TITLE: COMPOSITION AND METHOD FOR SYNTHESIS OF
HIGH-TEMPERATURE HgBa-Ca-Cu-O (HBCCO) SUPERCONDUCTORS IN BULK AND THIN FILM
Specification
U.S. Government Rights This invention was partially supported by grants from the U.S. Government. The Government has certain rights in this invention.
Background of the Invention 1. Field of the Invention.
This invention relates to a composition and method for synthesizing high temperature superconductors in bulk and thin film. 2. Discussion of the Related Art.
Superconductivity above 130 K has recently been observed in the multiphased Hg-Ba-Ca-Cu-0 (HBCCO) compound system. This multiphased HBCCO was found to consist of the homologous series HgBa 2 Ca n.1 Cu n 0 2n+2+s [Hg-12(n-l)n] with n = 1, 2, 3, ... and superstructures of well-defined stacking-sequences of members of Hg-12(n-l)n. Similar to the TlBa 2 Ca n.1 Cu n 0 2n+3+4 [Tl-12(n-l)n] structure, Hg-12(n-l)n has a general layered structure with repeated stacking of the rock-salt type layers of
(BaO)(HgOδ)(BaO) and the perovskite-type layers of (Cu0 2 )Ca(Cu0 2 ).... Unlike Tl-12(n-l)n, which contains the trivalent Tl and therefore the rock- salt type layers of (BaO)(T10)(BaO), Hg-12(n-l)n has the divalent Hg and thus the oxygen-deficient rock-salt type layers of (BaO)(HgOδ)(BaO). Soon after the reports of superconductivity above 130 K in the multiphase Hg-BaCa-Cu-0 (HBCCO) samples, efforts were made to identify the compound phase or phases responsible for superconductivity
at such high temperatures. Very recently, HgBa 2 CaCu 2 O β+8 (Hg-1212) and HgBa 2 Ca 2 Cu 3 0 8+g (Hg-1223) were successfully synthesized and characterized. Hg-1212, which has two Cu0 2 -layers per unit cell, displays a tetragonal structure with lattice parameters a = 3.862(1) A and c = 12.707(5) A, and a magnetically-determined sharp superconducting transition at ~ 112 K before oxygenation and up to ~ 120 K after oxygenation.
On the other hand, Hg-1223, which possesses three Cu0 2 -layers per unit cell, exhibits an orthorhombic structure with lattice parameters a = 5.451(2) A, b = 5.432(2) A, and c = 15.826(7) A, and a magnetically- determined sharp superconducting transition at ~ 120 before oxygenation and ~ 135 after oxygenation. The resistivity of Hg-1223 becomes zero at 134 K which is the highest reproducible resistively determined transition- temperature (T c ) ever observed. The irreversibility line H j (T j ) of Hg-1212 was carefully examined in the art and was found to follow the power law
Hj « (1 - TJT ' with α ~ 5/2 and T ∞ being the onset superconducting transition-temperature in the absence of a magnetic field.
A similar power law was also observed in the art to apply to YBaijCugO., (Y-123) and the double Bi0 2 . and TI0 2 .layer high-temperature superconductors (HTS's) Bi 2 Sr 2 Ca n . 1 Cu n 0 2n+4 and Tl 2 Ba 2 Ca n . 1 Cu π 0 2n+4 where v = 2 or 3, but with n ~ 3/2 and ~ 11/2, respectively. This suggests that the critical current density (J c ) for Hg-1212 should lie between those of Y-123 and the Bi/Tl-based HTS's at the same reduced temperature, due to the stronger coupling between Cu0 2 -blocks. Given the higher T c to comparison with that of Y-123, Hg-1212 may be a good candidate for large total current (I) or large J c applications provided that the compound can be made into practical forms and the compound stability is improved. One such practical form is important for electronic applications is thin film. The standard solid-state reaction technique has been employed to
prepare the superconducting phases of the HBCCO system. Oxides and/or carbonates of the cations have been used successfully as the starting ingredients. Because of the low decomposition temperature (~ 500° C) of HgO with respect to the high reaction-temperature (~ 800° C), and the high vapor-pressure and corrosive nature of Hg at the reaction temperature pose a serious challenge to the preparation of Hg-1212, Hg-1223 or other homologous members. The absence of Ca in (Hg-1201) makes the preparation of Hg- 1201 slightly easier. Other problems include the easy formation of the insulating layered compound of CaHg0 2 at ~ 500° C, which is low compared with the reaction temperature for the HBCCO compounds, and the easy reaction of the precursor with moisture and C0 2 in the air.
The unusual linear or dumb-bell coordination of Hg in HBCCO as demonstrated in HgBa 2 RCu 2 0 65+4 where R = rare earth and Hg-1201 show that there should exist a large number of vacant oxygen-sites in the Hg- layer. An even higher T c may be found in the HBCCO system by fine- tuning the doping levels, especially the anion doping. To carry out effectively the study, one needs samples of pure or nearly-pure phases of Hg-12(n-l)n. Unfortunately, the complexity of HBCCO compound chemistry and the toxicity of the Hg-containing ingredient make such a task extremely difficult.
Summary of the Invention The present invention involves a simple and safe route for the synthesis of high quality samples of Hg-12(n-l)n. For this invention, it has been found that: (1) the formation of Hg-12(n-l)n at ambient pressure undergoes a Hg- vapor/solid reaction process rather than a HgO-solid/solid diffusion that is similar to the solid-state reaction used for other-cuprate oxides; (2) there is competition between the formations of CaHg0 2 and Hg-12(n-l)n; (3) the formation of Hg-12(n-l)n depends critically on the
precursor used and the synthesis conditions such as the Hg-vapor pressure, the reaction temperature, the reaction time, and the reaction atmosphere; and (4) the Hg-12(n-l)n is only marginally stable at temperatures close to although lower than the formation temperature. It only recently became possible to synthesize samples with up to
95% Hg-1212 and 90% Hg-1223 by volume, using the proper precursor and controlling the synthesis conditions, such as the reaction atmosphere, the Hg-vapor pressure, and the reaction temperature, it was this modified solid-state reaction technique that was used to prepare the Hg-1212 thin films in the present invention.
Some parameters that affect the formation of Hgl2(n-l)n were examined. Based on this information, a controlled vapor/solid reaction CVSR technique has been developed and used to synthesize samples with up to 95% of Hg-1212 and 90% of Hg-1223 by volume with sharp superconducting transitions at ~ 120 K for Hg-1212 and ~ 135 K for
Hg-1223, after annealing in flowing oxygen at 300° C.
The present invention is also for a process for synthesizing Hg-1212 films by reacting a precursor film in a controlled Hg-atmosphere. The films so-prepared exhibit a preferential orientation of the c-axis perpendicular to the (100) SrTi03-substrate and a lattice parameter c =
12.50 A, which is slightly smaller than that of the bulk Hg-1212 material. They all display a superconducting transition with the mid-point at ~ 110 to 120 K, similar to bulk Hg-1212.
By preparing the precursor carefully, controlling the reaction temperature and time, and adjusting the Hg-vapor pressure, the present invention of a controlled vapor/solid reaction process to synthesize samples with up to 95% Hg-1212 and 90% Hg-1223 by volume in a simple and safe enclosed environment was developed. Samples so obtained display sharp superconducting transitions determined both magnetically and resistively. The magnetically-observed T c 's of Hg-1212 and Hg-1223
are, respectively, ~ 111 K and 112 K as-synthesized but increase respectively to ~ 120 and ~ 135 K after oxygenation. The technique has also been successfully used to process highly-oriented Hg-1212 films. The process of the present invention may be equally usable for Hg-1201. By reacting rf-sputtered thin-films of the precursor Ba-jCaCtLjO,
(which is not a compound) on SrTiOg.substrates in a controlled Hg- atmosphere, Hg-1212 thin-films have been successfully prepared by use of the process of the present invention. These films show a rather sharp superconducting transition determined magnetically at a temperature up to ~ 120 K, which is similar to that of bulk Hg-1212. They grow epitaxially with the c-axis preferentially oriented perpendicular to the surface of the substrate.
Brief Description of the Drawings Fig. 1 graphically depicts an x-ray diffraction pattern of sample PH- 60 with Hg-1212 as the major phase.
Fig. 2 graphically depicts χ vs. T Ph-60: O as-synthesized and V post-oxygenated.
Fig. 3 graphically depicts P vs. T for Ph-60: O as-synthesized and V post-oxygenated. Fig. 4 graphically depicts a diffraction pattern of sample PI-45 with
Hg-1223 as the major phase: • impurity peaks.
Fig. 5 graphically depicts χ vs. T for PI-45: • as-synthesized and V post-oxygenated.
Fig. 6 graphically depicts P vs. T for PI-45: ° as-synthesized and • post-oxygenated.
Figs. 7a and 7b are the scanning electron micrograms of thin films of: (a). Ba 2 Ca 112 Ba 2 . 1 0 x and (b) Hg-1212.
Figs. 8a and 8b are the x-ray diffraction patterns of thin-films of: (a) Ba^.^Ba^O,., and (b) Hg-1212.
Fig. 9 graphically depicts the χ(T) of several Hg-1212 thin films. Fig. 10 graphically depicts the M-H loops of Hg-1212 thin films at 10 and 77 K.
Detailed Description of the Preferred Embodiment I. BULK SEMICONDUCTOR MATERIAL SYNTHESIS.
The controlled vapor/solid reaction (CVSR) process of the present invention for the synthesis of Hg-12(n-l)n basically consists of four main steps: (1) preparation of the precursor; (2) control of the Hg-vapor release and its pressure; (3) formation of Hg-12(n-l)n with n = 2 and 3; and (4) oxygenation of Hg-12(n-l)n.
1. Preparation of the Precursor (BaoCa n l Cu n O ιr ). The precursor Ba^a^C^O, is usually a mixture of binary and tertiary oxides and therefore not a compound, except for n = 1, which is difficult to synthesize as a single phase. Therefore, it can readily absorb moisture, react with C0 2 in air and form carbonates and/or hydroxides of the cations. Reacting such a contaminated precursor with HgO in a closed environment, such as a sealed quartz ampoule, may result in a rupture of the sealed quartz ampoule (perhaps due to the released H 2 0 and/or C0 2 ) or in the absence of Hg-12(n-l)n (perhaps due to the difficult formation of the compounds from the stable carbonates and/or hydroxides). In fact, both types of failure to produce Hg-12(n-l)n were observed in controlled experiments with aged precursors, while fresh precursors invariably venerate Hgl2(n-l)n under the synthesis conditions to be described later. Therefore, preparation (especially the final step) of the precursor in an Ar- or N 2 -atmosphere in a dry-box or a simple glove-bag was performed to keep exposure of the precursor to air to a minimum. The prepared precursor pellets were either used immediate or immediately stored in a
desiccator for future use.
Precursors with three different sets of starting materials with the appropriate cation-proportions of Ba:Ca:Cu = 2:(n-l):n have been used. They are: (A) BaC0 3 (Alfa, 99.997%), CaO (Alfa, 99.95%), and CuO (Cerac, 99.999%); (B) Ba0 2 (Atomergic Chemicals Corp., 99%), CaO (Alfa, 99.95%) and CuO (Cerac, 99.999%); and (C) Ba(N0 3 ) 2 (Alfa, 99.95%), Ca(N0 3 ) 2 *4H 2 0 (Alfa, 99.995%), and Cu(N0 3 ) 2 »3H 2 0 (Alfa, 99.9%). Hg- 12(n-l)n has been made successfully with each of the three different sets of starting materials. However, the success rate increases with starting materials in the order of (A), (B) and (C). The difference may be related to the possible incomplete decomposition of the carbonate in (A) and the possible inhomogeneous mixing in (A) and (B).
When the (A)-set of starting materials was used, BaC0 3 , CaO, and CuO were mixed in an agate mortar, compressed into pellets of 1.3cmφx0.5cm thick, and then heated in an oxygen atmosphere for 24 to
48 hours at 900° C to decompose BaC0 3 to BaO and C0 2 and to rid them of possible H 2 0 and/or C0 2 contaminants. Sometimes, the steps were repeated two or three times to promote homogeneity. Compression was used to reduce the surface-to-volume ratio of the precursor to help reduce its degradation due to the inevitable exposure to air between some synthesis steps.
When the (B)-set of starting materials of Ba0 2 , CaO and CuO was used, preparation steps similar to those for the (A)-set of materials were adopted. The (C)-set of starting materials gave the highest success rate, perhaps due to better mixing at the molecular level of these ingredients.
Appropriate amounts of Ba(N0 3 ) 2 , Ca(N0 3 ) 2 »4H 2 0 and Cu(N0 3 ) 2 *3H 2 0 were all dissolved in deionized water inside a beaker. The solution was heated on a hot plate and stirred constantly with a magnetic stirrer. The solution was slowly dried into a blue powder in the beaker with the release of brown fumes of N0 2 .
The blue powder, which melts at - 540° C and boils at ~ 629° C, was then scraped out of the beaker, ground, then placed in an alumina crucible and slowly dried by heating it up to 500 to 550° C and maintaining this temperature for one-half hour. It was then further heated up to 600 to 620° C and kept at this temperature for approximately 1 hour until all nitrate gas was driven off. The resulting black mixture was then ground, compacted, and sintered in flowing oxygen at 900° C for 24 to 48 hours to prepare the precursor pellets.
2. Control of the Hg-Vapor Pressure. One of the crucial questions in the synthesis of Hg-12(n-l)n is whether the compounds form through a solid (HgO)/solid(precursor) diffusion or a vapor(Hg)/solid(precursor) reaction, although only solid-HgO and precursor materials are used. A very simple test was conducted by sealing a pre-reacted Hg-1212 pellet and a Ba a CaCα precursor pellet together, heating the composite slowly to 800° C and keeping it at this temperature for approximately 5 hours before slowly cooling to room temperature. It was found that a large volume-fraction of the Ba-jCaCujO,. pellet was transformed into Hg-1212. The observation suggests that HgO in the pre-reacted Hg-1212 (and also Hg-1223) must have decomposed into Hg and 0 2 and then combined with other ingredients to form compounds through vapor/solid reaction and not solid/solid diffusion prior to reaching the reaction temperature of ~ 800° C. This certainly is not a surprise, since HgO decomposes at 500° C.
Another test was performed where HgO and Ba j CaCu a O,, without mixing, were sealed and reacted. A large amount of CaHg02 was found with a trace of Hg-1212 at best. This demonstrates that the rapid release of Hg from HgO at ~ 500° C will result in the formation of CaHg0 2 before the reaction to form Hg-1212 enabled at ~ 800° C. Indeed, this is consistent with the formation temperature of CaHg0 2 at 500 to 550° C. Therefore, after the above tests, if the release of Hg-vapor and the Hg-
vapor pressure in the sealed quartz ampoule can be controlled where the reaction of the precursor with HgO takes place, Hg-12(n-l)n can be synthesized.
Based on this principle, several schemes have been contemplated by using a composite Hg-source which releases Hg slowly. Two such schemes are: (a) Fixed volume/varying reactant (FV/VR) method; and (b) Fixed volume/varying precursor-to-reactant ratio (FV/VP-R) method, where reactant represents the pre-reacted Hg-12(n-l)n. Both are further discussed below. The reactant pellet was made by compressing the thoroughly mixed powder of appropriate amounts of HgO and pulverized precursor. In both above-mentioned methods, the Hg-supply was provided by the composite source of a compacted HgO/precursor pellet which could be just a reactant pellet. It should be noted that a precursor with a composition of Ba/Ca/Cu different from 2/(n-l)/n can also be used in the composite Hg-source pellet.
However, using the specific composition of 2/(n-l)/n for making the composite Hg-source often turns the composite source itself into the desired Hg-12(n-l)n.
Summary of the Two Schemes (a) Fixed Volume/Varving Reactant (FV/VR) Method.
Since tests show that the HgO in the reactant will first decompose at 500° C into Hg-vapor and 0 2 , one may seal reactant pellets of varying masses in evacuated quartz tubes of fixed volume or reactant pellets of fixed mass in evacuated quartz tubes of varying volume. For the simplicity of experiment, the former was adopted, fixed volume/varying reactant method.
(b) Fixed Volume/Varying Precursor-to-Reactant Ratio
(FV/VP-R) Method.
Instead of varying the Hg-vapor pressure by using different amounts of the reactant described above, the Hg-pressure was controlled by sealing a precursor pellet with the reactant pellet inside the quartz tube and by varying the precursor-to-reactant mass ratio. The rationale for doing this is the hope to reduce the chance of formation of CaHg0 2 due to the close solid (HgO)/solid (precursor) contact in the reactant pellet. Indeed, using this method, the CaHg0 2 volume-fraction was reduced in comparison with the FV/VR method. 3. Formation of Hg-1212 and Hg-1223.
Pellets of reactants (Scheme (a)) and reactants/precursors (Scheme (b)) sealed in evacuated quartz tubes of fixed volumes, which were, in turn, encapsulated in stainless steel tubes for precautionary measures, were reacted inside a tubular furnace by slowly heating them at a rate of ~ 160°C/hour to 800 to 860° C and kept at this temperature for 5 hours before slowly cooling to room temperature. Results showed that Hg-1212 becomes unstable above ~ 820° C but difficult to form below ~ 750° C, and Hg-1223 forms more easily above 840° C.
4. Oxygenation. Oxygenation was carried out in flowing oxygen at temperatures from 100 to 310° C for 1 to 90 hours with a pronounced effect on T c but not on the structure. Hg-12(n-l)n degrades above 330° C.
5. Results.
All samples, both as-synthesized and oxygenated, underwent a quick resistive and magnetic screening before careful characterization by dc magnetization (χ), X-ray- and resistivity (p)-measurements was made. By using the CVSR method described above, samples were synthesized with up to 80 to 90% Hg-1212 and 65 to 75% Hg-1223 with a 100% yield. The phase-fractions of the various phases present were estimated by fitting their non-overlapping X-ray diffraction peaks with the standard patterns.
Later, the neutron-diffraction data taken at NIST on the same Hg-1212 samples suggested that the Hg-12(n-l)n phase-fractions listed in Tables 1-3 may be underestimated by 10 to 20%. The volume fractions of the superconducting Hg-12(n-l)n were also estimated by the magnetization measurements. The latter method usually gives a greater value. The volume of the quartz ampoules used for all results shown below was 1.655 cm 3 . The effect of the synthesis conditions on the formation of Hg-12(n- l)n are summarized below.
TABLE I Estimated phase -fractions in samples made by the FV VR method.
the T c of the as-synthesized simple* in the case of 2-step superconducting transitions, the minority transition is indicated in parentheses; in the case of Hg-94, the two transitions were of comparable strengths the volume-fraction before correction for demagnetization. • indicates that no daα was measured the estimated volume-fraction; • indicates that no data was measured
TABLE 2.
The es t imated phase-fraction in samples by FV/VP-R method IO obtain Hg- 1212.
* the T« of the as-synthesized samples; • indicates that no dau was measured t the volume-fraction before correction for demagnetization t the estimated volume-fraction
TABLE 3
The estimated phase-fractions in sample* prepared by the FV/VP-R method to obtain Hg-
* the T of (be sample after oxygenation t the volume-fraction before correction for demagnetization
8 the estimated weight fraction
I 10 hn reaction time and S hrs tor the others
Table 1 summarizes the volume fractions of various phases in samples prepared by the FV/VR method for Hg-1212. The reaction temperature was between 780 and 810° C for these samples. It is evident that, at the low Hg vapor-pressure associated with the low mass of the reactants, only Hg-1201 was found, together with large amounts of
CaHg0 2 and Ba-Cu-0 (BaCu0 2 and/or Ba jj CiigO,,.,,). As the Hg vapor- pressure increases, Hg-1212 increases to ~ 60% (sample Hg-75) at the expense of other phases. If it is assumed that all Hg in sample Hg-75 is released, the Hg-vapor pressure for sample Hg-75 would be ~ 49.6 bar at 800° C. The lattice parameters of Hg-1212 are a = 3.862(1) A and c =
12.707(5) A. No noticeable change in the phase-fraction of Hg-1212 or in its lattice parameters was detected after oxygenation at 300° C for up to 40 hours. The T c of the as-synthesized Hg-1212 ranges from ~ 110° to 120 K and it shifts up to 120 to 125 K after oxygenation. Table 2 summarizes the estimated volume-fractions of the various phases in representative samples prepared by the FV/VP-R method for Hg- 1212. The reactants and precursors used here were HgB& j CaCu j O,, and Ba- t CaCugO,, respectively. The reaction temperature was ~ 800° C. The mass of the reactant used is 1.75 gm which generated ~ 60% Hg-1212 by the FV/VR method. As a precursor of different masses was introduced to the ampoule, the equilibrium condition of reaction was changed, resulting in a change of the volume fractions of different phases. As the precursor mass increases, the volume fraction of Hg-1212 increased to - 75% and then decreased to ~ 70%, while CaHg0 2 continued to decrease, and Ba-Cu-0 remained about the same except for PH-19 where Ba-Cu-0 increased and HgO appeared. Often, but not always, the reactant and precursor pieces in the same run were found to be very similar in terms of the phase percentages. The X-ray diffraction pattern of PH-60 is shown in Fig. 1 and the magnetic susceptibility in Fig. 2 and resistivity in Fig. 3. Table 3 summarizes the estimated volume-fractions of the various
phases in representative samples prepared by the FV/VP-R method to obtain Hg-1223. The reactants and precursors used are HgBa- j Ca j CugO,. and respectively. By concentrating on the precursor-to- reactant ratio close to ~ 0.4, which produced the best Hg-1212 samples, it was found that samples with large volume fraction of Hg-1223 can be obtained by increasing the reaction temperature (T R ) to ~ 860° C for 5 hours. Apparently, longer reaction led to a deterioration of the sample, see Table 3. The obtained Hg-1223 was found to be orthorhombic with lattice parameters a = 5.451(2) A, b = 5.432(2) A, and c = 15.826(7) A and to have a sharp transition at ~ 112 K as-synthesized and as high as ~ 135
K after oxygenation for sample PI-45 (Figs. 4 and 5). The resistivity becomes zero, or nearly zero, below 134 K (Fig. 6) for the same sample after oxygenation. II. THIN FILM SUPERCONDUCTOR MATERIAL SYNTHESIS. The preparation of Hg-1212 thin films involves four main steps, which will be described below. They are: (1) the preparation of the sputtering target; (2) the sputtering of the thin-film precursor; (3) the formation of Hg-1212 through gas/solid diffusion; and (4) the characterization of the Hg-1212 thin-films. 1. Sputtering Target.
Carbon contamination was found to be detrimental to the formation of Hg-1212. Therefore, precautions were taken to avoid such contamination by using cation oxides or their equivalents instead of carbonates (even though carbonates work occasionally) as the starting ingredients and by carrying out all mixing in a glove box or glove bag in flowing argon. Since the sputtering and re-sputtering rates depend on the cation-species, preliminary results indicate that targets with the nominal composition of Ba/Ca/Cu = 2/1.2/2.1 appear to give the composition closest to the desired Ba/Ca/Cu - 2/1/2 for the thin-film precursor sputtered under conditions to be described in Step 2 below.
Each of the starting oxides of BaO 2 , CaO, and CuO was first heat- treated in an oxygen atmosphere at between 880 and 900° C for 48 hours to rid them of possible water vapor and/or carbonate contaminants. The heat-treated oxides were then pulverized and thoroughly mixed in the proportions cited above. The mixed powder was compressed at a pressure of ~ 2 kbar into disks of dimension 5.1 cmφ x 0.5 cm. The sputtering target was obtained by firing in air the compacted disk in a box furnace at a rate of ~ 50°C/hour from room temperature to between 900 and 920° C and maintained at that temperature for 20 to 24 hours, then slowly cooled at ~ 50° C/hour to room temperature. The slow cooling and heating were crucial to avoid cracking the target. The target was immediately transferred to a desiccator for storage before use.
2. Thin Film Precursor.
The Ba/Ca/Cu thin-film precursor was made by rf-magnetron sputtering on to (100) βrTiO^single crystalline substrates. During the sputtering, the incident power level was kept at 70 W while the reflected power was maintained at less than 2 W. The background pressure of the sputtering chamber was between 10 "7 and lO "6 torr. A 4:1 mixture of Ar and 0 2 . gases was used as the sputtering atmosphere which was maintained at a pressure of 80 mtorr during sputtering at flow rates of 32 and 8 seem, respectively. A thin-film precursor of ~ 1 μm thickness was therefore obtained. These films exhibit a very smooth surface, as shown in Fig. 7a by scanning electron microgram. No X-ray diffraction peaks characteristic of the film precursor are detected, as shown in Fig. 8a. Microprobe analysis showed the composition of the films to be
Ba 2 Ca 112 Cu 2ι O x . The thin-film precursor was stored in a desiccator immediately after preparation and before treating it in a Hg-atmosphere.
3. The Formation of Hg-1212 Films.
The direct reaction of the thin-film precursor with HgO in a sealed quartz tube failed to produce a Hg-1212 film due to the difficulty in
controlling the Hg vapor-pressure. The Hg, released through the decomposition of HgO at ~ 500 °C reacted with the CaO-component readily to form CaHg0 2 before the formation of Hg-1212 at ~ 800 °C. The same technique used to control the Hg-vapor pressure in the synthesis of bulk Hg-1212 and Hg-1223 was used to prepare the Hg-1212 films.
Basically, the release of Hg was slowed by using a pre-reacted pellet of HgBa a CaCu ), (instead of HgO) as the Hg-source and adjusting the Hg- of appropriate amount, e.g. in a ratio of = 3 to 10. The above composite Hg-source of HgBa 2 CaCu 2 0 ][ /Ba 2 CaCu 2 O x was then encapsulated together with the thin-film precursor of Ba s CaCU j O, in an evacuated quartz tube which was in turn sealed inside a stainless steel tube as a safety precaution. The whole assembly was slowly heated at a rate of ~ 160°C/hour to 800° C and maintained at that temperature for 5 hours before being cooled to room temperature. Some of the samples were then annealed in flowing oxygen at 300° C for 3 hours. Unlike the bulk, oxygenation seems not to generate too large an effect on the superconducting transition of the Hg-1212 films. 4. Characterization of Hg-1212. The Hg-1212 films have been characterized by X-ray diffraction, scanning electron microscopy, electron microprobe analysis, and magnetization measurements. After the reaction of the BaCaCu-precursor film in a controlled Hg-atmosphere, a change of the film surface morphology was clearly evident and a column-like structure appeared as shown in Fig. 7b. Three distinct phases in the over-layer were recognized in back-scattered electron images of these thin-film samples. Electron microprobe analysis and back-scattered imaging indicate that Hg-1212 was the most abundant phase present in the film.
A minor Hg-bearing phase with the composition of ~Hg 2 BaCu 3 O x (not necessarily a compound) perched on top of the Hg-1212 phase. A
second minor phase present was an oxide of Ba, Ca and Cu. It should be noted that, due to the irregular surface of the film and the small grain- sizes of the minor phases, chemical analysis was difficult. However, due to the epitaxy nature of the Hg-1212 films, the (000 diffraction peaks were easily observed as shown in Fig. 8b. Ac = 12.50 A was thus obtained.
This is slightly smaller than the value of 12.70 A obtained for bulk Hg-1212. The observation is similar to those on other HTS's, where the c-axes are often smaller in the thin-film form than in the bulk. The dc magnetization M was measured on several of these films.
They all show a superconducting transition with the mid-point T c varying from ~ 110 to 120 K when measured at the zero filed-cooled mode, as depicted in Fig. 9. Using the film sizes of ~ 2.4 mm x -mm x 1 μm, the estimated superconducting volume fraction of the films was ~ 3 to 96%. Unfortunately, the magnetization signal is rather small in the field-cooled mode, due to either the strong flux-pinning or a poor electrical connectivity of the film. The latter is consistent with a preliminary resistivity check that the film did not support a J c flowing through the whole sample. The M-H loop was also determined for the Hg-1212 thin film samples at both 77 and 10 K as shown in Fig. 10. A J c ~ A/cm 2 at 10 K was obtained according to the Bean Model for the films so-prepared if a grain size of 100 μm as suggested by the scanning electron micrograph was used for the Hg-1212 in the films. The foregoing disclosure and description of the invention are illustrative and explanatory of the preferred embodiments, and changes in the size, shape, materials and individual components, circuit elements, connections and construction may be made without departing from the spirit of the invention. What is claimed is:
