JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 384-386

Epoxy-modified Bi(Pb)SrCaCuO superconductors with improved mechanical properties

I. M. LOW, H. WANG, R. D. SKALA Materials Research Group, School of Physical Sciences, Curtin University of Technology, GPO Box U1987, Perth, WA 6001, Australia

The recent discovery of high temperature supercon- ductivity in the B i - ( P b ) - S r - C a - C u - O system [1-3] has offered the possibility of more economic practical applications of these ceramics. In contrast to the YBa2Cu307 (123) and other rare-earth substituted oxides, the more complex chemical nature of this system produces an array of phases with different structures, compositions and super- conducting properties. At least two superconducting phases in the Bi-based compound have been recog- nized; the high Te phase (222 3) superconducts at about l l 0 K while the (2212) phase has a Te of about 80 K.

Before the recently discovered high temperature superconductors (HTSC) can be developed for useful applications, a number of limitations inherent in these promising materials need to be solved. The limitations include poor critical magnetic field (Hc), low critical current density (J~), poor chemical resistance, low strength and poor fracture tough- ness. Hitherto, the mechanical properties of 2 2 2 3 ceramics remain virtually unknown, although those of 123 ceramics are widely known [4-8]. For instance, Cook et al. [1] and Low et al. [9] reported that 12 3 superconductors were very brittle, with fracture toughness of 1.3 MPa m 1/2 and 0.5 MPa m I/2, respectively. Similarly, poor strength and hardness were observed for 123 ceramics. Ihm et al. [2] and Low et al. [9] reported that flexural strengths of 123 bars ranged from 15.0 to 57.6 MPa, depending on porosity and processing history. They obtained Knoop hardness values ranging from 2 2 5 to 447 KHN. Recently, we [8, 9] successfulty fabric- ated epoxy-modified 123 superconductors with rauch improved hardness, strength and chemical resistance to watet.

In this letter, we report the improvement in mechanical and fracture properties of 2 2 2 3 ceram- ics by impregnation with epoxy resin. Properties such as hardness, strength, stiffness and fracture toughness are discussed in relation to microstruc- tural modification.

Samples were prepared via the standard solid- state reaction method with the molar composition of Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.0:3.0. The initial reagents used were commercial purity Bi203, Pb304, SrCO3, CaCO3 and CuO. Appropriate amounts of Sr and Ca carbonates were first mixed and ground for 30 min before the mixture was decomposed at 1200 °C for 20 h. The resulting powder was then mixed and ground with the rest of the metal oxides

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for another 40 min. This homogeneous powder was then uniaxially pressed at 150 MPa into pellet samples of diameter 15 mm and thickness 3 mm. Bar samples (60 mm × 12 mm × 3 mm) were also unia- xially pressed at 80 MPa. These samples were then annealed at 860 °C for 120 h, followed by furnace cooling to room temperature. The phase composi- tions of 2223 samples were confirmed by X-ray diffraction (XRD) using a Siemens D500 diffraeto- meter.

The sintered samples were subsequently impreg- nated for 10 min in a bath of epoxy resin (DOW D.E.R. 353) mixed with 32.3 parts per hundred of resin by weight of 4,4'-diamine-diphenyl-sulphone (DDS) as curing agent. The epoxy impregnated samples were then fully cured in an oven, initially at 120°C for 16h, followed by 4 h at 180°C. All sintered samples displayed the Meissner effect and had a Tc value of approximately 108 K [9].

A diametrical compression test was used to determine the splitting tensile strength of 2223 pellets [10]. Flexural strength and modulus of bar samples were performed using the three-point loading method. The flexural strength (S) and modulus (E) were calculated using the formulae:

S = 1 . 5 P f L / b d 2 (1)

E = PL3/4bd3e (2)

where P is the load at a particular beam deflection (e), Pf is the failure load, L is the support span, b is the sample width and d is the sample thickness. The deflections during bending were measured directly from the crosshead movement of the machine. Due allowance was made for the machine stiffness in calculating E. The fracture toughness (Kic) at extension of initial crack-length (a) was measured using the three-point bend test such that:

KI~ = ( PfL/dbB/2) f (a/b) (3)

where f ( a / b ) is a polynomial geometrical correction factor [11]. All of these tests were performed in ambient conditions with an Instron machine (model 1195) at a crosshead speed of 0.5mmmin < . A Zwick Charpy impact tester was used to evaluate the impact strength (IS) and impact toughness (IR) such that:

IS ~ ( 18EUe /V) ~/2 (4)

IR ~- Ue/(b - a )d (5)

where Ue is the energy absorbed and V is the

0261-8028 ©1995 Chapman & Hall

volume of the sample. Notching of samples for Klc and IR measurements was done by pressing a sharp razor blade to a depth of about 2 mm, prior to sintering at 860 °C and epoxy impregnation. Micro- hardness measurements of pellets were made on samples prepared by mounting and polishing on 1200 grit sandpaper. Hardness values were measured using a Tukon microhardness tester with Knoop indenter at a load of 200 g.

Load versus displacement results during KI~ measurement showed that the pure 2223 sample displayed a catastrophic fracture hut the epoxy- modified sample exhibited crack-arrest, followed by crack reinitiation prior to final fracture. The influ- ence of impregnation time on the hardness and tensile strength is shown in Table I. A comparison between pure and epoxy-modified samples in terms of flexural and fracture properties is given in Table II. When compared with pure 2 2 2 3, the addition of epoxy led to approximately a 1.5-fold increase in hardness, a 2-fold increase in tensile strength, IS and flexural modulus, a 4-fold increase in flexural strength and Klc, and a 1.3-fold increase in IR. Similar improvements in strength and elastic mod- ulus were also reported for the PMMA-modified 12 3 superconductors [7].

The presence of epoxy resin reduced the porosity of 2 2 2 3 from 40 to 15% [12]. The effect of the total porosity in the 2 2 2 3 matrix is to reduce its fracture surface energy, hardness, strength, elastic modulus and fracture resistance. Several relationships have been proposed to relate the strength and modulus of ceramic to its porosity [13]. The flexural strength and modulus of porous ceramic can be given by:

ac = CroF«(p) (6a)

Eo = EoFE(p) (6b)

where Fo(p) and FE(p) a r e a function of porosity (p) and satisfying the equation when Fo(0)= F~(0) = 1. Using Equation 6 to fit the test data for PMMA-impregnated 12 3 samples, Vipulanandan et al. [7] obtained the following function for Fo(p):

Fo(p) = (1 - Aop) (7a)

ùFE(p) = (1 - AEp) (7b)

TABLE I Knoop hardness and tensile strength of pure and epoxy-modified 2 2 2 3 pellets

Sample Strength (MPa) Hardness (KHN)

2223 26 62 Epoxy-2 2 23 50 79

TABLE II Flexura], fräeture and impact properties of pure and epoxy-modified 22 23 bar samples

cr E KÆ0 IS IR Sample (MPa) (GPa) (MPam 1/2) (MPa) (kJm -2)

2223 7.6 40 0.3 168 1.2 Epoxy-2223 27.2 82 1.3 273 1.6

They obtained values of 1.83 and 74 MPa for Ao and ao, and 2.85 and 50.4 GPa for AE and Eo, respec- tively.

Reduction of porosity therefore leads to bettet mechanical and fracture properties. However, it is difficult to control porosity by simply varying the compact pressure. Polymer infiltration may be a more effective method. The presence of epoxy has allowed open pores in the matrix to be filled, thereby improving the bond strength between the 2 2 2 3 grains. The improvement in hardness, tensile strength, flexural strength and modulus, and frac- tute toughness of epoxy-infiltrated samples is a direct reflection of improved bond strength between matrix grains coupled with a reduction in porosity via a void-filling mechanism [14, 15]. The scanning electron microscopy (SEM) revealed that the frac- ture surface of the control sample was more granular but there were more cleavage fractures of 12 3 grains in the epoxy-modified samples. This would support the proposal of a better bond strength between matrix grains as a result of epoxy infiltration. The formation of a continuous polymer phase within the matrix [8] serves to inhibit the initiation and propa- gation of microcracks. A similar explanation has been used to account for the strengthening and toughening of epoxy-modified cement mortars [16-19].

To ascertain whether the strengthening observed is a result of a pore size reduction and whether the maximum size pores act as critical flaws in control-

.ling the flexural strengths, the pore sizes from fracture surfaces were examined using SEM. The open pores near the surface had been filled but the closed-pore size did not vary much with the addition of epoxy resin. Both the control and epoxy-modified samples had approximate maximum closed-pore sizes of less than 5/~m (Fig. 1), which is rauch less than the critical crack size (a¢) calculated from the equation [20, 21]:

ac ~- Y2 EG1J~ra2 (8)

where Y is a geometric correction factor for the surface pore or crack; E and a are directly obtain- able from Table II. For a shallow semi-elliptical surface crack with a large aspect ratio, Y = 1.0. For a semi-circular surface crack, Y = Tr/2. Table III gives the a~ values for the control and modified 2 2 2 3 samples; clearly, they are at least several times larger than the maximum pore size of 5/~m. Thus, the pore sizes are too small to act as critical flaws in controlling the strength and fracture toughness. This observation concurs with the findings of Mai and Cotterell [21] and Knab et al. [22] that the critical crack in epoxy-modified mortars is not the largest pore. The improvement in strengths may be at- tributed to a reduction in open porosity together with an increase in bond strength of 2 2 2 3 matrix. The greater bond strength of matrix grains is believed to give rise to an improvement in the work of fracture (Glo or R) and the concomitant enhance- ment of fracture toughness.

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Figure I SEM micrograph showing the fracture sufface of (a) pure and (b) epoxy-modified 2 2 2 3 samples.

T A B L E III Critical crack size (a~) for control and epoxy-modi- fied 2 2 2 3 superconductors during bending

a~ (mm)

Shallow Sample Glc a (J m -2) Semi-circular semi-elfiptical

2223 2 1.1 0.4 Epoxy-2223 21 1.8 0.7

aCalculated from (Klo) 2 ~ E GI~.

Acknowledgement The financial support from the ARC Mechanism B Infrastructure Fund is acknowledged.

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Received 18 August and accepted 28 October 1994

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