 Al 1.5 O 12 [32].By applying Vegard's law to the crystallographic data for pure α-Zn 7 Sb 2 O 12 (a 0 � 859.4 pm) [33], Zn 6 Sb 1.5 Al 1.5 O 12 (a 0 � 846.4 pm) [32], and the end member of the homologous series, ZnAl 2 O 4 (�Zn 3 Al 6 O 12 ; a 0 � 809.1 pm), [34], an aluminum content of x � 1.25 was approximated (Figure 4) [35].
is corresponds to the composition Zn 6.17 Sb 1.58 Al 1.25 O 12 for the spinel secondary phase in the ZnO strut material.Consideration of this Al 3+ doping results in a significantly improved fit of the α-Zn 7 Sb 2 O 12 reflections in the Rietveld analysis of the diffraction data (Figure 3).e Bi 2 O 3 amount of 0.7 wt.%, corresponding to 0.1 at.%Bi (Table 1), in the ZnO strut material as detected by XRD and Rietveld analysis was significantly lower than in the initial powder mixture (2.7 wt.% Bi 2 O 3 /0.5 at.%Bi). is is an indication for the presence of an amorphous Bicontaining secondary phase, most likely formed during the liquid phase sintering process at the ZnO grain boundaries (Figure 2(c)) [25,26].As this amorphous phase is hidden for XRD and Rietveld analysis, an underestimation of the total Bi content is the result.Accordingly, a Bi content of 0.4 at.% was detected by EDS spectroscopy, which takes the amorphous material into account and is in good  Advances in Materials Science and Engineering agreement with the Bi amount in the initial powder mixture (Table 1).In addition, no indication for the partial loss of the sintering aid Sb 2 O 3 by formation of volatile, molecular compounds like Sb 4 O 6 was found [26].e Sb content in the ZnO strut material measured by XRD and EDS analysis was 0.8 at.% and 1.1 at.%, respectively, and in good agreement to the Sb amount of 1.0 at.% in the initial ZnO-Sb 2 O 3 -Bi 2 O 3 powder mixture.Furthermore, an aluminum concentration of 1.3 at.% was detected by EDS spectroscopy (Figure 5), which is in the same order of magnitude than the Al amount calculated from the results of the Rietveld analysis (0.7 at.%) and gives further evidence of the Al 3+ doping of α-Zn 7 Sb 2 O 12 .Nevertheless, it has to be noted that the accuracy of the EDS analyses for the trace elements (Sb, Bi, Al) is limited and a relative error of 4% to 12% has to be considered.

Macrostructure of ZnO Ceramic
Foams. e total porosity of the obtained ZnO ceramic foams is high with a value of 93.6 ± 0.4% (Table 2).According to the Archimedes measurements and in conformity with the microstructure observed in the SEM investigations, virtually no residual porosity is present in the ZnO strut material (<0.5% w. r. t. the strut volume).Only the hollow strut cavities originating from the PU template are present; they contribute a porosity of 32 ± 3% w. r. t. the total strut volume.is value is in good agreement to alumina foams with a densely sintered strut material made from the same PU templates [36].
An isotropic linear shrinkage of 16 ± 1%, equivalent to a volumetric shrinkage of 41 ± 4% in relation to the initial foam template dimensions, has been observed during the sintering process at 1100 °C. is is in a good agreement to sintering studies of dense ceramics in the ZnO-Sb 2 O 3 -Bi 2 O 3 system, for which a linear shrinkage of 17% is reported for the same powder composition as used within this work [11].
e open porous structure of the ZnO ceramic foams is confirmed by computed tomography; a total porosity of 94% and a closed porosity < 0.1% have been determined, which is in good agreement to the results of the Archimedes measurements.In addition, no significant pore window blocking is present resulting in a thoroughly open-cellular structure (Figure 6(b)).e cell size in the ZnO ceramic foams ranges Compressive strength Average strength 0.15 ± 0.03 MPa Weibull modulus 5.6 a Including the cavities resulting from the PU template burnout.b Related to the overall strut volume; V hollow strut /(V hollow strut + V strut material pores + V solid ) and V strut material pores /(V hollow strut + V strut material pores + V solid ).c Related to the volume of the strut material excluding the cavities resulting from the PU template burnout; V strut material pores /(V strut material pores + V solid ).d Extrapolated thermal conductivity of the bulk strut material according to Ashby's model [43].6 Advances in Materials Science and Engineering between 1.8 mm and 3.2 mm with an average of 2.7 ± 0.2 mm according to a morphometric analysis of the threedimensional reconstruction volume (Table 2, Figure 6(a)).e strut thickness and strut filament thickness amount to 0.38 ± 0.15 mm and 0.13 ± 0.08 mm on average.As expected, the morphological parameters of the ZnO ceramic foams are within the same range as previously reported for alumina cellular ceramics made from the same 20 ppi PU foam templates [21].

Mechanical and ermal Properties of ZnO Ceramic Foams.
e compressive strength data obtained on all 35 ZnO foam specimens could be satisfactorily modelled by a two-parameter Weibull distribution [18].An average compressive strength σ fc of 0.15 ± 0.03 MPa and a Weibull modulus m of 5.6 were calculated (Figure 7).e modulus m is in the expected range for brittle cellular ceramic structures [37,38].Essentially, the low compressive strength is a consequence of the high porosity of 93.6% in the ZnO ceramic foams; accordingly, the σ fc value is in the same order of magnitude as the compressive strength of cellular alumina with a similar microstructure and porosity range (σ fc � 0.19 MPa/0.34MPa at 94.0%/93% total porosity) [37].
For the evaluation of the strength-porosity correlation, the compressive strength data obtained for the cellular ZnO specimens were modelled with the Gibson-Ashby (GA) relation for the description of the crushing behavior of brittle, cellular materials (equation ( 1)) [39]: According to equation ( 1), the compressive strength σ fc of a brittle, cellular sample is a function of its relative density and the bending strength σ fs of the bulk strut material.e density exponent n describes the effect of the porosity on the mechanical strength; for open-celled foams, a value of 1.5 is commonly used.e constant C 6 is related to the cellular geometry of the foam; for cellular ceramics made by the polymer sponge replication, usually a value of 0.16 is applied for C 6 [40].e strength-porosity correlation for the ZnO ceramic foams is shown in Figure 8; the bending strength σ fs was varied to fit the Gibson-Ashby model to the compressive strength data of the ZnO foams.An adequate GA fit is the result, and a bending strength of 54 ± 2 MPa is approximated for the bulk ZnO strut material.is is in the same order of magnitude than the bending strength of compact zinc oxide varistor ceramics, which ranges between 80 MPa and 120 MPa [41,42].
e thermal conductivity of the obtained ZnO ceramic foams determined by the transient plane source technique was 0.82 ± 0.07 W•m −1 •K −1 at 93.6% total porosity.In order to evaluate the thermal properties of the ZnO strut   Advances in Materials Science and Engineering material, the bulk thermal conductivity λ b has been approximated.As the strut material contains virtually no microstructural porosity, a simple model derived by Ashby based on the rule of mixture was applied (equation ( 2)) [43]: e parameter λ g is the thermal conductivity of the gas phase (air; λ g � 0.0264 W•m −1 •K −1 ), [44] λ f is the thermal conductivity of the foam and P cell is the cell porosity of the ZnO foams including the hollow strut cavities (P cell � 93.5%) [36].e calculated bulk thermal conductivity of the ZnO strut material is 37 W•m −1 •K −1 , which is slightly lower compared to experimental and theoretical values reported for chemically pure ZnO (≈50 W•m −1 •K −1 ) [7,45].e reduced thermal conductivity of the ZnO strut material can be explained by the presence of the Bi 2 O 3 and α-Zn 7 Sb 2 O 12 secondary phases, which have a very low thermal conductivity of 1.1 W•m −1 •K −1 and 2.3 W•m −1 •K −1 , respectively [46].e thermal conductivity of 37 W•m −1 •K −1 of the bulk ZnO strut material is in good agreement with the value of 43 W•m −1 •K −1 extrapolated by Landauer's theory of effective percolation for a mixture of 90.2 vol.%ZnO and 9.8 vol% α-Zn 7 Sb 2 O 12 [47].Consequently, for commercial ZnObased varistor ceramics containing the same Sb 2 O 3 and Bi 2 O 3 sintering additives, a reduced thermal conductivity in the range between 20 W•m −1 •K −1 and 35 W•m −1 •K −1 is found as well [46].ZnO foams show a high total porosity of 93.6% including the characteristic hollow strut cavities typical for this processing technique.e strut material itself is almost fully densified; its microstructure consists of ZnO grains with an amorphous Bi-containing grain boundary phase.In addition, an Al-doped spinel secondary phase with the composition Zn 6.17 Sb 1.58 Al 1.25 O 12 was formed.e dopant Al 3+ originates from a contamination during the ball-milling process of the raw powders and leads to a stabilization of the thermodynamically metastable spinel phase at room temperature.

Conclusions
e mechanical and thermal properties of the obtained ZnO ceramic foams were evaluated, and an average compressive strength of 0.15 MPa and a thermal conductivity of 0.82 W•m −1 •K −1 were measured.e strength-porosity correlation was modelled with the Gibson-Ashby law for brittle, cellular structures; a bending strength of 54 MPa for the bulk ZnO strut material being in good accord to published data on comparable ZnO-based varistor ceramics was estimated.From the thermal conductivity data, a bulk thermal conductivity of 37 W•m −1 •K −1 was determined for the ZnO strut material, which is in good agreement with the thermal conductivity of ZnO varistor ceramics of similar composition.Advances in Materials Science and Engineering ) and 2(b)).e bright regions in the BSE images found at the grain boundaries indicate a homogeneously distributed Bi-containing secondary phase (Figures 2(b) and 2(c)).In accordance with literature data, this Zn-Bi-O grain boundary phase is formed during the liquid phase sintering process from the eutectic mixture of ZnO and the sintering aid Bi 2 O 3 melting at 750 °C

Figure 1 :
Figure 1: Open-cellular zinc oxide ceramic foams with a pore count of 20 ppi and 93.6% total porosity after sintering at 1100 °C.e yellow color is a consequence of structural defects in the ZnO lattice (oxide vacancies and interstitial zinc atoms).

Figure 2 :
Figure 2: SEM micrographs of the strut fracture surface in zinc oxide ceramic foams: (a) SE image, (b, c) BSE images revealing the Bicontaining secondary phase in the grain boundaries, (d) grain size distribution in the ZnO strut material, and (e, f ) BSE micrograph and EDS spectra of the strut surface (black ○) and of individual grains of a Sb-containing secondary phase (red ○).

Figure 5 :
Figure 5: EDS analysis averaged from 9 separate EDS spectra recorded on an uniaxially pressed slab of the ZnO strut material.Inset: Signals of the trace elements Al, Bi, and Sb.

Figure 6 :Failure
Figure 6: (a) Strut thickness and cell size distributions calculated from three-dimensional μ-CT reconstruction of a 20 ppi ZnO ceramic foam; (b) three-dimensional reconstruction of the same ZnO foam.

Figure 7 :
Figure 7: Compressive strength data of 20 ppi ZnO ceramic foams (□) and the corresponding fit with the two-parameter Weibull function (red line).

e
manufacturing of cellular ZnO ceramic foams with the established polymer sponge replication technique and an aqueous ZnO dispersion and Sb 2 O 3 and Bi 2 O 3 as sintering additives has been demonstrated.e obtained open-celled fit: σ fc = C 6 (ρ rel ) n σ fs // C 6 = 0.16, n = 1.5, σ fs = 54 ± 2MPa

Figure 8 :
Figure 8: Compressive strength data of 20 ppi ZnO foams (□) and the corresponding fit with the Gibson-Ashby model for the strength-porosity correlation (red line).

Table 1 :
Phase and elemental composition of open-celled zinc oxide ceramic foams determined by powder XRD with Rietveld analysis and a comparative EDS measurement.

Table 2 :
Properties of open-celled zinc oxide ceramic foams.e total porosity and cell porosity values refer to the geometric foam volume and the ratio V pores /V foam .e strut porosity is based on the strut volume and the ratio V strut pores /V struts as determined by the water immersion technique.