Cu composites with controllable thermal expansion coefficients

Materials and Design 54 (2014) 989–994

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Hydrothermal synthesis of ZrW2O8 nanorods and its application in ZrW2O8/Cu composites with controllable thermal expansion coefficients Z. Peng, Y.Z. Sun, L.M. Peng ⇑ CAS Key Laboratory for Mechanical Behavior and Design of Materials, Department of Modern Mechanics, School of Engineering Science, University of Science and Technology of China, Hefei 230026, PR China

a r t i c l e

i n f o

Article history: Received 25 December 2012 Accepted 5 September 2013 Available online 21 September 2013 Keywords: Negative thermal expansion Zirconium tungstate Controllable coefficients of thermal expansion Anisotropy in thermal expansion coefficients

a b s t r a c t High purity ZrW2O8 powders with negative thermal expansion were hydrothermally synthesized in HCl solution. Unreacted ZrW2O8/Cu was fabricated by hot-pressing. Sn addition enhanced the densification of composite compact. Low hot-pressing pressure and relative loose microstructure of the composites can effectively prohibit the formation of high pressure c-ZrW2O8. The thermal expansion curves were stable and uniform without anomalously large expansion. The coefficients of thermal expansion decreased with the ZrW2O8 volume fraction and a value of 1.6  106/°C was obtained for 68 vol.% ZrW2O8/Cu. Microstructural anisotropy led to obvious thermal expansion coefficient anisotropy. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Zirconium tungstate (ZrW2O8, abbreviated as ZT) exhibits an isotropic negative thermal expansion (NTE) behavior and the average coefficient of thermal expansion (CTE) reaches 7.2  106/°C over a wide temperature range of 273 to 777 °C [1–3]. The abnormal thermal contraction characteristic in ZrW2O8 is ascribed to the rotational vibrations of the rigid zirconium oxide (ZrO6) octahedral and tungsten oxide (WO4) tetrahedral, which are linked by shared oxygen atoms [4]. ZrW2O8 is only thermodynamically stable in the narrow temperature range of 1105–1257 °C while it is metastable from almost absolute zero to 777 °C. On heating above this temperature ZrW2O8 decomposes into ZrO2 and WO3. Simultaneously, ZrW2O8 undergoes two phase transformations: cubic a phase (space group P213) at ambient temperature and pressure to high temperature cubic b phase (space group Pa3) at 155 °C and a phase to high pressure orthorhombic c phase (space group P212121) under hydrostatic pressures higher than 0.2 GPa at ambient temperature [1,2]. The CTEs of the three phases (a, b, c) take values of 8.7  106, 4.9  106 and 1.0  106/°C, respectively. The a ? b transition is reversible upon cooling below 155 °C, while the c phase is metastable upon release of the hydrostatic pressure at ambient temperature and exist until heating above 120 °C at ambient pressure. There is a 5% volume contraction associated ⇑ Corresponding author. Tel.: +86 551 360 6964; fax: +86 551 360 6459. E-mail address: [email protected] (L.M. Peng). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.012

with the a ? c transition [5], while the a ? b transition is second order and is thus not associated with a volume change. The traditional solid state way of synthesizing ZrW2O8 is to sinter the mixed powders of WO3 and ZrO2 above 1105 °C, followed by rapid cooling to prevent the decomposition of ZrW2O8. However, it is usually difficult to control the procedure and obtain ZrW2O8 powders with homogeneous and small particle morphologies at such a high sintering temperature. Moreover, the high vapor pressure in the sintering temperature range inevitably results in a remarkable loss of WO3. As a result, it is quite difficult to achieve high purity ZrW2O8 by the solid state processing route. In contrast, two low temperature synthesis methods including sol-gel method [6,7] and hydrothermal reaction [8–10] have been utilized to prepare nanoscaled ZrW2O8 particles. In the latter route, pure ZrW2O8 phase is obtained by the dehydration of the precursor ZrW2O7 (OH)2(H2O)2 synthesized in a HCl solution. The most important application of ZrW2O8 is to serve as a CTE compensating component for creating composites with tailored thermal expansion. Up to date, there have been extensive reports on the ceramic composites of ZrW2O8, such as ZrO2/ZrW2O8 [11,12], cement/ZrW2O8 [13], SiC/ZrW2O8 [14], ZrW2O8/Zr2WP2O12 [15]. On the other hand, polymer matrix composites containing ZrW2O8 nanoparticles, i.e. ZrW2O8/Polyimide [16,17] and ZrW2O8/ cyanate ester [18] have been recently prepared with large reductions in CTE. Metal matrix composites are attractive materials for applications where the high thermal conductivity of metals and the low thermal expansion of ceramics are simultaneously

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desirable. For instance, Cu/ZrW2O8 [19,20] and Al/ZrW2O8 [21] composites have received considerable attentions. Among the metallic matrices, copper is considered as the prior candidate due to its excellent thermal and electrical conductivities. Such a composite could find its application in the electronic packaging [22,23]. Hot-pressing (HP) and hot-isostatic-pressing (HIP) have been employed to fabricate fully dense composites consisting of a Cu matrix containing 33–75 vol.% ZrW2O8 particles [24,25]. Nevertheless, processing temperature and time must be carefully controlled to avoid the chemical reaction between the two phases. Moreover, high pressing pressure employed during the HP or HIP (to reach full densification) and the thermal mismatch stresses generated in the composites usually caused the formation of high-pressure c-phase. As a result, thermal expansion coefficients of the composites were unstable and anomalously large due to the inversible transformation of c-ZrW2O8 to a-ZrW2O8 at 120 °C with a volume expansion of 5%. Accordingly, the present study describes the synthesis of highpurity nano-sized ZrW2O8 powders using hydrothermal method. The (40–68 vol.%) ZrW2O8/Cu composites are then prepared by means of hot-pressing at lower temperature and pressure to avoid the phase transition of a-ZrW2O8 to c-ZrW2O8. In order to enhance the densification of composite compact, a small amount of lowmelting-point Sn is used as sintering aids. The microstructures and thermal expansion behaviors of both the synthesized pure ZrW2O8 and composites are investigated. In particular, the thermal expansive anisotropy of sintered composite bodies will be examined by measuring the CTEs in both the parallel and perpendicular directions to the hot-pressing direction, on which there have been no reports available in the literature.

2. Experimental details Zirconium oxychlorid (ZrOCl28H2O), sodium tungstate dehydrate (Na2WO4  2H2O) and hydrochloric acid (HCl), all with analytical grade purity, were used as starting materials. Solutions of Zr (0.41 mol/L) and W (0.34 mol/L) were prepared by dissolving zirconium oxychlorid (13.05 g) and sodium tungstate dehydrate (22.27 g) in 200 and 100 mL distilled water, respectively. It should be noted that a little excess of Zr (the molar ratio of Zr:W = 1.2:2) is beneficial to eliminate the impurities (WO3) in the final products [10,26]. The two solutions were simultaneously added dropwisely into 25 ml distilled water under continuous mechanical stirring for 2 h at 60 °C. Then 125 mL HCl aqueous solution (10.8 mol/L) was added dropwisely into the mixture during stirring for another 2 h. The slurry was transferred to a Teflon-lined stainless steel autoclave and heated at 180 °C for 8 h. The cooled solution was filtered to obtain ZrW2O7(OH)2  2H2O precursor. The resulting precursor was washed several times with distilled water, dried at 70 °C and finally heated at 600 °C for 6 h. The obtained ZrW2O8 powders were mixed with commercially available Cu (99.7% pure, 200 mesh) and four composites containing 40, 50, 60, and 68 vol.% ZrW2O8 powders were prepared. Hereafter the abbreviated ‘‘ZTxxCuyy’’ was used to represent the composites (xx and yy mean the volume fraction of individual phase). 2 vol.% Sn (99.5% pure, 200 mesh) powders were added to the ZT68Cu30 as sintering aids. The mixtures were wet ball milled at 125 rpm in ethanol for 10 h. The dried powders with 3 wt.% PVA (polyvinyl alcohol) addition were then uniaxially cold-pressed under 550 MPa into £20 mm  12 mm cylinders. Hot-pressing was performed under 35 MPa in vacuum at 550 °C with a soaking time of 1 h. Pure ZrW2O8 powders were coldpressed into a £6 mm  12 mm rod, sintered at 1130 °C for 1 h and cooled in air. The ground rod was used for CTE measurement.

Phase constituent was identified by X-ray diffraction (XRD, Rigaku D/Max TTR-III) with Cu Ka radiation. TG-DTA measurements on ZrW2O7(OH)2  2H2O precursor and ZrW2O8 powders were carried out on a DTG-60H between 30 and 1200 °C with a heating rate of 10 °C/min. The morphology of the synthesized ZrW2O8 powders was examined using a scanning electron microscope (SEM, XL30 ESEM) and transmission electron microscope (TEM, JEM-2100F). The microstructures of composites were observed by an optical microscope. The densities of the composites were estimated using Archimedes’ principle and compared with theoretical ones to obtain the degrees of densification. The sintered composite bodies were cut into bars of 3 mm  3 mm  10 mm with the longitudinal direction or perpendicular to the hot-pressing direction. CTE measurements were conducted using a Q400 TMA at a heating rate of 5 °C/min from 20 to 500 °C under a N2 atmosphere. 3. Results and discussion Fig. 1a shows the XRD patterns of the precursor, the heat-treated product, and ZT68Cu30Sn2 composite. It can be seen that both the precursor synthesized in a HCl solution and final product exhibit sharp diffraction peaks which indicates the complete crystallinity of the powders. Their peak positions are well indexed to those of ZrW2O7(OH)2  2H2O (JCPDS28-1500) and cubic ZrW2O8 (JCPDS83-1005), respectively. It is documented that the precursor was formed during the hydrothermal process through the following reactions [27]:

ZrO2þ þ ðx þ 1ÞH2 O ! ZrO2  xH2 O þ 2Hþ

ð1Þ

þ WO2 4 þ 2H þ ðy  1ÞH2 O ! WO3  yH2 O

ð2Þ

ZrO2  xH2 O þ 2WO3  yH2 O ! ZrW2 O7 ðOHÞ2  2H2 O þ ðx þ 2y  3ÞH2 O

ð3Þ

Fig. 1b and c shows the TG-DTA curves of ZrW2O7(OH)22H2O and ZrW2O8 powders to examine their thermal transformation behavior. Three endothermic and one exothermic peaks are observed on the DTA curve of the precursor. A weight loss of 8.26% in the temperature range of 200 and 282 °C corresponds with the first endothermic peak, which demonstrates the dehydration of ZrW2O7(OH)22H2O and the formation of ZrW2O8 (see Eq. (4)). The measured weight loss is in quite good agreement with the calculated value (8.4%), indicating that ZrW2O8 product exhibits high purity. At higher temperatures, the two sets of TG-DTA curves display similar characters. Between 716 and 1128 °C, ZrW2O8 crystals decomposed into ZrO2 and WO3 (see Eq. (5)) while almost no weight loss was observed. The endothermic peak at 1128 °C is attributed to the evaporation of WO3. Above this temperature, ZrW2O8 was reformed (see Eq. (6)) corresponding with the exothermic peak at 1154 °C. As a result, the processes for the thermal transformation can be described as follows:

ZrW2 O7 ðOHÞ2  2H2 O ! ZrW2 O8 þ 3H2 O ZrW2 O8 ! ZrO2 þ 2WO3

ð4Þ ð5Þ

ZrO2 þ 2WO3 ! ZrW2 O8

ð6Þ

As is evident from the SEM and TEM images (Fig. 2a and b), the hydrothermally synthesized ZrW2O8 powders show nanorod-like morphologies with average length of 5 lm and diameter ranging from 100 to 500 nm. Fig. 2c and d presents the typical optical micrographs on the section parallel to the hot-pressing direction of Cu-composites with 50 and 68 vol.% ZrW2O8, respectively. The ZrW2O8 particles are homogeneously distributed throughout the Cu matrix. The measured individual relative densities of the four composite samples are 87%, 84%, 82%, and 86%: the higher the

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c-ZrW2O8 formed during high-pressure cold pressing can be reconverted to cubic a-ZrW2O8 by heating at 120 °C. The order-disorder transformation of a ? b at 155 °C is reversible [1,3]. However, previous investigations demonstrated that a small amount of Cu2O precipitates were formed via the reaction between Cu and ZrW2O8 when the composite system was subjected to HP or HIP at temperatures higher than 600 °C [11,22]. Furthermore, due to the very large CTE mismatch between Cu and a-ZrW2O8 (Da  27  106/ °C at ambient temperature), significant residual stresses were generated at ambient temperature after cooling from the fabrication temperature. During the past years, many different methods such as neutron diffraction and synchrotron diffraction have been developed to measure the residual stresses in different types of components [28]. However, special and expensive equipments are needed. Theoretical calculation is usually an effective approach to estimate the approximate value of residual stresses. By considering a mismatching spherical precipitate in an infinite and ideally plastic matrix as proposed by Lee et al. [29], the matrix undergoes plastic deformation at the particle-matrix interface when the particle hydrostatic stress p reaches two thirds of the matrix yield stress ry. The mismatch hydrostatic stress p can be calculated by [29]:

p ¼ 3Gm ceð1  bÞ

ð7Þ

where

ac aðc  1Þ þ 1 1 þ mm a¼ 3ð1  mm Þ b¼

c¼

Fig. 1. (a) XRD patterns of synthesized precursor ZrW2O7(OH)2  2H2O, final product ZrW2O8 and ZT68Cu30Sn2 composite, (b and c) TG–DTA curves of the precursor ZrW2O7(OH)2 2H2O and (b) ZrW2O8, respectively.

volume fraction of ZrW2O8, the lower the density. However, the 68 vol.% ZrW2O8 composite has almost the same density as that of the 40 vol.% ZrW2O8 composite due to the enhanced densification by a 2 vol.% Sn addition. No additional XRD peaks from other phases including c-ZrW2O8 except a-ZrW2O8 and Cu were detected (see Fig. 1), which indicates that no chemical reactions take place under the present hot-pressing conditions. The orthorhombic

Kp Km

ð8Þ ð9Þ ð10Þ

e is the radial mismatch strain identified as DaDTP(=(ap  am)(TR  TP), TR and TP denotes the room temperature and the processing/sintering temperature, respectively). am, ap, Gm, Km, Kp, and mm are the CTE, Young’s modulus, bulk moduli and Poisson’s ratio of the Cu matrix and/or ZrW2O8 particulate, respectively. Using the related material properties (GCu = 48.3 GPa, KZT = 74.5 GPa, KCu = 138 GPa and mm = 0.34 [30]), the tensile hydrostatic stress in the Cu particulates was estimated to be 509 MPa. It is obvious that this value far outnumbers the Cu matrix yield stress (200 MPa), and thus it can be safely deduced that the matrix is fully plastic at ambient temperature after cooling from the processing temperature. The thermal residual stress in a composite can be maintained only if its value is lower than that of matrix yield strength as it is well-documented in the literature [31]. Instead, the thermal mismatch stress can be relaxed via matrix plastic deformation and dislocation multiplication [29,32]. In addition, the present composites are not fully dense with some residual voids. However, the effect of voids on the magnitude of thermal residual stress is neglected for simplicity. In fact, the presence of porosity in a composite may increase or decrease thermal residual stresses dependent on the collocation of matrix and reinforcement since the Young’s modulus is strongly affected by the volume fraction, shape and orientation of pores [33,34]. It can thus be safely assumed that the matrix is fully plastic at ambient temperature and most of the thermal residual stresses were relaxed by the matrix plasticity and pores in the present composite system after cooling from the fabrication temperature. As a result, the allotropic transformation of a ? high-pressure c was not observed. Fig. 3a and b depicts the thermal expansion curves of hydrothermally synthesized ZrW2O8 and composites with a different volume fraction of ZrW2O8, respectively. A transition in the curve slope for the monolithic ZrW2O8 is clearly observed at around 150 °C. The two segments exhibit an individual average slope of

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(a)

(b)

(c)

(d) ZrW2O8

Cu 50 µm

50 µm

Fig. 2. (a and b) SEM and TEM images of ZrW2O8 particles, (c and d) optical micrographs on the section parallel to the hot-pressing direction of ZT50Cu50 and ZT68Cu30Sn2 composites, respectively.

the curves at about 150 °C for the composites containing 60 vol.% and 68 vol.% ZrW2O8 are attributed to the phase transformation from a-ZrW2O8 ? b-ZrW2O8. Thus, the mean CTEs of the two composites in the temperature range 20–120 °C are slightly lower than the corresponding values in the whole temperature range. As shown in the inserted table, the four composite samples containing 40 vol.%, 50 vol.%, 60 vol.% and 68 vol.% ZrW2O8 exhibit mean CTEs of 9.9  106/°C, 7.6  106/°C, 3.3  106/°C and 1.6  106/°C in the temperature 20–500 °C, respectively. In general, compared with the previous fully dense composites prepared under high pressure and sintering temperature [19,25], the expansion curves of the present composites are relatively stable and uniform without anomalously large expansion due to the absence of a M c reversible phase transformation. Several models are usually used to predict the CTE of particle-reinforced composites, including the simple linear rule-of-mixture (ROM), Turner and Kerner models [35]. However, large discrepancy between the predicted value and experimental results exists since the matrix/reinforcement elastic interactions are ignored in ROM (see the inserted table in Fig. 3b). In Turner and Kerner expressions, we have to distinguish the matrix phase and reinforcement phase, and therefore the predicted values usually depend on the choice of matrix phase. Another formula was recently established by means of thermoelastic mechanics and micromechanics of composites to predict the CTE of multi-phase particle-reinforced composites. For a composite containing n phases, the CTE is expressed as [36]

ac ¼

n X i¼1

Fig. 3. Thermal expansion curves of (a) pure ZrW2O8 and (b) ZrW2O8/Cu composites with different volume fraction of ZrW2O8 particulates (The table was inserted to summarize and compare the experimental results and the theoretically predicted values).

9.6  106/°C and 4.8  106/°C, corresponding with the CTE values of a and b phases, respectively. The CTE of the composite materials decreases with the amount of ZrW2O8. The dents on

fi K i ai 4Gc þ 3K i

, n X i¼1

fi K i 4Gc þ 3K i

ð11Þ

Table 1 Material properties of Cu and ZrW2O8 [30] used for predicting the CTE of composites. Property

Cu

ZrW2O8

Young’s modulus, E (GPa) Shear modulus, G (GPa) Bulk modulus, K (GPa) Poisson’s ratio, m CTE, a (106/°C)

128.9 48.3 138 0.34 17.7

88.3 33.9 74.5 0.303 8.7

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(b) (a)

Pressing direction

Pressing direction

50 µm

Measured direction

Measured direction

Observed direction

(c)

Pressing direction

Observed direction

Pressing direction

50 µm Fig. 4. (a) Thermal expansion curves of ZT60Cu40 composite along the directions parallel and perpendicular to the hot-pressing direction to show the thermal expansion anisotropy, (b) and (c) optical micrographs of ZT60Cu40 composite with microstructural anisotropy along the aforementioned two directions.

where fi, Ki and ai are the volume fraction, bulk modulus and CTE of the i th phase, respectively. Gc is the shear modulus of composite determined by the Young’s modulus Ec through n X Ec Gc ¼ fi mi Þ ðmc ¼ 2ð1 þ mc Þ i¼1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi .Xn f Xn i fE Ec ¼ i¼1 i i i¼1 E i

stable and uniform from room temperature to 500 °C without anomalously large expansion. References

ð12Þ

ð13Þ

where mi and mc are the respective Poisson’s ratio of the i th phase and composite. Using the data in Table 1, the theoretical CTE values in the measured temperature range describe more accurately the changing trend of the experimental composite CTEs with the volume fraction of ZrW2O8 particulates (see the table in Fig. 3b). Considering that structural anisotropy usually exists in the HP materials and there has not been any reports relating the CTE to the structural anisotropy of ZrW2O8/Cu composites, Fig. 4a compares the thermal expansion curves of ZT60Cu40 parallel and perpendicular to the pressing direction. The HP composite shows a considerably anisotropic expansion behavior. The mean CTE along the direction perpendicular to pressing (7.8  106/°C) is approximately 1.4 times higher than that along the pressing direction (3.3  106/°C). Similar anisotropy in the thermal expansion coefficients is also observed in the other three composites. The dependence of CTE on the measured direction can be easily understood from a perspective of structural anisotropy. As is evident from Fig. 4b and c, much more copper phases with higher CTE are distributed on the section perpendicular to the pressing direction due to the high temperature plastic flow of Cu matrix, resulting in remarkable anisotropy in thermal expansion coefficients.

4. Conclusions High purity nanorod-like ZrW2O8 powders were prepared by the dehydration of the hydrothermally synthesized precursor ZrW2O7(OH)2(H2O)2 in a HCl solution subjected to calcination at 600 °C for 6 h. ZrW2O8/Cu composites were prepared by hot-pressing method with low thermal expansion. Small amount of Sn additive can serve as sintering aid and increase the density and the strength of the composites. The low hot-pressing pressure and relative loose microstructure can effectively prohibit the formation of high pressure c-ZrW2O8. The thermal expansion curves were

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