BACKGROUND Superconductors are materials that can conduct electricity with little or no electrical resistance. Superconducting materials are generally characterized by a critical temperature, Te, below which the materials are in a superconducting phase in which they can conduct an electric current without electrical resistance. Above the critical temperature, superconductors are in a normal conducting phase in which they can conduct an electric current with an electrical resistance and concomitant electrical energy loss.

The superconducting phase transition in crystalline materials results from an attractive interaction potential between pairs of electrons, known as Cooper pairs, within the crystal lattice. While electrons in a crystal lattice ordinarily experience a net repulsion for one another, it is possible for them to experience an overall net attraction for one another as a result of each electron's attractive interaction with the positive ions located at the lattice points in the crystal lattice.

Hydrogen and its isotopes can be dissolved in many metals, and can occupy the interstitial sites in the host metal crystal lattice. When hydrogen and its isotopes are dissolved in palladium (Pd), the resulting chemical compound is known as a palladium hydride, and is represented by the chemical formula PdYHx, where YH is hydrogen (H), deuterium (2H), or tritium (3H) and x is the stoichiometric ratio of YH to Pd. As discussed below, in a palladium crystal lattice there are a maximum of 3 interstitial sites per palladium atom that can be occupied in a unit cell of the palladium crystal lattice.

Consequently, palladium hydrides PdYHx can be formed having stoichiometric ratios x < 3.

Palladium naturally occurs in metallic form, and has a crystal lattice that can be classified as a face centered cubic (fcc) Bravais lattice as shown in Fig. 1. A unit cell of an fcc lattice contains 4 atoms, thus a unit cell of a palladium crystal contains 4 Pd atoms.

When hydrogen ions dissolve in palladium's fcc lattice, they can occupy one of two types of interstitial sites in a unit cell of the lattice, respectively referred to as octahedral (O) and tetrahedral (T) sites. At low temperatures, dissolved hydrogen ions tend to

preferentially occupy the tetrahedral interstitial sites in the palladium lattice, while at higher temperatures dissolved hydrogen ions tend to preferentially occupy the octahedral sites.

The octahedral sites, marked by an (O) in Fig. 1, are located at the center of the fcc unit cell, and along any edge of the unit cell at a point that is equidistant between two lattice vertices. Thus, there are four octahedral interstitial sites per unit cell, or one octahedral site per palladium atom in a palladium crystal lattice. The octahedral sites are so named, because each site has six nearest neighbor palladium atoms that can form the vertices of an octahedron surrounding the site.

The tetrahedral sites, marked by a (T) in Fig. 1, are located in each of the eight corners of the fcc lattice unit cell. One such corner is shown in an exploded view in Fig.

1. Thus, there are eight tetrahedral interstitial sites per unit cell, or two tetrahedral sites per palladium atom in a palladium crystal lattice. The tetrahedral sites are so named, because each site has four nearest neighbor palladium atoms that can form the vertices of a tetrahedron surrounding the site.

Palladium hydrides, PdYHX, have been known to undergo superconducting phase transitions and to exhibit the phenomenon of superconductivity since 1972. In particular, PdlHl is known to have a superconducting critical temperature of 9K ; while Pd2Hl is known to have a superconducting critical temperature of 11 K ; and Pd3Ho. s is known to have a superconducting critical temperature of 4K. While the palladium hydride system PdYHx has thus been known to possess superconducting properties at or near stoichiometric ratios approaching x-1, no one has thus far been able to explore whether the system exhibits superconducting properties at large stoichiometric ratios, characterized by x > l.

One reason for this, is that a pure palladium hydride system is unstable for stoichiometric ratios x 1. While it is possible to dissolve hydrogen ions into a palladium crystal lattice at such high concentrations, the hydrogen ions readily diffuse out of the lattice over a short period of time on the order of a few minutes. Thus, to produce a palladium hydride system with stoichiometric ratios x >-1, it is necessary to develop a method to stabilize PdYHx compounds for x 1.

SUMMARY The invention discloses a palladium hydride superconducting material, PdyHx where YHX is lHx, 2HX, or 3Hx and x is the stoichiometric loading ratio of YHx to Pd in the

material. The superconducting material has critical temperatures Tc > l lK for stoichiometric loading ratios x > : 1. The critical temperature linearly depends upon a power of the stoichiometric loading ratio. Samples have been produced having critical temperatures of 51. 6K, 80K, 82K, 90K, 100K, and 272.5K.

The surface of the superconducting material is coated with a stabilizing material to stabilize the stoichiometric loading ratio of isotopic hydrogen to palladium in the material. The stabilizing material can be a metal, metal oxide, ceramic, or polymer that can be bonded to, amalgamated to, or alloyed with the palladium hydride superconductor to prevent the isotopic hydrogen loaded into the palladium lattice from degassing or deloading. Examples of stabilizing materials include mercury, aluminum, nickel, copper, gold, silver, platinum, zinc, steel, cadmium, lead, PTFE, nickel oxide, or copper oxide.

The stabilizing material allows PdYHx samples to be produced having stoichiometric loading ratios x 1 that are stable over periods exceeding 24 hours, over temperature cycles from 4K to 400K, and down to pressures of 1 mbar.

In one embodiment, the palladium hydride superconducting material is made by electrochemically loading isotopic hydrogen into a palladium lattice in an electrochemical cell. In this embodiment, a power supply sends an electric current through an electrochemical cell filled with an acidic solution. The electromotive force created by the power supply causes hydrogen isotopes in the acidic solution to be reduced at a palladium cathode, and to subsequently dissolve into the cathode. The stoichiometric ratio of isotopic hydrogen loaded into the palladium lattice can be monitored in real time by monitoring the resistance of the sample relative to its resistance prior to hydrogen loading. When a desired stoichiometric loading ratio is reached, a stabilizing compound can be bonded to, amalgamated to, or alloyed with the palladium cathode, and the power supply turned off.

The anode in the electrochemical cell can in principle be made from any metal, however certain advantages are achieved when the anode is made from a non-corrosive metal such as platinum, nickel, tungsten, gold, copper, or titanium. The anode and palladium cathode in the electrochemical cell can be of arbitrary shapes and sizes, and placed in the cell in an arbitrary geometry.

The acidic solution used in the electrochemical cell can be any electrolytic solution that can serve as a source of isotopic hydrogen ions. In one embodiment it is made by dissolving an electrolyte in 10-5 M light or heavy water (1H20, 2H20, 3H20).

However, solutions which are more or less acidic can be made by dissolving an electrolyte in light or heavy water having higher or lower molarities.

The electrolyte dissolved in the electrolytic solution can be any salt that can be made from a cationic species and an anionic species. The cationic species can be chosen from among the elements lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, tin, scandium, cerium, and lead. The anionic species can be chosen from among the sulfates, phosphates, chlorates, carbonates, borates, bromates, chromates, oxalates, nitrates, perchlorates, iodates, permanganates, acetates, sulfites, nitrites, and chlorites. In one embodiment, the electrolyte dissolved in the acidic solution is a SrS04 salt.

The stabilizing compound can be any compound that can bond to the surface of the palladium cathode to prevent the isotopic hydrogen loaded into the cathode from degassing or deloading. The stabilizing compound can be the salt of any metal which can be bonded to, amalgamated to, or alloyed with the palladium cathode such as a salt of mercury, aluminum, nickel, copper, gold, silver, platinum, zinc, steel, cadmium, or lead.

In addition, the stabilizing compound can be a solution of any insulator that can be bonded to the surface of the palladium cathode such as a polymer, a ceramic, or a metal oxide. For example, the insulator can be PTFE, nickel oxide, or copper oxide. In one embodiment, the stabilizing compound is a 10-5 M mercurous sulfate solution. The mercury in the solution is electrochemically amalgamated to the surface of the palladium hydroxide.

The power supply can be a direct current or alternating current power supply, and can supply current to the electrochemical cell at a current density or root-mean-square current density between 1 and 1000 mA per cm~2. The power supply can also be a pulsed current power supply and supply current pulses to the electrochemical cell at repetition rates of up to 25 kHz. The current pulses can have rise times on the order of 50-500 nsec, widths on the order of 0.5 to 5. 0 msec, and peak currents of up to 6000 A.

In another embodiment of the invention, uniform thin film palladium hydride superconductors can be made by allowing gas phase hydrogen isotopes to diffuse into a thin film palladium sample. In this embodiment, a thin film sample is placed inside a pressure chamber that is pressurized with a source of gas phase hydrogen isotopes to pressures up to 100 bar. Over time, the hydrogen isotopes dissolve into the thin film

palladium sample. The stoichiometric loading ratio of the thin film sample can be measured in real time by measuring the relative resistance of the sample. When the relative resistance indicates a desired stoichiometric loading ratio, a stabilizing material can be deposited onto the surface of the sample using well known thin film deposition techniques such as chemical vapor deposition or molecular beam epitaxy.

In still another embodiment of the invention, uniform thin film palladium hydride superconductors can be made by injecting gas phase hydrogen isotopes onto a thin film palladium sample. In this embodiment a source of isotopic hydrogen in the gas phase is delivered to a vacuum chamber. The hydrogen isotopes are ionized by an ionizing source such as an x-ray source. The resulting hydrogen isotope cations are then accelerated toward a thin film palladium cathode where they are absorbed. The stoichiometric loading ratio of the thin film sample can be measured in real time by measuring the relative resistance of the sample. When the relative resistance indicates a desired stoichiometric loading ratio, a stabilizing material can be deposited onto the surface of the sample using well known thin film deposition techniques such as chemical vapor deposition or molecular beam epitaxy.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS FIG. 1 is a schematic illustration of a unit cell in a palladium crystal lattice.

FIG. 2 is a schematic illustration of an apparatus that can be used to load a palladium lattice with hydrogen or its isotopes at stoichiometric ratios exceeding x = 1.

FIG. 3 is a prior art plot of the measured relative resistance of PdYHx for lH and 2H and stoichiometric ratios x < l.

FIG. 4 is a plot of the relative resistance of PdYHX for lH and 2H and stoichiometric ratios x 2, based on a phenomenological model of resistance in the PdYHx system.

FIG. 5 is a plot of the relative resistance of PdlHx as a function of time for a 1H to Pd stoichiometric ratio of x = 0.98.

FIG. 6 is a plot of the resistance of a PdlHx sample as a function of temperature, showing a superconducting phase transition at a temperature of 51. 6K.

FIG. 7 is a schematic illustration of an YHX loaded palladium wire having a plurality of domains, each of which has a different stoichiometric loading ratio x.

FIG. 8 is a plot of the temperature dependence of a critical magnetic field that destroys superconductivity in a sample via the Meisner effect.

FIG. 9 is a log-log plot of the critical temperature versus stoichiometric loading ratio of a palladium sample loaded with YH at a stoichiometric loading ratio x.

FIGS. l OA and l OB are plots of the resistance of a PdlHx sample as a function of temperature, showing superconducting phase transitions at temperatures of around 80K that are persistent over a period of at least 24 hours.

FIGS. I lA, 11B, and 11C are plots of the resistance of a PdlHx sample as a function of temperature, showing superconducting phase transitions at temperatures of 90K, 1 OOK, and 272.5K, respectively.

FIG. 12 is a schematic illustration of a pressure chamber apparatus that can be used to load a thin film palladium lattice with gas phase hydrogen isotopes at stoichiometric ratios exceeding x = 1.

FIG. 13 is a schematic illustration of a vacuum chamber apparatus that can be used to load a thin film palladium lattice with gas phase hydrogen isotopes at stoichiometric ratios exceeding x = 1.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION As previously mentioned, palladium hydride systems PdYHX for average stoichiometric ratios x 1 are known in the art. However, the palladium hydride system becomes intrinsically unstable for stoichiometric ratios approaching an average stoichiometric ratio x-1. As a result, little is known about the properties of such systems for average stoichiometric ratios x > l.

Fig. 2 is a schematic illustration of an apparatus (200) capable of producing a stable palladium hydride system at average stoichiometric ratios x h 1. Apparatus 200 consists of a power supply (205), an electrochemical cell (201) having an anode (202), a palladium cathode (203), and an acidic solution (204), and means (208,209) for adding an electrolyte to acidic solution (204). Power supply (205) serves as a source of electric current, and can either be a direct-current (DC) power supply or an alternating-current (AC) power supply. In one embodiment, power supply (205) is a DC power supply and supplies an electrochemical current density of between 1 and l 000 mA per cm-2 between

anode (202) and palladium cathode (203). In another embodiment, power supply (205) is an AC power supply and supplies a root-mean-squared electrochemical current density of between 1 and 1000 mA per cm~2 between anode (202) and palladium cathode (203).

Acidic solution (204) serves as a source of YH+ ions in electrochemical cell (201), and conducts the current generated by power supply (205) through the cell. In one embodiment, acidic solution (204) is made by dissolving an electrolyte in 10-5 M light water or 1H20. The 10-5 M 1H2O acts as a source of 1H+ ions and yields an acidic solution having a pH-5. 0. Other acidic solutions (204) having higher or lower pH's can be used in electrochemical cell (201), and can be produced by dissolving an electrolyte into light water having respectively lower or higher molarities such as 10'or 10 M water. Heavy water, 2H2O or 3H20, can be used instead of 1H20 to act as a source of 2H or 3H ions, respectively.

In one embodiment, the electrolyte dissolved in acidic solution (204) is a SrS04 salt. Other electrolytes can be used however, and still be within the scope of the invention. For example, and by way of example only, the electrolyte can be any salt that can be made from a cationic species and an anionic species, where the cationic species is chosen from among the elements lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, tin, scandium, cerium, and lead ; and where the anionic species is chosen from among the sulfates, phosphates, chlorates, carbonates, borates, bromates, chromates, oxalates, nitrates, perchlorates, iodates, permanganates, acetates, sulfites, nitrites, and chlorites.

The anode (202) in electrochemical cell (201) can in principle be made from any metal, however certain advantages are achieved when anode (202) is made from a non- corrosive metal. Thus, in one embodiment, anode (202) is made from platinum wire, however, in alternative embodiments anode (202) can be made from alternative non- corrosive metals such as nickel, tungsten, gold, copper, or titanium. Anode (202) and palladium cathode (203) can be of arbitrary shapes and sizes, and placed in electrochemical cell in an arbitrary geometry. However, in one embodiment, anode (202) is made from a 30 cm long, 1 mm diameter platinum wire, while palladium cathode (203) is made from a 30 cm long, 50 urn diameter palladium wire, and anode (202) and cathode (203) are oriented parallel to one another and separated from one another by 7 cm in electrochemical cell (201). Of course, other geometries for the position, size, and

configuration of anode (202) and palladium cathode (203) are possible and within the scope of the invention.

When power supply (205) is turned on a positive electromotive force develops in acidic solution (204) between anode (202) and palladium cathode (203). As a result of this force, an electric current flows through electrochemical cell (201) and YH+ions are attracted to palladium cathode (203). At cathode (203) the YH+ ions are reduced by the free electrons in palladium cathode (203) to form neutral YH atoms. The YH atoms then dissolve in palladium cathode (203) where they occupy either octahedral or tetrahedral interstitial sites in the palladium lattice. This process of depositing YH atoms into a palladium lattice, whether performed by electrochemical or other means, is referred to as hydrogen loading.

To measure the stoichiometric ratio of YH atoms dissolved into palladium cathode (203) during the hydrogen loading process, a relative resistance measurement across palladium cathode (203) can be made. The relative resistance measurement is a ratio of the resistance of palladium cathode (203) after a given amount of YH has been loaded into it to the resistance of palladium cathode (203) before any YH has been loaded into it. The relative resistance of a PdYHx system is known to be a function of the average stoichiometric ratio x of YH to Pd in the system, and is shown in Fig. 3 for stoichiometric ratios x 1. The relative resistance of the palladium cathode (203) can thus be used to . determine the average stoichiometric ratio of YH to palladium in palladium cathode (203) while it is being loaded with YH in electrochemical cell (201).

It should be noted that the relative resistance measurement technique can only measure the average stoichiometric loading ratio x in palladium cathode (203) because the dissolution of YH in palladium cathode (203) does not uniformly occur throughout the cathode. In particular, since dissolution of YH in palladium cathode (203) primarily occurs along the outer surface of the cathode, the distribution of YH throughout palladium cathode (203) is expected to have a radial dependence and to be much larger along the outer layer of cathode (203) than along its core. However, neither the homogeneity nor the gradient of the loaded YH atoms inside palladium cathode (203) can be known from the average measured stoichiometric loading ratio x.

The resistance across palladium cathode (203) can be measured in any number of ways known in the art. In one embodiment, shown in Fig. 2, the resistance is measured by generating an AC signal with an AC signal generator (206), passing the generated signal through palladium cathode (203), and measuring the voltage drop in the signal

across the cathode. Prior to measuring the voltage drop across cathode (203), the DC bias added to the generated AC signal by the current produced in electrochemical cell (201) is filtered out through high-pass filters (207). Using this technique, the relative resistance of palladium cathode (203) can be determined from measurements of its resistance before power supply (205) is turned on and at some subsequent time during the loading process after an amount of YH has been loaded onto palladium cathode (203) to achieve a stoichiometric loading ratio x.

Referring again to Fig. 3, the relative resistances of YH loaded Pd lattice systems at room temperature are known for systems having average stoichiometric loading ratios x 1. However, no relative resistances are known for PdYHx systems having average stoichiometric loading ratios of x s1 because of the inherent instability of such systems which rapidly and spontaneously undergo YH deloading. Consequently, to measure the stoichiometric loading ratio for Pd YHx systems having an average stoichiometric loading ratio x ru using a relative resistance measurement technique, a model for the relative resistance as a function of stoichiometric loading ratio must be developed so that the relative resistance measurements for xi 1 can be extrapolated to the region 1 : x 3.

As shown in Fig. 4, a phenomenological model has been developed. The model predicts the relative resistance of Pd YHX systems at room temperature will exponentially decrease with increasing stoichiometric loading ratio for average stoichiometric loading ratios x >-1. Moreover, the model predicts the relative resistance will decrease to essentially 0 resistance at an average stoichiometric loading ratio of x ; 1. 6, such that the Pd YHX system will be in a room temperature superconducting state at such loading ratios.

The details of the phenomenological model are as follows. The palladium hydride system Pd YHX is modeled as having two electron transport mechanisms available in parallel, one superconducting and the other normal or non-superconducting. Each electron transport mechanism is modeled to provide a unique conductive pathway for electrons in the Pd YHX system, such that each unique conductive pathway has a corresponding resistance that uniquely varies with the stoichiometric loading ratio x of the system. Because the two conductive pathways are modeled to be present in parallel, their corresponding resistances are added in parallel to obtain the overall resistance of the PdYHx system.

The normal or non-superconductive electron transport mechanism is modeled to provide a conductive pathway having a relative resistance that linearly depends upon the stoichiometric loading ratio. This linear dependence is believed to result from the linear

increase in the number of scattering centers in the system with the increase in the stoichiometric ratio. Thus, the relative resistance of the normal electron transport mechanism is modeled as: (Eq. 1) RIRO =1+x where R is the resistance at stoichiometric loading ratio x, Ro is the resistance of a pure palladium system at stoichiometric loading ratio 0, X is a linear constant of proportionality, and x is the stoichiometric loading ratio.

The superconductive electron transport mechanism is modeled to provide a conductive pathway having a resistance that exponentially depends upon the stoichiometric loading ratio x. This exponential dependence is believed to result from fluctuations of the Pd YHX system from the superconducting state. Thus, the resistance of the superconducting transport mechanism is modeled as: (Eq. 2) R/R0 = #e-γ(x-xc)/kBT <BR> <BR> <BR> <BR> where R and Ro are as defined in Eq. 1, ß is a constant of proportionality, y (x- xc) is the condensation energy of a superconducting state as a function of the stoichiometric loading ratio x and a critical stoichiometric ratio xc at which the Pd YHx system condenses into the superconducting state, kB is Boltzmann's constant, and T is the absolute temperature in degrees Kelvin.

Adding the relative resistances from Eq. 1 and Eq. 2 in parallel, one obtains a phenomenological expression for the relative resistance of the PdyHx at room temperature as a function of the stoichiometric loading ratio of YH to Pd in the system. The expression can be written as: <BR> <BR> <BR> <BR> (1+#x)#e-γ(x-xc)/kBT<BR> <BR> <BR> <BR> <BR> <BR> <BR> (Eq. 3) R/R0 =<BR> 1+#x+#e-γ(x-xc)/kBT where R, Ro, X, ß, y, x, xc, ka and T are as defined in Eq. 1 and Eq. 2. The expression in Eq. 3 was fit to experimental data for the relative resistance R/Ro at stoichiometric loading ratios x < 1 to determine the free parameters #, p, y and xc.

Using the fitted free parameters, the resulting curve of the relative resistance of a Pd YHX system as a function of stoichiometric loading ratio up to a maximum loading ratio of x = 2 is shown in Fig. 4 for a temperature T of 300K, and for IH (401) and 2H (402).

As can be seen from Fig. 4, at 300K the Pd YHx system is predicted to be a superconductor <BR> <BR> <BR> at stoichiometric loading ratios greater than a critical stoichiometric loading ratio xc 1.6.

At different temperatures, the phenomenological relative resistance curves plotted in Fig.

4 are compressed so that the critical stoichiometric loading ratio decreases with decreasing temperature. For example, Fig. 4 also shows the relative resistance of a Pd YHx system as a function of stoichiometric loading ratio for lH (410) and 2H (420) at a temperature of 66K. In these curves, the system is seen to be a superconductor at critical stoichiometric loading ratio xc > 1. 2 Referring back to Fig. 2, YH loading of palladium cathode (203) by electrochemical cell (201) can be performed until the relative resistance measurement of the cathode (203), as measured by the AC signal produced by AC signal generator (206), indicates from the plot of Fig. 4 that the palladium cathode (203) has been loaded with sufficient YH to yield an average YH to Pd stoichiometric ratio of x > l. Thus the YH loading of palladium cathode (203) can be performed until the relative resistance of the cathode rises to its maximum of-1. 8, then descends to negative relative resistance values indicating an average stoichiometric loading ratio of x > 1 as shown in Fig. 4.

When the relative AC resistance measurements of palladium cathode (203) indicates that it has been loaded with YH at an average stoichiometric ratio exceeding x 1, a stabilizing compound (210) is added to the electrochemical cell (201) using addition means (208) and (209). The addition means (208) and (209) can be any means for adding stabilizing compound (210) to electrochemical cell (201). For example, the addition means can be a sample holder (208) and a valve (209) that is opened to allow stabilizing compound (210) to gravitationally fall into the electrochemical cell (201).

Stabilizing compound (210) can be any compound that can bond to the surface of the YH loaded palladium cathode (203) to thereby prevent the YH loaded into the cathode from degassing or deloading. In one embodiment, stabilizing compound (210) is a 10-5 M mercurous sulfate (Hg2SO4) solution. Other stabilizing compounds (210) can be used however, provided they can prevent the loaded YH from degassing. For example, stabilizing compound (210) can be the salt of any metal which can be bonded to, amalgamated to, or alloyed with palladium cathode (203). Thus, for example, stabilizing compound (210) can be any salt of mercury, aluminum, nickel, copper, gold, silver, platinum, zinc, steel, cadmium, or lead. In addition, stabilizing compound (210) can be a solution of any insulator that can be bonded to the surface of palladium cathode (203) such as a polymer, a ceramic, or a metal oxide. For example, the insulator can be PTFE, nickel oxide, or copper oxide. Other insulating materials can be used as well, provided they can prevent the loaded YH from degassing out of palladium cathode (203). Once

stabilizing compound (210) has been bonded or electroplated onto the surface of palladium cathode (203) the DC power supply (205) is turned off.

Stabilizing compound (210) prevents spontaneous degassing or deloading of YH from the Pd YHx at stoichiometric ratios exceeding x : 1. The stability of YH loaded Pd YHX samples produced by apparatus (200) for stoichiometric loading ratios approaching x-1 can be seen in Fig. 5. As shown in Fig. 5, the relative resistance of a palladium cathode (203) loaded with YH is seen to dramatically increase in a region (501) to a maximum of 1.8 while the current source (205) is supplying an electromotive force to the electrochemical cell (201). The relative resistance then suddenly decrease in a region (502) when a mercurous sulfate stabilizing compound (210) is added to the cell. The Hg from the mercurous sulfate is allowed to amalgamate the palladium cathode (203), thereby preventing spontaneous deloading of YH until a region (503) of stable relative resistance less thanl. 2 is reached, at which time the current source (205) is turned off.

As shown in Fig. 4, the stable relative resistance of 1.2 at region (503) can result from either a YH to Pd stoichiometric ratio of 0.2 or 0.98. The stoichiometric ratio is known to be 0.98 because of the regions of dramatic rise (504) and subsequent decay (505) of the relative resistance that occur when the palladium cathode (203) finally deloads. The region of dramatic rise (504) corresponds to the stoichiometric ratio region between 0.7 x < 0.98 during which the relative resistance increases with decreasing stoichiometric ratio. The region of subsequent decay (505) corresponds to the stoichiometric ratio region between 0.0 x < 0.7 during which the relative resistance decreases with decreasing stoichiometric ratio and returns to unity at a stoichiometric ratio of x = 0. Referring again to Fig. 5, the region (503) of stable relative resistance 1.2 of the YH loaded palladium cathode (203) at a stoichiometric ratio of x = 0.98 is seen to last for a period of approximately 12000 seconds or 3. 3 hours before deloading occurs.

Once the samples (203) of Pd YHX for stoichiometric ratios x : : 1 were prepared in apparatus (200) as described, they were laid on a copper sample holder to ensure temperature uniformity, quenched to liquid helium or liquid nitrogen temperatures, and then slowly allowed to warm to room temperature. While the samples (203) were slowly warming, four point AC electrical resistance measurements of the samples were taken at 8 second intervals both in the presence and absence of constant magnetic fields. The four point AC electrical resistance measurement technique is well known in the art, and is used to measure the electrical resistance of a sample without measuring the resistance of the electrical contacts on the sample or of the lead wires from the sample to the resistance

measuring equipment Once the samples warmed to room temperature, they were re- quenched to liquid helium or liquid nitrogen temperatures, and there electrical resistances were measured a second time as they warmed to room temperature. Thus, the characteristics of the electrical resistances of the samples were tested over cyclical temperature variations ranging from 4K to 300K, and were found to be independent of such temperature cycles. Finally, the electrical resistance measurements were repeated on the samples after a period of 24 hours had elapsed, and some of the samples were found to be stable for more than 24 hours.

One sample (203) of Pd YHx prepared in apparatus (200) was observed to have the electrical resistance shown in Fig. 6. Two plots of electrical resistance versus temperature are shown in Fig. 6, namely plot 601 which was made in the absence of an external magnetic field, and plot 602 which was made in the presence of a constant 1 Tesla magnetic field. These two plots show two interesting properties that suggest that the YH loaded palladium sample (203) is a superconductor with a critical temperature of 51.6K.

The first interesting property seen in Fig. 6, is that the Pd YHx sample (203) has an electrical resistance in plot 601 that linearly depends on temperature above 56K, exponentially depends on temperature in a region between 40K and 53K, and again linearly depends on temperature below 40K. This pattern of temperature dependence is indicative of the temperature dependence of a multi-domain superconductor having at least one domain with a critical temperature of 51.6K. Here, and throughout this application, the critical temperature is taken to be the temperature at which the electrical resistance exponentially drops to 90% of its initial value in the region where it has an exponential dependence on temperature.

The electrical resistance of the Pd YHx sample (203) in plot 601 does not drop identically to 0 Q below critical temperature Tc, because sample (203) is a multi-domain sample in which different domains have different stoichiometric loading ratios corresponding to different critical temperatures. As previously mentioned, sample (203) is known to have an average stoichiometric loading ratio of x l from the relative resistance measurements made during its preparation, however, the distribution of YH within sample (203) is known to be non-homogeneous. As a result, sample (203) is expected to have different domains having different stoichiometric loading ratios x.

Referring again to Fig. 4, for a given critical temperature Tc, a sample domain will have a superconducting phase transition at that temperature if it has a stoichiometric

loading ratio x that is greater than some critical stoichiometric loading ratio xc. Thus, sample (203) may have some domains having a stoichiometric loading ratio x >-Xethat will already be in a superconducting state at Tc, other domains having a stoichiometric loading ratio x = xc that will have a superconducting phase transition at Tc, and still other domains having a stoichiometric loading ratio x # xc that will not exhibit a superconducting phase transition at Tc. Thus, the resistance of sample (203) immediately above Tc will have contributions from the resistivities of the domains in the sample that have stoichiometric loading ratios x : x,, while the resistance of the sample below Te will only have contributions from the resistivities of those domains having stoichiometric loading ratios of x < xc. As long as the sample has one or more domains with stoichiometric loading ratios of x-< xc, its resistance below To will not go identically to zero.

This can perhaps be better explained in reference to Fig. 7, which shows a Pd YHx sample (203) having a plurality of regions or domains (701-706) having different stoichiometric loading ratios x. Each stoichiometric ratio x corresponds to a critical stoichiometric ratio for a corresponding critical temperature Tc. The domains (701-706) can vary arbitrarily in length, size, and geometry due to the non-homogeneity of the YH loading process. A given domain within the sample, such as domain (706), will have a given stoichiometric loading ratio x6. That stoichiometric loading ratio is the critical stoichiometric loading ratio xc6 for the onset of superconductivity at some critical temperature Te6. If the remaining domains (701-705) have stoichiometric loading ratios x ; < x6 where i = 1,5, then all domains (701-706) will contribute to the resistance of sample (203) at temperatures above Te6, while only domains (701-705) will contribute to the resistance of sample (203) at temperatures below TC6. Thus, even after the superconducting phase transition of domain (706) at temperature Te6, sample (203) will exhibit a finite resistance since domains (701-705) remain in a normal conducting state.

The length of domain (706) having a superconducting phase transition at critical temperature Tc6 can be determined from the jump in electrical resistance in the sample near the critical temperature. At temperatures above Tc6, all domains (701-706) will contribute to the resistance of sample (203), which can be written as: (Eq. 4) Rhigh = ##L/A; (T#Tc6) where Rhigh is the resistance, # is the resistivity, L is the contributing length, and A is the cross-sectional area of the sample at temperatures T > Tc6. At temperatures below Tc6,

only domains (701-705) contribute to the resistance of sample (203) since domain (706) is in a superconducting state. Consequently, the resistance of sample (203) can be written as : (Eq. 5) Rhigh-#R=##L-l/A; (T#Tc6) where Rhigh, #, L, and A are defined as in Eq. 4, #R is the drop in sample resistance around critical temperature Tc6, and I is the length of the domain (706) that has undergone a superconducting phase transition at critical temperature Te6. Subtracting Eq. 5 from Eq.

4, an expression can be developed for the length I of superconducting domain (706), namely : (Eq. 6) l-OR. A. p Using this relationship, the length of the domain of the PdyHx sample (203) undergoing a superconducting phase transition at a critical temperature of 51.6K as shown in Fig. 6 is estimated to be around 1mm.

Referring again to Fig. 6, the second interesting property seen in the figure is that the phase transition in the electrical resistance seen in plot 601 at 51.6K disappears when a IT external magnetic field is applied as seen in plot 602. This suggests that the superconducting phase in the domain having a superconducting phase transition at 51. 6K has been destroyed by the applied magnetic field according to the well known Meisner effect. In the Meisner effect, the critical temperature of a superconducting phase transition is a function of an externally applied magnetic field, H. As shown in Fig. 8, the critical temperature of a superconducting phase is a maximum in the absence of an external magnetic field, and decreases with increasing magnetic field up to a critical field He (0) at which the critical temperature reaches OK, and the superconducting phase is completely destroyed.

Thus, the sample domain in plot 601 of Fig. 6 having a critical temperature Tc3 (0) = 51. 6K could be associated with a superconducting phase having a critical field Hc3 (0) # 1T as shown in Fig. 8. Consequently, the superconducting phase in that domain is destroyed when the 1T magnetic field is applied to the sample as shown in plot 602.

The domain having a critical temperature Tcl (l) = 31. 3K in plot 602 of Fig. 6, could be associated with a superconducting phase having a critical temperature Tel (0) >-55K, and a critical field Hc3 (0) S 1T as shown in Fig. 8. Consequently, the superconducting phase of the domain is not destroyed when the IT magnetic field is applied to the sample, although

the critical temperature is lowered to a value TCI (1) = 31.3K as shown in plot 602.

Similarly, the domain having a critical temperature Tc2 (l) = 18. 8K in plot 602, could also be associated with a superconducting phase having a critical temperature TC2 (0) >-55K, and a critical field Hc2 (0) >-IT, so that the superconducting phase of the domain is not destroyed when the IT magnetic field is applied to the sample, although the critical temperature is lowered to a value Tc2 (1) = 18. 8K as shown in plot 602.

In the discussion of Figs. 6 and 7, the phase transition in the electrical resistance of sample (203) at 51.6K can be explained as a superconducting phase transition of a multi-domain sample having at least one domain with a critical temperature of 51.6K. As previously mentioned, while sample (203) is known to have an average stoichiometric loading ratio x, g based on the relative resistance measurements made in apparatus (200), the actual stoichiometric loading ratios xi of the various domains within the sample are not known. Thus, the actual stoichiometric loading ratio x of the domain in sample (203) that is undergoing a superconducting phase transition at 51.6K is not known.

To determine this loading ratio, prior art data showing the dependence of the critical temperature on the stoichiometric loading ratio for loading ratios x < 1 have been extrapolated using the phenomenological resistance model shown in Fig. 4. In particular, the phenomenological resistance model shown in Fig. 4 predicts that a YH loaded palladium sample will have a superconducting phase transition at a temperature of 300K if the sample is loaded with a stoichiometric ratio x that exceeds a critical stoichiometric loading ratio Xe = 1.6. As shown in Fig. 9, this data point has been plotted as a point (901) on a log-log plot of critical temperature Tc versus critical stoichiometric loading ratio xc. In addition to data point (901), data are plotted in Fig. 9 from prior art data points (910) of critical temperature versus stoichiometric loading ratio. As can be seen from Fig. 9, the plotted data points can be well fit with a linear relationship. This relationship can then be used to determine the stoichiometric loading ratio of the domain in sample (203) having the superconducting phase transition at 51.6K. From Fig. 9, the domain is seen to have a stoichiometric loading ratio of x ~ 1. 29.

In Fig. 6, the electrical resistance of a PdYHx sample made in apparatus (200) and having at least one superconducting domain with a critical temperature of 51.6K was disclosed. Other samples were made in apparatus (200) having superconductor domains with even higher critical temperatures. For example, Fig. 1 OA shows the electrical resistance curve (1001) of a sample that had a phase transition at 80K that is indicative of a multi-domain sample having a superconducting domain with a critical temperature of

80K. Referring to Fig. 9, this superconducting domain is seen to have a stoichiometric loading ratio x-1. 33. The electrical resistance curve of the sample of Fig. I OA was measured again 24 hours later, and resulted in curve (1010). The phase transition of a superconducting domain with a critical temperature of 80K is apparent in curve (1010), and indicates that the superconducting domain persisted in the sample for a period of at least 24 hours. The absolute resistance of curve (1010) is somewhat higher than the absolute resistance of curve (1001) because some of the hydrogen isotopes have been able to escape from the sample, thereby increasing its absolute resistance as shown in Fig. 4.

As shown in Fig. 10B, another sample had an electrical resistance curve (1002) with a phase transition at 82K that is indicative of a multi-domain sample having a superconducting domain with a critical temperature of 82K. The electrical resistance curve of this sample was measured again 24 hours later, and resulted in curve (1020) which shows the persistence of the superconducting domain with the critical temperature of 82K for a period of at least 24 hours. The absolute resistance of curve (1020) is somewhat higher than the absolute resistance of curve (1002) because some of the hydrogen isotopes have been able to escape from the sample, thereby increasing its absolute resistance as shown in Fig. 4.

Still other samples were produced with superconducting domains at even higher critical temperatures. Fig. 1 lA shows the electrical resistance measurements of a sample having a phase transition at 90K that is indicative of a multi-domain sample having a superconducting domain with a critical temperature of 90K. Fig. 11B shows the electrical resistance measurements of a sample having a phase transition at 100K that is indicative of a multi-domain sample having a superconducting domain with a critical temperature of 100K. And finally, Fig. l lC shows the electrical resistance measurements of a sample having a phase transition at 272.5 K that is indicative of a multi-domain sample having a superconducting domain with a critical temperature of 272. 5K. For each of these samples, the length of the superconducting domain can be determined from the size of the resistance drop near the critical temperature using Eq. 6, and the stoichiometric loading ratio can be determined from the log-log plot of critical temperature versus stoichiometric ratio shown in Fig. 9.

While the invention has been described in terms of a method and apparatus for making a PdYHX sample having a stoichiometric loading ratio x > 1 using an electrochemical loading process in which hydrogen isotopes are loaded into a palladium lattice using a DC or AC current, other methods and apparatus for making the PdYHx

sample are possible and within the scope of the invention. For example, the isotopic hydrogen loading can be done using a, us pulsed electrolysis technique in which a series of electrical pulses are supplied to electrolysis cell (201). The pulses can have rise times on the order of 50-500 nsec, widths on the order of 0.001 to 5. 0 msec, peak currents of up to 6000 A, and repetition rates of up to 25 kHz.

In addition, other methods and apparatus can be used to load gas phase hydrogen isotopes into thin film palladium samples. These methods and apparatus allow homogeneous PdYHX samples having large stoichiometric loading ratios x > 1 to be produced. Because the samples are homogeneous, the number of domains per sample can be small, and can even be as small as a single PdYHx domain. Thin film samples having a single domain with a high stoichiometric loading ratio x will have a single high-Tc critical temperature. Thus, these samples will exhibit no appreciable resistance below the critical temperature, and will be able to conduct electricity below the critical temperature without electrical losses.

One method and apparatus for uniformly loading gas phase hydrogen isotopes onto a thin film palladium sample (203) is shown in Fig. 12. As shown in the figure, an ion gun is used to inject isotopic hydrogen into a thin film palladium sample. A source (1201) of isotopic hydrogen in the gas phase is delivered to a vacuum chamber (1202).

The hydrogen isotopes are ionized by a high energy x-ray or other ionizing source (1203).

The resulting hydrogen isotope cations are then accelerated toward a thin film palladium cathode (1204) where they are absorbed. The stoichiometric loading ratio of the thin film sample can be measured in real time by measuring the relative resistance of the sample.

When the relative resistance indicates a desired stoichiometric loading ratio, a sealant (1205) capable of bonding to the loaded thin film palladium sample is deposited onto the surface of the sample using well known thin film deposition techniques. For example, the sealant can be deposited onto the surface of the sample using chemical vapor deposition and molecular beam epitaxy deposition techniques.

An alternative method and apparatus for uniformly loading gas phase hydrogen isotopes onto a thin film palladium sample is shown in Fig. 13. A thin film palladium sample (1301) is placed inside a pressure chamber (1302). The chamber is pressurized with a source of hydrogen isotopes in the gaseous phase to pressures up to 100 bar. The hydrogen isotopes dissolve into thin film palladium sample (1301). The stoichiometric loading ratio of the thin film sample can be measured in real time by measuring the relative resistance of the sample. When the relative resistance indicates a desired

stoichiometric loading ratio, a sealant (1303) capable of bonding to the loaded thin film palladium sample is deposited onto the surface of the sample using well known thin film deposition techniques. For example, the sealant can be deposited onto the surface of the sample using chemical vapor deposition and molecular beam epitaxy deposition techniques.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, these and other embodiments of the invention are within the scope of the following claims.