Thermal diffusivity, heat capacity and thermal conductivity of porous

JOURNAL

OF MATERIALS

SCIENCE

LETTERS

5 ( 1 9 8 6 ) 799 802

Thermal diffusivity, heat capacity and thermal conductivity of porous partially stabilized zirconia M. V. S W A I N

Division of Materials Science, CSIRO, Clayton, Victoria, Australia 3168 L. F. J O H N S O N ,

R. SYED, D. P. H. H A S S E L M A N

Department of Materials Engineering, Virginia Polytechnic, Institute and State University, Blacksburg, Virginia 24061, USA

Partially stabilized zirconia (PSZ), which relies on the tetragonal-to-monoclinic phase transformation for a significant increase in fracture toughness [1, 2], is an excellent candidate material for service conditions which require high resistance to abrasive wear and high load-bearing ability, such as extrusion dies, seals, valves and engine components [3]. Because of its high chemical stability, PSZ also is eminently suited for service under highly corrosive conditions, such as those encountered in the chemical and materials processing industries. Its very low value of thermal conductivity [4-7] compared to many other ceramic and metallic structural materials, also makes PSZ very suitable for service under conditions which involve spatially non-uniform temperature distributions for which heat losses should be kept to a minimum. It is for this latter reason that PSZ is a particularly effective substitute material for components of internalcombustion or turbine engines with greatly improved fuel-efficiency. The thermal conductivity of PSZ can be lowered even further by incorporating within its microstructure discontinuities such as cracks [8] or pores [4, 9-11] which interfere with the direct flow of heat and also serve to enhance thermal shock resistance [12]. In the case of cracks the volumetric heat capacity is maintained such that the heat storage capacity is unaffected. Pores, in contrast, will lower the volumetric heat capacity in direct proportion to the volume fraction of the pores. A reduced volumetric heat capacity serves to reduce the energy requirements for reaching specified levels of temperature. The purpose of this letter is to report experimental data for the effect of a pore phase on the thermal diffusivity, specific heat and thermal conductivity of a partially stabilized zirconia. Specimens of the PSZ with 3.3 wt % MgO as stabilizing agent with a range of density were prepared by cold-pressing at room temperature appropriate mixtures of zirconia (Harshaw Chemical Co., Electronic Grade) and magnesia (British Drug House, Analar Grade) powders over a range of pressures followed by firing at approximately 1700 ° C over a period of 1 h. The specimens were prepared directly in the form of circular disks approximately 10ram in diameter by approximately 2 mm thick, suitable for measurement of the thermal diffusivity. This eliminated the need for cutting and grinding and assured that the data obtained were not affected by the tetragonal-to-monoclinic 0261-8028/86 $03.00 + .12 © 1986 Chapman and Hall Ltd.

phase transformation induced by surface preparation. The crystallographic phase composition following firing was determined by X-ray analysis using CuK radiation and a scanning speed of 0.5°min -1. The density and pore content were determined from the mass and volume assuming a value for the full density of 5.81gcm 3. Table I lists the data for the cold-pressure, fired density, approximate value of porosity and phase composition. It should be noted that the ratio of the tetragonal-to-monoclinic phase content decreases strongly with decreasing density. This effect is thought to arise from the presence of the pores which reduces the mechanical constraint provided by the matrix and which inhibits the tetragonal-to-monoclinic phase transformation. Fig. 1 shows optical micrographs of samples with density of 5.13 and 4.42gcm -3. The pore phase consists primarily of isolated pores with randomly oriented non-spherical pore geometry. The thermal diffusivity was measured by the flashdiffusivity method [13]. Experimental details were described in previous papers [14, 15]. For the measurement of thermal diffusivity the maximum temperature was limited to ~ 1000 ° C, in order to avoid permanent irreversible changes due to destabilization reactions or other causes, observed in a previous study, for temperature > 1000°C [16]. The specific heat was measured from room temperature to 600 ° C, by means of differential scanning calorimetry. The thermal conductivity (K) was calculated from K =

xOc

(1)

where x is the thermal diffusivity, e is the density and c is the specific heat. Fig. 2a, b, c and d shows that the experimental data for the thermal diffusivity as a function of temperaTABLE I Cold pressure, density, porosity and phase composition of porous partially stabilizedzirconia Pressure Density Porosity Monoclinic Tetragonal Cubic (MPa) (gcm-3) (%) (%) (%) (%) 20 5 3 2

5.56 5.13 4.75 4.42

4 12 18 24

5 15 15 20

35 30 30 25

60 55 55 55 799

Figure 1 Optical micrographs of porous partially stabilized zirconia with densities of: (a) 513 gcm -3 and (b) 4.42 gcm -3.

ture for the values of density of 5.56, 5.13, 4.75 and 4.42 g cm -3. The data obtained on return to room temperature were found to be identical to those obtained prior to heating within the estimated experimental accuracy of ~ 3% heating. This indicates that at least I

I

I

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over the duration of these measurements of ~ 6 h for a single specimen, no significant permanent changes in thermal diffusivity occurred. The data in Fig. 2. indicate that, in general, the thermal diffusivity decreases with decreasing density.

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Figure 2 Experimental data for the temperature dependence of the thermal diffusivity of partially stabilized zirconia with values of density of: (a) 5.56; (b) 5.13; (c) 4.75 and (d) 4.42gcm -3.

800

T A B L E II Density and specific heat ( J g - I K J) of porous partially stabilized zirconia

t..,,) 4 0

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Temperature

Density (gcm -3)

(°C)

5.56

5.13

4.75

4.42

25 100 200 300 400 500 600

0.475 0.527 0.566 0.592 0.612 0.629 0.642

0.475 0.524 0.562 0.587 0.606 0.623 0.636

0.477 0.530 0.570 0.595 0.615 0.630 0.643

0.480 0.513 0.571 0.597 0.617 0.634 0.649

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DENSITY (gcm -3) 5.56 5.13 4.75 4.42

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THEORY

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--~ Z 0