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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 33 (2013) 3231–3241

Solid state synthesis of nano-boehmite-derived CuAlO2 powder andprocessing of the ceramics

K. Vojisavljevic a,∗, B. Malic a, M. Senna a,b, S. Drnovsek a, M. Kosec a

a Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Sloveniab Keio University, Yokohama 223, Japan

Received 14 February 2013; received in revised form 25 May 2013; accepted 28 May 2013Available online 22 June 2013

bstract

he delafossite CuAlO2 powder was prepared from the nano-boehmite AlOOH·xH2O and Cu2O by the solid state synthesis at 1100 ◦C in argon.he inherently slow solid state reaction was accelerated by introducing rod-like boehmite nano-particles which fully covered the 1 �m sized Cu2Oarticles in the reactant mixture, and decomposition of the nano-boehmite upon calcination. In contrast, the reaction between the Cu2O and Al2O3,ntroduced as a reference, resulted in mixed phases under the same experimental conditions. Sintering of the nano-boehmite derived CuAlO2

◦

owder compact at 1100 C for 2 h in air resulted in the delafossite ceramic with 86% of theoretical density, without any impurities detectable by-ray diffraction analysis. The analysis of the microstructure by scanning electron microscopy confirmed that the bulk of the sintered sample waselafossite phase with uniformly distributed porosity, with only traces of Cu-rich impurities at the surface.

2013 Elsevier Ltd. All rights reserved.

eywords: Delafossite CuAlO ; Boehmite; Bulk sintering; Calcination atmosphere

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. Introduction

Due to an increasing interest in functional oxides for transpar-nt electrodes, a considerable effort has been recently devotedo the research and development of different n- and p-type oxideemiconductors. P-type oxide semiconductors with high enoughonductivity and transparency across the visible spectrum arecarce and hence, special attention has been paid to their produc-ion. A series of p-type delafossite materials CuMO2 (M = Al,r or Y) is of particular interest. Copper aluminate (CuAlO2)lms prepared by physical vapour deposition have been reported

o exhibit the p-type behaviour and have already been usedn various applications in optoelectronics.1–4 Undoubtedly, thehase-pure targets with a high relative density are prerequisitesor physical vapour deposition of high quality CuAlO2 films.

The delafossite CuAlO2 has been usually prepared by con-

entional solid state synthesis route from CuO or Cu2O andl2O3 powders,3–7 where milling in different liquid media for a

ong period were at first used to achieve homogeneous reagent

∗ Corresponding author. Tel.: +386 1 477 3936; fax: +386 1 477 3887.E-mail address: katarina.vojisavljevic@ijs.si (K. Vojisavljevic).

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955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2013.05.025

ixtures, and then multiple calcinations at high temperaturesith intermediate wet-milling steps were applied.2,8,9 However,

econdary phases and low relative densities have been reportedor the solid-state synthesized delafossite CuAlO2, despite highrocessing temperatures (1200–1350 ◦C) and extremely longimes, such as six days, have been used.9,10 The pure delafos-ite structure is difficult to achieve by starting from the Cu2Ond Al2O3 powder mixture, due to the tendency of oxidation ofuprous (Cu+) ions to cupric (Cu2+) during calcination, result-ng rather in decomposition than in formation of a single phaseaterial. Furthermore, if high temperatures are involved in the

roduction of delafossites, the volatilization of cuprous ionsould take place,11 leading to the formation of CuAl2O4, aetrimental spinel phase. The presence of secondary phases,temming either from the degradation of the CuAlO2 duringhe cooling cycle of the thermal treatment or volatilization ofuprous ions at high temperatures, can drastically affect elec-rical and dielectric properties of material. Therefore, a properelection of the temperature, time and atmosphere of the solid-tate reaction is of utmost importance. Particularly, the valence

tate of the Cu-ions in the powder mixture during the heatingnd cooling cycles should be controlled. The oxidation of theu+ to Cu2+ ions could occur during heating in air atmosphere at

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elatively low temperatures (200–500 ◦C). Afterwards, the CuOtarts to react with the Al2O3, yielding the CuAl2O4 phase (Eq.1)) at the temperatures below 1100 ◦C:

uO + Al2O3 → CuAl2O4 (1)

From the literature12 it is inferred that upon further heat-ng, the CuAlO2 should be formed according to the followingeaction (Eq. (2)):

uO + CuAl2O4 ↔ 2CuAlO2 + (1/2)O2 (2)

Furthermore, the CuO gradually decomposes and evaporatesrom the powder mixture upon heating above 1100 ◦C. Due tohe latter process, on one hand, and presence of oxygen, on thether, the equilibrium is shifted towards the backward reactionn Eq. (2), leading to re-appearance of the spinel phase in thealcined powder.

Besides, the literature data also suggest that the reductionu2+ → Cu+ takes place at temperatures above 1020 ◦C,11,13,14

.e., CuO and CuAl2O4 undergo the reduction (Eqs. (3) and (4)):

CuO → Cu2O + (1/2)O2 (3)

CuAl2O4 → 4CuAlO2 + 2Al2O3 + O2 (4)

nd therefore the second possible mechanism of CuAlO2 phaseormation can be described by the following reaction (Eq. (5)):

u2O + Al2O3 → 2CuAlO2 (5)

According to Jacob and Alcock,14 who provided the mostxtensive information about the Gibbs free energies of forma-ion of CuAlO2 and CuAl2O4 in the system Cu2O–CuO–Al2O3,he most limiting factor in formation of delafossite phase cane attributed to relatively high temperature of total reductionf the spinel phase. A standard Gibbs free energy change,G◦ (in cal) for the reaction presented by Eq. (4), is given

y �G◦ = 33,400 − 20.02T,14 and when the reaction reachedhe equilibrium, the Gibbs free energy change, �G becomesqual to zero, i.e., �G◦ = −RTlnK = 33,400 − 20.02T. The equi-ibrium constant, K should be expressed in terms of thebserved oxygen partial pressure, pO2 . Calculated tempera-ure of 1171 ◦C corresponding to the three-phase equilibriumuAl2O4–CuAlO2–Al2O3 at pO2 = 0.21 atm. Thus, the tem-erature interval between 1020 ◦C and 1171 ◦C is important forhase pure delafossite.

Still, it is possible to expect the degradation of the as-formedelafossite to the CuAl2O4 and CuO during the cooling cycle inir. To avoid the formation of the undesired CuAl2O4 phase, its necessary to preclude the oxidation of the Cu+ ions. The inerttmosphere, such as N2 or Ar, provides the best conditions forhe calcination of the mixture of the Cu2O and Al2O3, allowinghe formation of the delafossite through the reaction (Eq. (5))nd its stabilization upon cooling.

Otherwise, the delafossite formation could be promoted by

eplacing one of the oxides with a more reactive compound,uch as Cu- or Al-nitrate, hydroxide or oxyhydroxide, whichhermally decomposes upon heating. Only in a few reports theuthors proposed the usage of Al(OH)3 instead of Al2O3

15–17

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Ceramic Society 33 (2013) 3231–3241

or production of pure and Mg- or Ca-doped CuAlO2 targets forF magnetron sputtering, however processing details and phaseomposition of the targets were not discussed therewith.15,16

Within this work, the focus was laid on the solid-state syn-hesis of the CuAlO2 using the boehmite (�-AlOOH·xH2O)ano-powder. In contrast to systematic studies of the CuAlO2ynthesis from the oxide mixture, no data exist for the reactionsith the boehmite. Gamma-AlOOH can be easily decomposed

o �-Al2O3 at relatively low temperatures. Due to the high spe-ific surface area and presence of the hydroxyl groups at theurface18,19 the gamma-phase is more reactive than the oxide,.e., �-Al2O3. The expected advantages of using the nano-oehmite in the delafossite solid state synthesis: (1) a goodomogenization with the Cu2O and thus short reaction paths,nd (2) its higher reactivity as compared to the oxide as a con-equence of the thermal decomposition (Hedvall effect20), canignificantly promote the synthesis of phase pure delafossiteowder at lower temperatures than in the case of �-Al2O3. Foromparison, the delafossite powder was also synthesized fromhe mixture of the oxides. Further, the sintering of as-preparedowder compacts was studied, with the aim to obtain the CuAlO2eramics with the as high as possible density without any sin-ering additives.

. Experimental

.1. Processing of CuAlO2 powder and ceramic

The Al-reagents were the �-Al2O3 (AKP-50, 99.5%,umitomo Chemical, Tokyo, Japan) and the nano-boehmitelOOH·xH2O (99.99%, expressed as the purity of the �-Al2O3hase at 600 ◦C, SSA of the �-Al2O3: 140.9 m2/g, SkySpringanomaterials, Houston, Texas). The Cu2O (99.9%, Alfa Aesar,arlsruhe, Germany) was used after preliminary wet milling for

h in a planetary mill (Retsch PM400, Retsch GmbH, Haan,ermany, zirconia vials and balls) in isopropyl alcohol (IPA).The Al2O3 and Cu2O powders were mixed in a stoichiometric

olar ratio (Cu/Al = 1.0), homogenized in a planetary mill in IPAor 4 h, and dried at 90 ◦C. The powder mixture was denoted asAO. The nano-boehmite-Cu2O powder mixture, prepared in

he same manner, is denoted as nano CAO. Several calcinationsf both powder mixtures were performed in both oxidizing (air)nd inert (argon) atmospheres.

The nano CAO powder mixture was pressed into pellets with0 MPa and calcined twice at 1100 ◦C for 10 h in argon atmo-phere. The pellets were crushed in a mortar and milled in aixer mill for 30 min (Retsch MM400, Retsch GmbH, Haan,ermany) in IPA between two calcination steps. The as-calcinedowder was uniaxially pressed into pellets of 8 mm in diame-er at 100 MPa and subsequently by cold isostatic pressing at00 MPa, and sintered in air to 1100 ◦C, followed by a holdingime of 2 h, with heating and cooling rates of 5 ◦C/min.

.2. Characterization

The morphology of the CuAlO2 powder was examined by theeld emission scanning electron microscope FE-SEM (JEOL

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SM 7600F, Tokyo, Japan). The powder was ultrasonically dis-ersed in IPA and the suspension was spread on the surface of theample holder. After alcohol evaporation a 5 nm thick layer ofarbon was deposited on the powder surface under the vacuumnd analyzed. The microstructure and the chemical compositionf the sintered sample were analyzed on polished cross-sectionssing the scanning electron microscope, SEM (JEOL JSM 5800,okyo, Japan) equipped with a LINK ISIS EDS 300 energy dis-ersive system (Oxford Instruments Analytical Ltd., Abington,K).The particle size, expressed as the median value (d50), and

ize distribution (PSD) of all powders were determined from therea distribution measured by a laser granulometer (Microtrac3500 Particle Size Analyzer, Montgomeryville, PA).

The thermal behaviour of the nano-boehmite, CAO andano CAO powder mixtures was followed by thermogravimet-ic and differential thermal analysis (TG/DTA, NETZSCH STA09). Around 50 mg of the powder was placed in an alumina cru-ible and heated up to 1150 ◦C with a heating rate of 10 ◦C/minn an atmosphere of flowing air or argon.

The phase composition of the powder and ceramic werenalyzed by PANalytical diffractometer (X’Pert PRO MPD,lmelo, Netherlands, Bragg-Brentano geometry using theuK� radiation and X’Celerator detector configured in reflec-

ion geometry). The data acquisition was done in the step scanode (2θ = 0.034◦, integration time 100 s) in angular rangeθ = 10–70◦. The phases were identified with the X’Pert Highcore package,21 using the PDF-2 reference patterns database.22

he crystallite size of the CuAlO2 powder was calculated usinghe Debye–Scherrer equation (Eq. (6)).

= kλ

β(2θ) cos θ(6)

here L is the crystallite size, λ the wavelength (CuK�1,.15406 nm), k the dimensionless shape factor (=0.94), θ theragg angle. The half-width of the diffraction line β(2θ) in radi-ns was taken as the experimental half-width (βexp.) and wasorrected for experimental broadening (βins.) according to:

(2θ) = (β2exp . − β2

ins.)1/2

(7)

The βins. was measured experimentally by a highly crystallineaB6 powder.

The dimensional changes of the powder compacts in differenttmospheres were analyzed by the heating stage microscopeLeitz V.1A; Leitz, Wetzlar, Germany) at the rate of 10 ◦C/min inowing gas (Ar or air). Around 25–30 mg of a selected powderas uniaxially pressed at 100 MPa into a compact of 6 mm iniameter.

The density of the sintered compact has been calculated from

he mass and dimensions of the sample. The relative densityas calculated using the theoretical value of 5.10 g/cm3 (PDF0-035-1401, R3-m).23

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Ceramic Society 33 (2013) 3231–3241 3233

. Results and discussion

.1. Morphology of the reagents, CAO and nano CAOowder mixtures

The micrographs of the reagents, and CAO and nano CAOowder mixtures, along with their PSD, are collected in Fig. 1.he FE-SEM image of the Cu2O powder (Fig. 1a) indicates theresence of particles different in shape and size, from roundedarticles below 100 nm in size, to sharp and irregularly shapedarticles of several 100 nm in size, which adhere to one anothernd form micron sized agglomerates. The granulometric resultndicates that the median particle size (d50) is 0.5 �m and 90%f the particles (d90) are smaller than 1.4 �m. The �-Al2O3articles have spherical to elliptical shapes, with the size ofpproximately 100–300 nm (Fig. 1b). The median particle sized50) is 0.36 �m, while the value of d90 is 1.1 �m, which indi-ates that the primary particles form agglomerates.

In contrast to Cu2O and �-Al2O3 powders, the AlOOH·xH2Os in the form of granules of ∼7 �m, see Fig. 1c. The micrographaken at a higher magnification (see the inset in Fig. 1c) revealshat the granules are built of bunches consisting of nanorods withengths of 100–200 nm and diameters of 10–20 nm. According toranulometric analysis, the median particle size (d50) is 6.75 �m,hile the d90 is 20.57 �m.The difference between the morphologies of the CAO and

ano CAO powder mixtures, see Fig. 1d and f, is in the sizend geometry of the Al-reagent particles. In the nano CAO,he rod-like nano-boehmite particles fully cover the surface ofhe Cu2O particles, indicating the higher density of the contactoints between the reagents as compared to the CAO sample.he median particle sizes of the CAO and nano CAO powderixtures are 0.206 and 0.208 �m, respectively. However the

omparison of their PSDs reveals that the latter sample exhibits slightly broader bimodal size distribution (see Fig. 1e and g).

.2. Reaction between Cu2O and Al2O3 in air and Artmospheres

The thermal behaviour of the CAO powder mixture in airnd argon were analyzed by TG–DTA. Furthermore, the CAOowder mixture was also pressed into the pellets and heated toemperatures between 600 ◦C and 1100 ◦C and analyzed by XRDo obtain a better insight into the process of phase formation. Theesults are collected in Fig. 2.

The weight loss of the CAO powder mixture upon heating inir from room temperature to 200 ◦C is 0.54% (Fig. 2a) and it cane related to evaporation of the physisorbed water. The weightain in two steps between 200 ◦C and 500 ◦C is due to oxidationf the Cu2O, and it is coupled with exothermic peaks observed inhe DTA curve at 284 ◦C and 390 ◦C. The respective weight gainf 5.72% is close to the theoretical value of 6.5%. Upon heatingbove 500 ◦C, the reaction between CuO and Al2O3 takes place

Eq. (1)), see Fig. 2b. The diffraction peaks of the spinel phase arebserved in the XRD-pattern of the sample heated at 900 ◦C. At000 ◦C, the coexistence of the CuO and CuAl2O4 is confirmed.ccording to literature data,11,24 CuO and CuAl2O4 undergo

3234 K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241

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ig. 1. FE-SEM micrographs of the reagents (a) Cu2O, (b) Al2O3, (c) AlOOano CAO.

he reduction upon further heating (Eqs. (3) and (4)). These tworocesses result in the partial formation of the delafossite, and aeight loss of 5.19%, which is close to the theoretical value of.33%, accompanied by an endothermic event at approximately060 ◦C, could be related to the above mentioned mechanism.esides the delafossite, only CuAl2O4 and CuO were found in a

ample heated to 1100 ◦C. The absence of Al2O3 indicates thathe higher temperatures are necessary for total decompositionf the spinel, and formation of pure delafossite.

The total mass loss upon heating of the CAO powder mixturen argon atmosphere from the room temperature to 1000 ◦C isnly 1.16%, see Fig. 2c. It is related to evaporation of waternd reduction of the residual CuO to Cu2O phase, which iss confirmed by XRD of the sample heated to 800 ◦C, seeig. 2d. At 900 ◦C, the reaction between Cu2O and Al2O3 takeslace, see Eq. (5), and an appreciable amount of delafossiteolymorphs, rhombohedral 3R and hexagonal 2H, is alreadyormed at 1000 ◦C. However, the presence of unreacted Cu2Ond Al2O3 could be observed even in the sample heated to thenal temperature of 1100 ◦C.

The shrinkage vs. temperature curves of the CAO powderompact upon heating in air and argon atmospheres have beenecorded. The results are collected in Fig. 3, together with theRD patterns of the powdered compacts, heated in the micro-

cope to the final temperature of 1150 ◦C.In both cases the samples show almost no shrinkage, as

vident from Fig. 3a. The presence of rhombohedral CuAlO2

dow

2O and the powder mixtures with respective PSD (d), (e) CAO and (f), (g)

PDF 35-1401; R-3m), cubic CuAl2O4 (PDF 33-0448; Fd3m),nreacted rhombohedral Al2O3 (PDF 88-0826, R-3c) and mono-linic CuO (PDF 89-5897; C2/c) phases was confirmed in theample heated to 1150 ◦C in air, see Fig. 3b.

The XRD pattern of the sample heated in argon (Fig. 3b)hows the presence of rhombohedral CuAlO2 (PDF 35-1401;-3m), hexagonal CuAlO2 (PDF 40-1037; P63mmc), rhom-ohedral Al2O3 (PDF 88-0826, R-3c) and cubic Cu2O (PDF8-2076, Pn-3m) phases. In contrast to aerobic conditions, theu-ions remain in the +1 state during heating and cooling cycles

n Ar, leading to the formation of the delafossite without thendesired spinel phase.

Based on the above results, the temperature of 1100 ◦C andong reaction times of 10 and 24 h were selected for the calcina-ion of the CAO powder mixture in Ar atmosphere (the results areot presented here). Besides the CuAlO2, both unreacted oxidesn appreciable amounts were found in the powder calcined evenfter the 24 h, therefore the Cu2O and Al2O3 mixture was notonsidered for further study.

.3. Reaction between Cu2O and AlOOH in air and Artmospheres

First, the thermal behaviour of the �-AlOOH·xH2O pow-er was analyzed by TG–DTA, see Fig. 4a. Upon heatingf the �-AlOOH·xH2O powder to 1100 ◦C in air the totaleight loss of 20.66% is recorded. The weight loss of

K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241 3235

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ig. 2. TG–DTA data of the CAO powder mixture and XRD patterns collectetmospheres. Note that the Bragg reflections of different phases are collected as

9.40%, observed between 30 ◦C and 550 ◦C corresponds toehydration–dehydroxylation of the reagent and it can be sep-rated into two distinct steps. The first step (30–200 ◦C) withhe weight loss of 3.55% is mainly attributed to removal ofhe physisorbed water. The second step (200–550 ◦C) with

he weight loss of 15.27% is related to the removal of thehemisorbed water and to the decomposition of the boehmiteo �-Al2O3. It is coupled with two endothermic events at 453 ◦C

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ig. 3. Shrinkage vs. temperature (a) and XRD (b) of the CAO powdered compacts

tmospheres. Note that the Bragg reflections of different phases are collected as tick

m the samples heated to different temperatures in air (a), (b) and Ar (c), (d)bars below the patterns.

nd 480 ◦C. The weight loss of 1.07% from 500 ◦C to 1000 ◦Cs associated with further elimination of the residual hydro-yls in the crystalline structure of the alumina.25 Furthermore,he broad endotherm in the range from 500 ◦C to 1000 ◦C islso indicative for the formation of various transition alumina

hases before the final conversion to �-Al2O3.26 Based onhe TG–DTA result, the calculated molar weight of the nano-oehmite powder is 61.0532 g/mol, which can be described

after heating in the microscope to the final temperature in (a) air and (b) argonbars below the patterns.

3236 K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241

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ig. 4. TG–DTA (a) and XRD patterns before and after heating to 600 ◦C (b) ore collected as tick bars below the patterns.

ith the chemical composition – AlOOH·0.059H2O. The phaseurity of the nano-boehmite powder was checked by XRD anal-sis, see Fig. 4b, confirming the orthorhombic �-AlOOH phasePDF 01-083-2384, Amam). In addition, the nano-boehmiteowder compact was heated in air to 600 ◦C. The XRD patternf the as-heated specimen reveals the presence of the �-Al2O3PDF 00-047-1770), confirming the decomposition of nano-oehmite to �-Al2O3. Broad diffraction peaks of the �-Al2O3ndicate the low crystallinity and possibly the presence of themorphous phase. Soled and co-workers27 suggested that the-Al2O3 contains a significant amount of amorphous phase.

To analyze the thermal behaviour of the nano CAO powderixture in different atmospheres, the TG–DTA curves have been

ecorded, along with the XRD patterns of the powder mixtureeated to different temperatures (Fig. 5). The total mass losspon heating in air from room temperature to 1100 ◦C is 8.53%

Aa

ig. 5. TG–DTA data of the nano CAO powder mixture and XRD patterns collectedtmospheres. Note that the Bragg reflections of different phases are collected as tick

lOOH·xH2O powder in air. Note that the Bragg reflections of different phases

Fig. 5a). The oxidation of the Cu2O undergoes in two stepsetween 200 ◦C and 600 ◦C, coupled with exothermic peaks at22 ◦C and 298 ◦C. In addition, the weight loss of 2.65%, accom-anied by an endothermic event at 520 ◦C indicates the removalf chemisorbed water and decomposition of �-AlOOH to �-l2O3. According to TG–DTA data the total weight loss uponeating to 600 ◦C is 4.21%, which is close to the theoreticalalue of 4.65%, according to the reaction (Eq. (8)):

u2O + 2AlOOH·0.059H2O + (1/2)O2

↔ 2CuO + Al2O3 + 0.118H2O (8)

Upon heating above 600 ◦C, the reaction between CuO and �-l2O3 takes place (Eq. (1)), and only the CuAl2O4 and CuO exist

t 800 ◦C and 900 ◦C, see Fig. 5b. The weight loss in between

from the samples heated to different temperatures in (a), (b) air and (c), (d) Arbars below the patterns.

K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241 3237

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stpas shown in Fig. 7a, therefore, the temperature was increased to1100 ◦C, and after heating for 10 h the CuAlO2 phase with onlytraces of the Cu2O was obtained (Fig. 7b).

ig. 6. Shrinkage vs. temperature of the nano CAO powder compacts (a) and Xo the final temperature in air and argon atmospheres. Note that the Bragg reflec

00 ◦C and 900 ◦C is only 0.58% and it could be related to theemoval of the residual hydroxyls from alumina. The weight lossf 3.74% between 900 ◦C and ≈1057 ◦C does not agree well withhe theoretical value of 6.13%, expected for the reaction betweenuO and CuAl2O4 (see Eq. (2)). The result indicates that pureelafossite phase formation is far from being completed. Besidehe rhombohedral and hexagonal delafossite phases, the pres-nce of CuAl2O4 and CuO was also confirmed in the sampleeated to 1100 ◦C.

The total mass loss upon heating the nano CAO powder mix-ure in argon atmosphere from room temperature to 1100 ◦C is.68%, see Fig. 5c. The weight loss of 8.25% recorded uponeating to 600 ◦C corresponds to removal of the chemisorbedater and decomposition of �-AlOOH to �-Al2O3, which is in

good agreement with the value of 8.23% calculated accordingo the following reaction (Eq. (9)):

u2O + 2AlOOH·0.059H2O → Cu2O + Al2O3

+ 1.118H2O (9)

The nano CAO powder, heated to 800 ◦C in argon, is theixture of the CuAlO2 and unreacted Cu2O and Al2O3 phases,

ee Fig. 5d. Almost pure delafossite is obtained upon heatingo 900 ◦C. A weight loss of 0.43%, recorded between 600 ◦Cnd 900 ◦C, can be related only to the removal of the residualater. Traces of Cu2O could be detected in the XRD pattern of

he sample heated to 1100 ◦C.The shrinkage vs. temperature curves of the nano CAO pow-

er compacts have been recorded in air and argon atmospheres.he results are presented in Fig. 6, along with the XRD pat-

erns collected from the powdered compact heated to the finalemperature of 1150 ◦C in the microscope.

Both shrinkage curves could be divided into three regions.pon heating to 450 ◦C the shrinkage was not observed. The

econd region between 450 ◦C and 800 ◦C could be related to

he removal of the chemisorbed water and decomposition of the-AlOOH,28 and partial reaction between the reactants, as veri-ed by XRD analysis at different temperatures shown earlier. Ateep slope was detected in both shrinkage curves above 800 ◦C,

F(

tterns (b) collected from the powdered samples after heating in the microscope of different phases are collected as tick bars below the patterns.

ndicating the reaction between the components. The shrinkagef the sample heated in air at the temperature of 1120 ◦C was5.2%, while for the sample heated in argon was only 4.9%.ccording to the XRD pattern (see Fig. 6b), almost pure CuAlO2as formed during heating in argon, in agreement with the reac-

ion presented by Eq. (5). The observed lower intensity of theuAlO2 diffraction peaks as compared to those of the sampleeated in air could be related to a slower crystallite growth innert atmosphere than in air. The sample heated in air atmosphereontained also the spinel phase (see Fig. 6b), therefore the pow-er obtained by calcination in Ar atmosphere was consideredor further study.

.4. Synthesis and sintering of nano-boehmite deriveduAlO2

Based on the results obtained by TG–DTA and the heatingtage microscope, the temperature of 1000 ◦C was selected forhe calcination of the nano CAO powder mixture in argon. Thehase-pure delafossite was not obtained after heating for 10 h,

ig. 7. XRD patterns of the nano CAO mixture calcined for 10 h at (a) 1000 ◦C,b) 1100 ◦C in Ar atmosphere.

3238 K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241

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ig. 8. XRD patterns between 36◦ and 40◦ obtained after the 1st and the 2nd calalcined 2 × 10 h at 1100 ◦C in Ar.

The phase pure delafossite powder was synthesized by two-tep calcination of the nano CAO powder mixture at 1100 ◦C for0 h in Ar atmosphere. The XRD results, presented in Fig. 8a,onfirm that the traces of the Cu2O, observed after the firstalcination, completely disappeared after the second calcina-ion. The crystallite size of the CuAlO2 powder was calculatedrom the broadening of the delafossite reflections using theebye–Scherrer equation (Eq. (6)). It was found that the crys-

allite size of the CuAlO2 is 46.2 nm.The powder consisted of loose agglomerates of particles of a

ew 100 nm in size, see Fig. 8b. The median particle size of thenal powder product, as determined by laser granulometry, was.35 �m. The presence of particles with sizes between 0.1 to.2 �m, as observed by SEM, indicated that the small particlesorm agglomerates.

Sinterability of the CuAlO2 was checked in air and Ar atmo-phere. The CuAlO2 powder compacts with the green densitiesf ≈58% were heated to 1150 ◦C in the heating stage micro-cope. In Fig. 9, the shrinkage vs. temperature curves andRD patterns obtained after heating of the samples in air and

rgon atmospheres are collected. Upon heating the compact to150 ◦C in Ar, the shrinkage was only 2%, while heating in airesulted in almost 10% shrinkage. The calculated density of the

Ata

ig. 9. Shrinkage vs. temperature of the CuAlO2 compacts (a) and XRD patterns (bnal temperature in air and argon atmospheres.

steps of the nano CAO mixture (a) and FE-SEM (b) of the nano CAO mixture

ample was 71% TD. The XRD confirmed that the pure delafos-ite phase was obtained in both cases.

The powder compacts with 62% TD, obtained by cold-sostatic pressing at 700 MPa, were sintered at 1100 ◦C for 2 hn air and Ar atmospheres. Sintering in argon resulted in almosto densification; the relative density was 63% TD, while theeramic, sintered in air reached 4.36 g/cm3, or 86% TD. Thesexperiments revealed that sintering of the delafossite is influ-nced also by the atmosphere. As almost no shrinkage wasbserved in the sample heated in argon atmosphere, we assumehat the material transport was hindered in inert atmosphere.

The phase composition and microstructure of the CuAlO2intered at 1100 ◦C for 2 h in air were analyzed and the resultsre collected in Fig. 10. The ceramic is a phase pure delafossiteccording to XRD. The analysis of the microstructure revealedhat the bulk of the sample had a dense microstructure with aniform distribution of porosity within the delafossite matrix.races of Cu-rich impurities could be identified at the surfacef the pellet, as confirmed by EDXS, note some white inclu-ions in the inset of Fig. 10b, which consist of Cu: 44.2 at% and

l: 5.5 at%. Consequently, the amount of the Cu-rich impuri-

ies, identified only at the surface of the sample by SEM/BEInalysis, could be estimated to be less than 1%, which is within

) collected from the powdered samples after heating in the microscope to the

K. Vojisavljevic et al. / Journal of the European Ceramic Society 33 (2013) 3231–3241 3239

F 00 ◦C( samp

tsnip

bCti

C

sc

scapbaa

dThaA

TSa

A

L

P

D

T

RD

ig. 10. XRD (a) and SEM micrographs of the CuAlO2 ceramic, sintered at 11b), and interior (c) regions of the pellet, and (d) image of the sintered CuAlO2

he detection limit of the XRD technique. The presence of thepinel phase in the sintered sample was not detected by XRDor by SEM/EDXS analysis. The colour of the sintered samples blue–grey, which is typical for the samples without any spinelhase (see Fig. 10d).

The presence of Cu-rich traces at the ceramic surface coulde tentatively explained by intercalation of the oxygen into theuAlO2 crystal lattice during the thermal treatment in air, when

he off-stoichiometric delafossite phase could be formed accord-ng to the following reaction:

uAlO2 + �

2O2 →

(Cu+

1−2�Cu2+2�

)AlO2+� (10)

As it has been shown for the copper-iron delafossite powder,uch off-stoichiometric phase may decompose into a slightlyopper-deficient delafossite and CuO upon heating in air.30 It

iat

able 1intering temperatures (Ts) and times (ts), densities (ρ) and theoretical densities (TD)

nd in this work.

uthors Method Ts (◦C) ts

iou et al.10 RS 1200 2–1350 2–

ark et al.12 SS 1160 2 ×1200

ura et al.29 SS + MA/36 h 550 –a

1100

his work SS + S 1100 2

S: reactive sintering; SS + S: solid state synthesis and sintering; MA: mechanical ac: delafossite, traces of the following phases; A: Al2O3; C: CuO and S: spinel. In alla The sintering time was not reported

for 2 h in air: critical edge zone with Cu-rich defects and enlarged part of edgele.

eems that a similar sequence of processes takes place in ourase, when the surface of the ceramic is directly exposed to their atmosphere during thermal treatment. In order to trace theossible oxygen intercalation, the phase stability and thermalehaviour of the CuAlO2 under air atmosphere in a wide temper-ture range should be analyzed by thermogravimetric analysisnd Rietveld refinement as reported in Ref. 30.

In Table 1, our results are compared with the literatureata. Liou et al.10 obtained CuAlO2 ceramics with 49.7–52.6%D by reactive sintering at 1200 ◦C for 2–6 h in air. Slightlyigher values of 55.6–59.7% TD were obtained after sinteringt 1350 ◦C. The XRD revealed the presence of the unreactedl O phase. Park and co-workers12 obtained CuAlO ceram-

2 3 2

cs with 73.5–75% TD after sintering for 2 × 20 h at 1160 ◦Cnd 1200 ◦C, respectively. The XRD confirmed the presence ofraces of CuO in the sintered samples. Dura and co-workers29

of the CuAlO2 ceramics prepared by different methods reported in the literature

(h) ρ (g/cm3) TD (%) Phases

6 2.53–2.68 49.7–52.6 D + A6 2.83–3.04 55.6–59.7

20 3.74 73.5 D + C3.82 75

4.43 87 D + S4.96 97

4.36 86 D

tivation. cases, the phase composition was analyzed by XRD.

3 opean

pscTTt

arvspomut

fee

4

tbsc

-

-

-

pdsadbei

A

r4pA

R

1

1

1

1

1

1

1

1

1

1

2

240 K. Vojisavljevic et al. / Journal of the Eur

repared the CuAlO2 powder by several milling and calciningteps of the Cu2O and Al2O3 mixture and subsequent mechani-al activation for 36 h and sintering at 550 ◦C and 1100 ◦C in air.he resulting CuAlO2 ceramic samples with 87% TD and 97%D were obtained. Note however, that the material contained

he detrimental CuAl2O4 phase as evident from the XRD.29

In our work, 86% dense ceramic was obtained by sintering inir atmosphere at 1100 ◦C for 2 h. In comparison to the results,eported in Table 1, the achieved density was higher than thealues obtained by reactive sintering or conventional solid stateynthesis of the oxide-mixture and sintering of the powder com-act in air, however these two studies reported the presencef secondary phases in the ceramics, detected by XRD. Theechanochemically activated material reached a higher density

pon sintering at 1100 ◦C, but according to XRD it containedhe spinel phase.

The boehmite-derived delafossite powder was found suitableor preparation of ceramic targets for sputtering with high-nough density and mechanical stability of the ceramic whichnabled final surface-finishing to the required dimensions.

. Conclusions

The single-phase CuAlO2 powder has been synthesized byhe solid state synthesis from the stoichiometric mixture ofoehmite AlOOH·0.059H2O nano-particles and Cu2O by a two-tep calcination at 1100 ◦C for 10 h in argon. The followingonclusions could be drawn:

owing to the rod-shaped and nano-sized gamma-AlOOH par-ticles, a high density of the contact points is achieved betweenthe reagents in the powder mixture, providing a large numberof nucleation sites important for enhancement of the solid-state reaction;

the thermal decomposition of the nano-AlOOH at tempera-tures below 600 ◦C to �-Al2O3, a more reactive compoundthan the �-Al2O3 phase, due to the presence of the hydroxylgroups at the surface and a higher specific surface area, con-tributed to the formation of the almost pure delafossite phaseat 900 ◦C in argon atmosphere; when the �-Al2O3 was usedinstead nano-AlOOH, only a small amount of delafossite wasobtained under the same experimental conditions;

the calcination of the powder mixture in argon precludes theoxidation of the Cu+ into Cu2+ ions and possible formation ofthe detrimental spinel phase.

The solid-state reaction of the boehmite and Cu2O has beenroven as a better and faster approach for the single-phaseelafossite powder as compared to the conventional oxide-basedolid state synthesis. The single phase ceramics were obtainedfter sintering at 1100 ◦C for 2 h in air according to XRD. Theetrimental spinel phase was not observed in the ceramic either

y XRD or with the SEM–EDXS analysis. Heating in air wasssential for enhancement of densification and resulted in ceram-cs with 86% of theoretical density without using any additives.

2

2

Ceramic Society 33 (2013) 3231–3241

cknowledgments

We acknowledge the financial support of the Slovenianesearch agency (research programme P2-0105 and projects J2-273 and 1000-11-780007) and the EC within the 7FP ORAMAroject: Oxide materials for electronics applications, Grantgreement NMP3-LA-2010-246334.

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