The most interesting crystal chemical aspect of the high temperature superconductivity (HTSc) is that for those based on Tl, there exists not only variable numbers of Cu-O layers but also variable numbers of Tl-O layers, which provide the possibility of many compounds, in what is really an infinitely adoptive series through the process of substitution and the interrelated variation of its elements. The whole series can be written as: TLmBa2Can−1CunOm+2n+2 and TLmSr2Can−1CunOm+2n+2  ; the subscript m defines how many rock salt layers are present. The number of copper layers is remarkably variable, with n = 1, 2, 3 and 4 presently claimed to be prepared as bulk material. Even more remarkably, all compounds, synthesized in this series, are claimed to be superconducting with Tc’s greater than 80 K. The desirable phase is the m = 2 and n = 3 phase, Tl2Ba2Ca2Cu3O10+δ (2223) because of its higher Tc  . Discoveries of Tl-based system have not only set new Tc records with zero resistance up to 125 K, but also have provided a new insight into the mechanism of high-Tc oxide superconductivity  .
Two superconducting phases, Tl2Ca2Ba2Cu3O10+δ and Tl2Ca1Ba2Cu2O8+δ were identified in their samples by Hazan et al.  . They found that these phases have a pseudo tetragonal until cell (5.40 × 5.40 × 36.25) Å, corresponding to a, b and c lattice dimensions, respectively. In the same year, Parkin et al.  found that Tc in Tl2Ba2Ca2Cu3OX system prepared by a solid state reaction method varies from 118 K to 125 K depending on preparation conditions. The transmission electron spectroscopy shows that the unit cell is a body-centered tetragonal and contains trilayer Cu perovskite units separated by bilayer Tl-O units with a = 3.82 and c = 12.00 Å. Tc (zero) was found to be 78, 96, 106, 98, 45 and 25 K for Tl0.5Ca1−xPrxSr2Cu2 oxide sample with x = 0, 0.1, 0.2, 0.5 and 0.7, respectively and for x = 1, the sample exhibited a semiconductor behavior.
Kaell et al. (1991)  studied the variation of Tc due to partial Sr substitution, for Ba in Tl2Ba2−xSrxCa1Cu2O8 (Tl-2212), (x = 0 - 0.6), and Tl2Ba2−xSrxCu1O6 (Tl-2201) (x = 0 - 0.6). They showed that, Tc decreases rapidly with increasing Sr content for both Tl-2212 and Tl-2201 systems.
Bulk polycrystalline Tl2Ba2CaCu2Ox system has been fabricated by Ossandon et al. (2001)  . Results showed that the crystalline unit cell is tetragonal with lattice constant a = 3.8550 Å and c = 29.318 Å (cell volume of 2.18 × 10−23 m3) containing two sets of adjacent oxygen copper layers. The samples were irradiated at room temperature in air with 0.8 GeV protons. The superconductive properties of the virgin and irradiated materials were investigated magnetically.
Eu-substituted (Tl0.6Pb0.4) (Sr0.9Ba0.1)2 (Ca1−xEux)Cu2O4, for x = 0.2, 0.4, and 0.6 compounds were prepared by Kuzman et al. (2004)  . They demonstrated that Eu can be substituted for Ca in separating the layer between Cu-O conducting layers, Cu atoms are in the center of the basal plane of pyramids of the conducting layers, in the spacing layer Ba atoms are shown, and oxygen atoms are situated both in the corners of the pyramids and in the insulating layer, surrounded by Tl atoms.
Maysoon et al.  studied the effect of sintering time on superconducting properties of Tl1.6Hg0.4Sr2Ca2Cu3O10+δ and Tl1.8Bi0.2Sr2Ca2Cu3O10+δ compounds. They found that the optimum sintering time which was characteristic for the growth of high-Tc phase equal to 3 h and prolonging the sintering time to 6 h lead to decrease the transition temperature.
In this paper the impact of quenching in air was investigated on the structural and electrical properties of Tl2−xHgxSr2Ca2Cu3O10+δ system type 2223.
The solid state reaction technique has been used to prepare homogeneous and well-defined single phase superconducting samples with uniform oxygen stoichiometry evolution of accurate physical parameters. Solid state reaction is the least expensive and largest batch size of any powder fabrication process, the procedure involves a simple blending of the required precursors, followed by one or more reactions at a high temperature  .
So appropriate amounts of high purity Tl2O3, Hg2O, Sr (NO3)2, CaO and CuO powders have been weighed stoichiometrically. Precursor Sr2Ca2Cu3O10+δ was first prepared using high purity powders of Sr (NO3)2, CaO and CuO as starting materials. Then, Tl2O3 and Hg2O were added to the mixture and grinding them in agate mortar to obtain a very fine powder. The mixtures were pressed into a pellet of (0.2 - 0.3) cm thickness and 1.3 cm diameter, under a pressure of about 3 ton/cm2. The samples were sintered in air atmosphere at temperture 1133 K for 3 hrs in order to make the particles of the materials joined together and gradually reduce the volume of pore spaces between them. The most common method of determining Tc of a superconductor is the resistivity measurement by using the four-point probe method in a cryostat with presence of liquid nitrogen. The cryostat system joined to a rotary pump to get a pressure of (~10−2 mbar) inside the cryostat by using (Edward Prini 12) gage, and joined also to a sensor of a digital thermometer type (RTD), and thermocouple type (T) near the sample position. Four wires have been connected to the cryostat, the outer connection were used for supplying the sample with current from D.C power supply type (6236A triple output), while the inner connections were used to measure the voltage drop by a digital nanovoltmeter type (KETHLEY 180) with sensitivity of about ( nanovolt).
The structure of the prepared samples was obtained by using X-ray diffractometer (XRD) type Philips having the following features (source: Cukα, voltage: 40 kV, current: 20 mA, wavelength: 1.5405 Å). A computer program  was used to calculate the lattice parameters a, b and c. This program is based on Cohen’s least square method  . Scanning electron microscope (SEM) type, Jeol model JSM-5600 has been used to study the nature of grains and to analyze the surface morphology of the samples. The oxygen content in the oxide ceramic samples was determined by the simple iodometric titration technique  .
3. Results and Discussion
Two sets of samples were produced for each superconducting compound, the first set of samples was allowed to cool to room temperature in a tube furnace that has programmable controller with a cooling rate 275 K/min, while the second set of samples was removed from the furnace and quenched in air. Figure 1 and Figure 2) illustrate the variation of resistivity with temperature for Tl2−xHgxSr2Ca2Cu3O10+δ, samples. It is found that the substitution of 0.1, 0.2, 0.3 and 0.4 Hg content to the composition raise the transition temperature Tc and more addition (x = 0.5) decrease the critical temperature, and when x ≥ 0.6 our apparatus could not help us to obtain the value of Tc(off) because it is less than the liquid nitrogen temperature. This result indicates that the Hg has catalytic effect on the reaction to form the high-Tc phase (HTP) within the range x = 0.1 - 0.4. However, certain amount of Hg is necessary for the occurrence of this reaction, while excessive Hg substitution promotes another reaction, which is likely to assist the formation of low-Tc phase instead of the high-Tc phase    .
From these figures and Table 1 the role of quenching enhancement on the critical transition temperature has been seen.
The above results demonstrate that it is interesting to prepare variety of Tl-2223 samples in order to obtain the optimum high-Tc phase. Similar explanation was reported by Hermiz et al.  . Thus, such comprehensive and complementary procedures should be used for investigating TlSrCaCuO system to explore if there will be a light at the end of the channel to arrive to or near room temperature; high temperature is required for the initiation of the formation reaction of superconducting compound; a short duration heating minimizes the decomposition and quenching stabilizes the superconducting phase.
Figure 1. Temperature dependence of normalized resistivity for Tl2−xHgxSr2Ca2Cu3O10+δ samples for different Hg content (samples furnace cooled).
Figure 2. Temperature dependence of normalized resistivity for Tl2−xHgxSr2Ca2Cu3O10+δ system with different Hg content (samples quenched in air).
Table 1. Variation in Tc values, lattice parameters and oxygen content with different cooling condition.
Some representative XRD patterns of the samples furnace cooled and quenched in air are shown in Figures 3-6. It is seen from these figures that the quenching enhances crystallinity and, the main peaks of these samples are attributed to the Tl-2223 phase with intensity higher than the furnace cooled samples peaks. The values of the lattice parameter a and c for these samples are listed in Table 1. This table shows an increase of the c-axis lattice constant for Hg-doped samples as compared with the undoped samples, the reason is due to the substitution of Hg for Tl where the ionic radii of Hg2+ (1.11 Å) is longer than that of Tl3+ (0.95 Å) as mentioned previously which renders c-parameter to be longer or get deformed  .
Most of the physical properties are very sensitive to the oxygen content in high-Tc superconductors. Therefore, it is important to report the oxygen content of the sample on which the physical measurements are made.
The variation of cooling condition reflected on the Tc and δ values, whereas higher Tc and δ values are recorded in the quenched in air samples rather than the furnace cooled samples. This indicates that the increase in the Tc and δ values are attributed to the increase of oxygen absorption during crystallization process of the superconductors  .
Figure 3. X-ray diffraction patterns of Tl2Sr2Ca2Cu3O10+δ samples sintered at 1133 K for 3 hrs in air (samples furnace cooled). H—High Tc phase, L—low Tc phase, —impurity phase (Sr2Ca2Cu7O).
Figure 4. X-ray diffraction patterns of Tl2Sr2Ca2Cu3O10+δ samples sintered at 1133 K for 3 hrs (samples quenched in air). H—High Tc phase, L—Low Tc phase.
The important feature which can be inferred from the above result, the quenched in air samples improvement of links on intergranular inter faces of high-Tc phase (2223) and enhances the transition temperature.
It can clearly be seen from the micrographs which are shown in Figure 7 and Figure 8 that the quenched in air sample has less porous, high growth and bigger grain than that of the furnace cooled sample.
Figure 5. X-ray diffraction patterns of Tl1.6Hg0.4Sr2Ca2Cu3O10+δ samples sintered at 1133 K for 3 hrs (samples furnace cooled). H—High Tc phase, L—Low Tc phase.
Figure 6. X-ray diffraction patterns of Tl1.6Hg0.4Sr2Ca2Cu3O10+δ samples sintered at 1133 K for 3 hrs (samples quenched in air). H—High Tc phase, L—Low Tc phase.
Figure 7. SEM micrographs of the fracture surfaces of Tl1.6Hg0.4Sr2Ca2Cu3O10+δ sample.
Figure 8. SEM micrographs of the fracture surfaces of Tl1.6Hg0.4Sr2Ca2Cu3O10+δ quenched sample.
A partial substitution of Tl by Hg was found to promote the formation and grain growth of Tl-2223 phase and then enhance the Tc value. XRD pattern analyses have shown a tetragonal structure, and there were at least two superconducting phases. The dominant phase showed the mean composition corresponding to Tl-2223. Samples quenched in air showed a higher Tc value, less porous, higher growth and bigger grains than the furnace cooled sample.