An Acoustic Emission As a Perspective Method for Investigation of High- Tc Superconductors

Characterization of Ceramic Materials by Acoustic Emission

M. Roth, E. Dul'kin and E. Mojaev

Department of Applied Physics, The Hebrew University, Jerusalem 91904, Israel.

The acoustic emission (AE) method has been used as a powerful characterization tool for studying the structure and preoperties of technologically important ceramic materials, such as high-Tc superconductors and relaxor ferroelectrics. With regard to the super-conducting YBCO (YBa2Cu3Ox) ceramics and BISCCO (Bi2Sr2CaCu2Ox) composite tapes, we show that by monitoring the acoustic emission bursts it is possible to measure the temperature hysteresis of phase transitions and to reveal their order, to determine the temperature of maximal oxygen absorption as well as to measure the lower critical magnetic field Hc1 and the full penetration field under electrical current transport. The restored strain energy in PbZn1/3Nb2/3O3 (PZN) and 9%PbTiO3-doped PZN (PZN-9%PT) relaxor crystals has been studied by means of AE as well. Two types of AE activity signals have been recorded: (i) related to temperature- or electric field-induced macroscopic phase transitions and (ii) associated with formation/disappearance of intrinsic polar nanoregions. Monitoring of AE under varying [001] electric fields has allowed a unique in situ observation of a low-field (1 kV/cm) irreversible orthorhombic-to-MC phase transition within the morphotropic phase boundary region of PZN-9%PT. The cumulative results demonstrate that acoustic emission method is an indispensable tool for studying the structure and properties of ceramic materials.

1. Introduction

Currently, there is a growing interest in the class of phenomena whereby transient elastic waves (in the ultrasound range) are generated by the rapid release of energy from localized sources within a material. It is known as acoustic emission (AE), which is associated with structural reconstructions within the solid state under the influence of external forces [1]. AE is a nondestructive method for investigating the kinetics of defect production, such as movement and accumulation of dislocations accompanying plastic deformation and their annihilation, twinning and movement of twin walls and of phase boundaries (PB) as well as the generation and propagation of cracks in solid state materials subjected to mechanical stress [2].

Another extensively studied source of AE includes martensitic phase transitions (PT) in metals and alloys under thermal ramping [3-6]. The greatest contribution to the AE accompanying martensitic PT is made by processes associated with the generation (or annihilation) and movement of dislocations originating from crystallographic mismatch between the original and the new phases at the PB. The AE activity, (sec-1), measured by means of a piezoelectric transducer reveals the temperature of the martensitic PT. It can be also a measure of dislocation density changes in course of durable thermal cycling through the PT, called phase work hardening (PWH).

Martensitic-like AE responses are observed in course of phase transitions in ferroelectric and ferroelastic crystals as well. By employing the AE method it has been possible to detect all ferroelectric-ferroelectric-paraelectric PTs in the BaTiO3 and SrTiO3 ceramic materials [7]. AE has been applied to studying the dependence of the dislocation density in PbTiO3 crystals on the PB orientation, in relation to the direction of the thermal field gradient; an AE maximum has been found when the PB is oriented at about 45°angle relatively to the direction of thermal field gradient [8]. Similarly to the martensitic PT in NiTi-based alloys [6], PWH has been detected in PbTiO3 crystals [9] as well as in (Na1-xLix)NbO3 binary solid solution ceramics [10] during prolonged thermal cycling. In (Na1-xLix)NbO3, AE accompanies two high-temperature ferroelectric-ferroelectric and ferroelectric-paraelectric PT, which have not been detected by the traditional dielectric method [11]. A similar uniqueness of the AE method has been demonstrated in the case of Ba0.85Sr0.15TiO3 posistor ceramics, where the PT cannot be detected by resistance measurements [12]. On the other hand, in PbZrO3 and PbHfO3 crystals, a strong AE signal accompanies only the ferroelectric-paraelectric PT due to the corresponding incoherent PBs [13]. A similar AE effect is observed in Pb(Fe0.5Nb0.5)O3 crystals through the ferroelectric-ferroelectric-paraelectric PT [14]. The appearance of PWH is also observed in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 crystal through a diffusive PT [15]. It is well known, that high-Tc superconductors also undergo PTs, including the superconducting transition. Some of the high-Tc superconductors have a perovskite-like crystallographic structure, like the ferroelectric crystals, and they undergo structural PTs similar to the martensitic-like transitions. This explains the extensive efforts that have been made during the last decade to apply the AE method to investigating the high-Tc superconducting phenomena.

The phenomena described above show clearly that AE is an indispensable method of studying many kinetic features of the structural PTs in solid state materials, such as determining the Tc values and characterizing the ΔTchysteresis, identifying the order of PTs, estimate the degree of inter-phase coherence at phase boundaries, defining the degree of hardening based on the PWH data [16], etc. Below, we review the main results recently obtained with the best characterized YBa2Cu3Ox (YBCO) and Bi2Sr2CaCu2Ox (BISCCO) high-Tc superconductors as well as Pb(Zn1/3Nb2/3)O3 with PbTiO3 (PZN-xPT) as a representative of relaxor ferroelectrics.

2. Experimental

The common experimental procedure of AE measurements is simple, and the basic setup is presented schematically in Fig. 1. Due to an external force of mechanical, thermal or electromagnetic nature, the investigated material produces elastic (ultrasonic) waves, which are converted to electrical signals by direct coupling to a piezoelectric sensor. Then the output of the piezoelectric sensor is amplified through a frequency-selective low-noise preamplifier, filtered and additionally amplified through an amplitude discriminating amplifier and converted to voltage pulses through an amplifier-multivibrator, which are counted and displayed in time units. Usually, three parameters of the AE are being measured: (i) total signal amplitude ∑A, (ii) total number of pulses ∑N and (iii) activity ΔN/Δt = (s-1). The latter parameter is most commonly determined.

Fig. 1. Basic setup for AE measurements under mechanical, temperature or electromagnetic loading

It is noteworthy that both in the case of low- and high-temperature experiments it is undesirable to subject the AE sensor to nonambient temperatures. Therefore, a quartz glass waveguide is usually introduced as a buffer transmitting the ultrasonic waves from the studied material to the AE sensor [2]. In the high-temperature setup, the sample is glued with a high-temperature epoxy resin to the polished end of the fused quartz acoustic waveguide. A piezoelectric PZT-19 ceramic sensor is glued to the opposite end of the waveguide and connected to a 500 kHz band-pass preamplifier. The sample comprising the top part of the waveguide is mounted in a resistance furnace. A Ch/Al thermocouple is attached to the waveguide near the sample. Two pinned rods connected to an external differential dilatometer are monitoring the sample size. The AE activity and thermal expansion ΔL can be simultaneously measured during heating and cooling at a rate of about 1-2 K/min with or without an oxygen flow. In the low-temperature setup,the configuration of the experiment is the same, but liquid nitrogen vapor is used for cooling the sample and the temperature is monitored by a Cu-K thermocouple attached to the waveguide near the sample.An induction coil is added to measure the magnetic susceptibility χ at the frequency of 1 MHz. The AE activity , susceptibility χ and thermal expansion ΔL are then simultaneously measured during heating and cooling at a rate of about 1-2 K/min. In another low-temperature setup, the sample is glued to the bottom end of the acoustic waveguide, while the piezoelectric sensor is adhered to its top end. A similar 500 kHz band-pass preamplifier is used. The sample with the lower part of the waveguide is submerged into liquid nitrogen. DC electric current is applied through two silver epoxy contacts on opposite side ends of the sample. The liquid nitrogen Dewar flask is mounted between the two poles of a DC magnet. AE activity is then measured at 77K in the presence of an electric current flow or magnetic field.

3. Sintering and oxygenation of high-Tc superconductors

Practical application of oxide superconductors requires bulk materials with greatly improved current-carrying capacities. Since it is well known that the critical current density of sintered YBCO ceramics appears to be limited by intergrain resistance, it is essential to control the grain growth. During the process of ceramics sintering, spontaneous grains growth is observed experimentally. In YBCO, the grains appear in the temperature range of about 800-900ºC and they continue to grow in size with a rate proportional to the temperature gradient in the material volume [17]. The anisotropic expansion and contraction of the grains during thermal processing produce an AE signal, which carries information about the size of the grains formed. The AE activity of YBCO displays three characteristic stages [18]. AE is initiated above 810°C, and a relatively sharp activity peak is observed in the 820-840ºC temperature range. This is followed by a narrow second stage, where essentially no AE can be detected. However, above 850ºC, the AE activity reemerges and increases nearly linearly with further temperature ramping. Just above this temperature, regular grain growth commences. These results may be interpreted within the framework of a qualitative model [19] suggesting that sintering proceeds in three stages. During the first stage, the pellet of pressed powder shrinks, and the material density increases. The shrinkage is accompanied by considerable mechanical stresses in the powder generating AE. During the second stage, a fluid glass phase, a "lubricant", appears at a higher temperature facilitating the material’s further shrinkage. Such "lubrication" decreases the friction in the system, which is confirmed by absence of AE. During the third stage, grain growth starts due to recrystallization. Unstrained crystallites take up material and grow into the neighboring strained (heavily plastically deformed) areas of the same phase, being gradually increased in size. This results in an increase in the area of the stressed intergranular boundaries, which involves climb or cross-slip of dislocations as they rearrange into the moving boundary. For larger grains, or larger intergranular area, the relief of plastic deformation strain is accompanied by an increased AE associated with the recrystallization process.

Sintering is only the first important step of the YBCO preparation. The second necessary step is oxygenation, which is crucial for obtaining the material in its superconducting state. Oxygen content determines the actual superconducting PT temperature, or so-called critical temperature (Tc). On cooling, after the sintering, of the initially tetragonal YB2Cu3OX phase in oxygen atmosphere the supercondicting orthorhombic II-phase (O-II) appears at about 650ºC. In this process, inflowing oxygen ions engage in occupying some of the vacant sites to form O-Cu-O chains. When the oxygen stoichiometric coefficient x ~ 6.5, most of the alternating O-Cu-O chains are filled with oxygen. The Tc of O-II is close to 60 K. The lattice strain associated with the incorporation of extra oxygen ions and the consequent T→O-II phase transition is accommodated by crystallographic twinning in the (110) plain. The 60K O-II phase nucleates and grows gradually in the tetragonal matrix. On further oxygenation, oxygen ions fill the vacant sites in the O-Cu-O chains completely as x reaches the value of 7. The lattice parameter a becomes half of that in the O-II phase implying that the O-II phase transforms to a new phase, O-I, with the Tc increasing to about 90K. Since the O-II→O-I phase transition is similar to the martensitic-like PT in FE materials, AE is expected to be a suitable method of studying the oxygenation processes in the YB2Cu3OX material. Our studies have revealed that the AE has actually two peaks of on cooling after sintering: at around 650ºC and 605ºC [20]. The first peak corresponds to the T→O-II PT, while the second is obviously attributed to the maximum adsorption of oxygen associated with the O-II→O-I phase transition.

AE exhibits interesting features also during sintering of superconducting BISCCO tapes [21]. Tapes composed of the highest-Tc Bi-2223 phase ceramics enclosed in silver cladding have been studied most extensively. The specific feature of such tapes is that cracks arise between primary and secondary sintering, due to intermediate rolling. Therefore, the secondary sintering of Bi-2223/Ag tapes after rolling up to a strain of  ~18% has been studied by both the AE and magnetic susceptibility methods for comparison. During the second (post-rolling) thermaltreatment of the tape, a broad band of AE from isdetected in the temperature range from 570 to 660°C. This broad band can be interpreted on the basis of other data obtained during in situ studies of crack generation and healing in Bi-2223/Ag tapes [22]. According to the BISCCO phase diagram, a liquid phase exists in the 400-660°C temperature range. Magnetic susceptibility (χ") measurements have been carried out during heating through this temperature interval. The results show a clear narrowing of the χ" peak, which is characteristic of enhanced electrical connectivity between the ceramic grains due to healing of the cracks [23]. Consequently, theprocess of liquid-phase healing of the rolling-induced cracks in thetapes can be regarded as the source of AE.

On cooling the Bi-2223/Ag tape after secondary sintering at 730°C, an AE signal is detected below 230°C as well [24]. The AE activity is weak near 230°C,but increases in intensity with decreasing temperature down to room temperature. The appearance of AE signals has been explained in terms of the relative deformation of the silver clad and the ceramic core of the Bi-2223/Ag composite tape. The thermal expansion coefficient (α) of silver and Bi-2223 ceramics are αAg = 20.5·10-6 K-1 and αc = 13.6·10-6 K-1 respectively, which implies stronger contraction of silver on cooling. In view of the high plasticity of silver (yield stress 65 MPa) and sufficiently large strength of the sintered ceramic (yield stress 150 MPa), the silver envelope cannot contract the underlying ceramics effectively and experiences considerable tensile strain. The associated plastic deformation causes the formation and movement of dislocations and is accompanied by AE. The observed strong AE activity ( values reach several hundreds of s-1 at RT), is about an order of magnitude larger than that of martensitic phase transitions in metals.

4. Superconducting phase transition in YBCO

We address now the results of AE measurements employed for studying the superconducting PT in YBa2Cu3OX. Over a decade ago, one of us has conducted a series of combined measurements comprising AE, thermal expansion (ΔL) and magnetic susceptibility (χ) using a low-temperature set up described above [25]. The results of these measurements are shown in Fig. 2. The dilatometric curve has two inflection points, at 83 and 92K, exhibiting no discontinuity. The first inflection is accompanied by AE, the second - by a 'jump' of χ. The relative change in χ is considerably large, about 50%, and the superconducting PT width is about 1K. The magnitude of in the AE spectrum is too small to represent a 1st-order PT. These results imply, therefore, that a 2nd-order structural PT takes place around 82 K, below the Tc, in agreement with the results of ultrasonic wave velocity measurements [26]. This structural PT can be understood in terms of the interaction between ultrasonic phonons and conduction electrons, which produces attenuation in the normal state and decay below Tc as the electrons become paired [27]. The symmetry of the subsystem associated with paired electrons may coincide or not with the symmetry of the crystallographic lattice [28]. Therefore, the superconducting transition is able to induce certain lattice instability and, as a consequence, lead to a structural PT below Tc [28]. A fit of the BCS theory to experimental results of elastic energy dissipation due to electron-phonon interaction gives an average value of Tc 86.5K, while the electrical resistivity measurements performed on the same YBCO ceramic sample yield Tc = 94K. This provides an additional explanation to our AE results indicating that a 2nd-order PT occurs at 82K, below the Tc, in the YBa2Cu3OX material.

Fig. 2. Simultaneous measurements of AE, thermal expansion and magnetic susceptibility in the superconducting PT range of YBa2Cu3OX ceramics

5. Flux penetration and mixed state in YBCO

In his fundamentalc paper, Abrikosov has demonstrated [29] that the magnetic field (MF) flux lines (FL) start penetrating the type-II superconductor when the applied MF reaches the lower critical value, Hc1. Above this field, each FL is accompanied by a vortex of persistent current (supercurrent) surrounding a normal resistance core. These FLs thread the material’s volume along the applied MF direction. Upon entering the material, the FLs are trapped by the pinning centers (defects, including grain boundaries, twins, etc.), initially at the surface and then in the volume of the sample. Under the transport current (I) condition, the repulsive Lorentz force (IμH) acts on the FLs in such a way as to drive them deeper into the material if they can overcome the opposing pinning forces, and the mixed state begins to form [30]. On the other hand, trapping at pinning centers forces the FLs to transfer their kinetic energy to the defects, and the latter begin to vibrate inducing the acoustic waves in the material as a whole. Excitation of AE may be expected therefore as the MF penetrates the material. Indeed, AE is observed in Nb-Ti wire under electrical current transport [31].

Recently, we have applied the AE method to studying the FL penetration process in high-Tc superconductors, such as YBa2Cu3OX. The low-temperature experimental setup has been used. The self-induced MF enhances as the DC current I flowing through the sample wire increases (external MF is absent). Fig. 3 shows that two AE activity peaks appear at 0.5 and 2.7A values of the DC transport current. The first peak is less intense, and it is assumed to mark the onset of MF penetration into the sample [32]. We have shown [33] that the magnetic field H(r) induced by the transport current just outside the wire, or the sample surface (r  r0), is given by

(1)

This result implies that the MF tends to infinity, yet the flux can penetrate the sample as deep as the coherence length, ξ = 1.2·10-9m [34]. Substituting the current value of 0.5A determined by AE (Fig. 3) into eq. 1 and taking (r–r0) ξ, we obtain H 100 Oe. This value of the magnetic field is in a good agreement with Hc1 = 90 Oe deduced from the internal friction measurements [31]. Thus, AE is a very convenient and useful tool for determining the Hc1 values in superconductors.