Electrochemical loading enhances deuterium fusion rates in a metal target

Materials

Potassium carbonate (K₂CO₃, 99%) was purchased from Thermo Fisher Scientific. K₂CO₃ was kept in a Napco 5831 vacuum oven at 120 °C for at least 24 h to eliminate the residual water. Deuterium oxide (D₂O, 99.9%) was purchased from Cambridge Isotope Laboratories and used as received. Iridium wire (0.5 mm, 99.95%) was obtained from Taobao. Ag/AgCl reference electrodes (CHI111) were purchased from CH Instruments. Potassium chloride saturated in H₂O was purchased from Fisher Chemical and used as received. Potassium hydroxide (KOH, ≥85%, pellets) was purchased from Sigma-Aldrich and used as a ⁴⁰K radioactive gamma-ray-emitting source during the neutron detector calibration process.

Plasma thruster

The plasma thruster is custom made from SS-316 stainless steel and includes three ring magnets and a set of hexapole magnets. A type N feedthrough, located below the plasma thruster, functions as an antenna to transmit microwaves into the plasma thruster (Supplementary Fig. 14). The type N feedthrough transmits the microwaves by means of a coaxial cable and is controlled through a proportional–integral–derivative (PID) loop. The deuterium flow rate is adjusted by a mass flow controller and fed into the gas inlet on the side of the thruster through SS-304 stainless-steel pipes. The detailed components of the plasma thruster, along with their make and model, are as follows: microwave antenna (Kurt J. Lesker, SS, IFTNG012033M, Type N Feedthrough), ring magnet (CMS Magnetics, NR011-42N, N42 neodymium), bar magnet (20 × 10 × 10 mm N35 neodymium), mass flow controller (ASERT, AST10-DLCMX) and microwave generator (Wattsine, WSPS-2450-200M).

Vacuum chamber

The vacuum chamber is a 6-inch 6-way standard ConFlat flange cube (Supplementary Fig. 15) used to provide an environment for the operation of the plasma thruster and PIII into the palladium target. The vacuum chamber physically connects the plasma thruster and the electrochemical cell together. The vacuum system consists of a turbo pump, two dry scroll pumps and a pressure gauge and can reach a minimum pressure of 5 × 10⁻⁸ torr. Furthermore, the vacuum system is equipped with a residual gas analyser to monitor the types of residual gas in the vacuum chamber. The vacuum chamber pressure is maintained between 1 × 10⁻⁵ and 2 × 10⁻⁵ torr during beam loading. The lowest pressure of the vacuum chamber during the electrochemical loading was approximately 4 × 10⁻⁵ torr. The detailed components of the vacuum chamber, along with their make and model, are as follows: dry scroll pump (Agilent, IDP-7), turbo pump (Agilent, TwisTorr 305 FS), pressure gauge (Edwards, WRG-S-DN40CF), turbo controller (Edwards, Turbo and Instrument Controller), residual gas analyser (SRS, RGA100), plasma thruster holder (Ideal Vacuum, SS, SWIFT-SEAL P1011088), manual gate valve (Kurt J. Lesker, SS, GV0400MVCF), pneumatic bellows sealed angle valve (Kurt J. Lesker, SS, SA0150PVCF) and vacuum chamber (Kurt J. Lesker, SS, CU6-0600, 6-inch CF UHV Cube).

Electrochemical cell

The housing of the electrolysis chamber was custom made from Macor (Machinable Glass Ceramic) and machined in-house using lathes and mills. A Viton O-ring holds the palladium target in place, physically separating the interface between the electrolyte and the vacuum. A brass rod, travelling the length of the electrochemical cell through a channel, connects the palladium target to the high-voltage power supply needed to drive fusion reactions (Supplementary Fig. 16). The detailed components of the electrolysis chamber, along with their make and model, are as follows: electrochemical cell holder (Taobao, SS, KF50 to 50 mm Compression port), spring probe (QA Technology, 100-PLN1609L), viton O-ring (McMaster-Carr, 1284N116), DC–DC converter (Walfront, Buck Boost ZK-4KX), lead-acid battery (Zeus, PC5-12F1-5), remote control switch (eMylo, R121A) and high-voltage power supply (JIAMAN, H2105N-30-17).

Because the target is held at a high negative voltage (−30 kV), the electrochemical reaction is driven by a floating galvanostat. The galvanostat consists of a DC–DC converter and is powered by a 12 V lead-acid battery. The galvanostat is set to drive the electrochemical reactions at 200 mA between the cathode and the anode for the electrochemical reaction (Supplementary Fig. 17).

As well as connecting the galvanostat, the brass rod is also connected to a negative high-voltage power supply to drive the fusion reaction. The current value recorded by the high-voltage power supply is returned to the PID system as feedback data for automatic control of the PIII current.

Fusion reactions in the Thunderbird Reactor

The annealed and cleaned palladium target was installed into the electrochemical cell, which was then installed into the vacuum chamber. Vacuum was applied to the vacuum chamber until a pressure of less than 5 × 10⁻⁶ torr was reached. Once a high vacuum was reached, deuterium gas was supplied to the plasma thruster at a flow rate of 0.5 sccm by a mass flow controller. The reactor control software was started after the mass flow controller read a steady flow rate of 0.5 sccm. The microwave generator was turned on with an input power of 10 W. The gate valve to the vacuum chamber was closed, and the vacuum decreased from 5 × 10⁻⁶ torr to 1 × 10⁻² torr. We found that reduced vacuum conditions facilitate easier ignition of the plasma. The microwave generator power was increased to 200 W to ignite the plasma, which was verified visually by observing light at the reactor pinhole opening (Supplementary Fig. 18a). The gate valve was then opened to the vacuum chamber. The vacuum chamber pressure stabilized between 1 × 10⁻⁵ and 2 × 10⁻⁵ torr. The PIII thruster current was controlled through a PID loop (see the above ‘Plasma thruster’ section), which was set to 0.5 mA, with lower and upper thresholds of 0.3 and 0.6 mA, respectively.

The background rate of neutrons was collected for 5 min using the CAEN CoMPASS software⁵⁰. The reactor was then started by applying −30 kV to the palladium target using a high-voltage power supply. When the high negative voltage is applied to a target immersed in plasma, electrons are repelled, generating a plasma sheath (Supplementary Fig. 18b). The plasma sheath, an electron-depleted region, appears darker than the plasma jet because it is difficult for ions to recombine with electrons. This sheath has a strong electrostatic field. As ions from the plasma jet enter this region, they are accelerated by the sheath voltage and implanted into the target. A 6.67-mm pinhole is placed at the exit of the plasma thruster during the experiment. The purpose of the pinhole is to limit the PIII current to the 0.5 mA set for the reactor conditions.

For a beam-loading experiment, the neutron production rate was collected for approximately 2 h. In a typical experiment, a stable state (when the rate of neutron production is no longer changing as a function of time) was reached after approximately 30 min. We define steady state as the period during which all data points remain within ±5% of their mean value over a 30-min interval following the application of high voltage.

For an electrochemically enhanced experiment, the procedure was the same as above except that the electrochemical cell was turned on after approximately 1 h of reactor operation (see the next section, ‘Electrochemistry in the Thunderbird Reactor’). The electrochemical cell was operated until the stable state was observed after the initial increase in the rate of neutron counts. The vacuum chamber pressure increased over time during the operation of the electrochemical cell, and the final pressure was usually between 3.5 × 10⁻⁵ and 4 × 10⁻⁵ torr.

Electrochemistry in the Thunderbird Reactor

All electrochemistry in the Thunderbird Reactor was conducted galvanostatically at a total current of 200 mA. A combination of a 12 V battery and a DC–DC converter was used to power the electrochemical cell. At a cell current of 200 mA, the cell voltage was approximately 3.8 V (Supplementary Fig. 19). The electrochemical cell was manufactured from a cylindrical piece of Macor (Machinable Glass Ceramic; see the above section, ‘Electrochemical cell’) with a diameter of 50 mm, a length of 265 mm and a 1-cm-diameter opening on the bottom facing the vacuum chamber. The palladium target, 300 μm in thickness with a geometric surface area of 1.5 cm² exposed to the electrolyte, served as the cathode. The palladium target was sealed with an O-ring at the bottom of the electrochemical cell, with a geometric surface area of 0.785 cm² exposed to the vacuum chamber. The preparation of palladium targets is described in the ‘Palladium target/cathode preparation’ section. The anode was an iridium wire, 320 mm in length and 0.5 mm in diameter. The iridium wire was rinsed with deionized water, dried with a Kimwipe and heated with a propane torch to eliminate adsorbed water. The cleaned iridium wire was placed into the electrochemical cell at a distance of 1 mm from the cathode. The electrochemical cell was filled with 17 g (13 ml) of 2 M K₂CO₃ in D₂O. The galvanostat was connected to the Pd target and the iridium wire using alligator clips to complete the electrochemical set-up.

Electrochemical cycling in the Thunderbird Reactor

The effect of power cycling of the electrochemical cell was tested by turning the galvanostat on and off. An electrochemically enhanced experiment was conducted as described above (see the ‘Fusion reactions in the Thunderbird Reactor’ and ‘Electrochemistry in the Thunderbird Reactor’ sections), with the electrochemical cell being activated after 60 min of beam loading and the neutron production rate stabilizing at 172.6(4) n s⁻¹. The electrochemical cell was then turned off and on, at intervals of 30 min, while the beam loading of the Pd target with plasma continued (Extended Data Fig. 7).

Palladium target/cathode preparation

The palladium target was rolled from a palladium bar (100 g, 99.95% purity, purchased from Valcambi). The bar was first manually cold-rolled to <500 μm using a Pepetools 90MM Flat Rolling Mill, then automatically cold-rolled to a final thickness of 300 μm using an MTI EQ-MR100A Electric Roller Press. The final thickness was measured with a Mitutoyo digital micrometer to an accuracy of ±1 μm. Targets were cut from the 300-μm palladium sheet into a disc shape with a diameter of 2.4 cm using a die cutter. The disc-shaped targets were cleaned with deionized water and annealed at 400 °C for 1 h at 10⁻⁵ torr in a 50-mm quartz vacuum tube installed in an MTI OTF-1200X-S tube furnace. After annealing, the targets were polished with sandpaper (CW 1200) and washed with isopropyl alcohol using a Kimwipe before use in the reactor. The annealing and cleaning procedures were performed between each fusion experiment to remove deuterium from the palladium. The targets were characterized with XRD to confirm the absence of deuterium from the palladium lattice.

The annealing procedure was based on Sieverts’ law. According to Holleck⁴³, the relationship between the hydrogen concentration in metals and hydrogen gas pressure is given by:

in which P_H2 is the partial pressure of hydrogen, P₀ is 1 torr, K(T) is Sieverts’ constant at a specified temperature (for example, 2.1 × 10³ at 400 °C) and n is the molar ratio of hydrogen in the metal.

We assumed the volume fraction of hydrogen gas in the vacuum chamber to be the same as that in ambient air, 5 × 10⁻⁵ vol% (ref. ⁵¹), which results in P_H2 = 5.0 × 10⁻¹² torr for the total pressure of 10⁻⁵ torr in the vacuum tube furnace. The molar ratio of hydrogen (n) at 400 °C is:

This low value of n suggests that the palladium targets will be completely deloaded of hydrogen atoms at a temperature of 400 °C. The deloading was validated experimentally using XRD (Supplementary Fig. 10).

XRD characterization of palladium targets

XRD spectra were collected on a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (1.5406 Å). A 2θ-omega scan for 20–90° with a scan speed of 5° min⁻¹ and a step size of 0.05° was performed using parallel beam optics. The XRD spectra of the Pd targets were collected at three different stages of the experiment: (1) before a fusion experiment; (2) after a fusion experiment; and (3) after annealing the target to remove deuterium from the palladium lattice. For each stage, XRD spectra were measured on both sides of Pd targets: the side exposed to the electrolyte in the electrochemical cell and the other exposed to the D⁺ beam in the vacuum chamber. Before a fusion experiment, the Pd targets were scanned after the preparation method described above. After a fusion experiment, the Pd targets were removed from the Thunderbird Reactor and transferred to the XRD in ambient air within 20 min for scanning. The Pd targets were scanned again after they were subsequently annealed at 400 °C for 1 h at 10⁻⁵ torr to remove deuterium from the Pd lattice.

Typical XRD spectra of Pd targets are shown in Supplementary Fig. 10. In this example, the fusion experiment included beam-loading for 60 min with a sheath voltage of −30 kV and a plasma current of 0.5 mA. This was followed by electrochemical loading for 60 min with a constant current of 200 mA across the electrochemical cell. Before fusion, the Pd target exhibited diffraction peaks for α-Pd (D/Pd < 0.01 (ref. ⁵²)) phase on both the electrochemical cell and the beam side. The lattice constants for the α-Pd phase before fusion were determined by Bragg’s law for (111), (200), (220) and (311) peaks to be 3.894 ± 0.001 Å for the electrochemical cell side and 3.8941 ± 0.0006 Å for the beam side. These values are consistent with the value of 3.889 Å of pure palladium⁵³. After fusion, the Pd target exhibited diffraction peaks for α-Pd and β-Pd (D/Pd > 0.6 (ref. ⁵²)) on the electrochemical cell side and α-Pd on the beam side. Although we speculate that the absence of β-Pd on the beam side is attributed to the instability of deuterium in the Pd lattice at high temperature caused by the ion bombardment, we do not at present have the infrastructure to provide direct evidence through in situ characterization of the Pd target in a beam-loading environment. The lattice constant of the α-Pd phase on the beam side was 3.896 ± 0.003 Å. After annealing, the Pd target exhibited α-Pd on both the electrochemical cell and beam sides, identical to the Pd target before the fusion experiment. The lattice constants for α-Pd after annealing were 3.8896 ± 0.0004 Å for the electrochemical side and 3.8925 ± 0.0004 Å for the beam side.

In situ XRD characterization of palladium targets

In situ XRD spectra were collected on a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (1.5406 Å) while the electrochemical cell was turned on. Two types of custom 3D-printed electrochemical cell fitted for in situ XRD measurements were used on the sample stage of the X-ray diffractometer. One cell exposed one side of the Pd target to ambient air while electrochemically loading the other side of the Pd target (Supplementary Fig. 11). The other cell exposed one side of the Pd target to vacuum throughout the experiment while electrochemically loading the other side of the Pd target (Supplementary Fig. 12). These cells were designed in-house using SolidWorks computer-aided design (CAD) software and 3D-printed using a Formlabs Form 3 stereolithography 3D printer with Formlabs clear resin (urethane dimethacrylate, methacrylate monomer and photoinitiator). The 200-mA constant current between the Pd target cathode and iridium wire anode was supplied by B&K Precision model 1550 switching DC bench power supply. All in situ XRD experiments were performed at room temperature (25 °C) with a 2 M K₂CO₃ solution in H₂O as an electrolyte.

We chose the Pd(H) (111) and (200) peaks located at 2θ = 40.1° and 46.6° for in situ XRD measurements. We performed looped scans from 35° to 50° with a scan speed of 5° min⁻¹ and a step size of 0.05°. For the electrochemical loading cycle, the DC power supply and XRD looped scans were initiated at the same time. The DC power supply was kept on at 200 mA for 60 min to load the Pd target, during which 17 scans of XRD measurements were performed. The voltage during the loading was approximately 12 V.

After the electrochemical loading cycle, we subsequently performed either a natural outgassing cycle in ambient air or under vacuum (10⁻² torr). To start the natural outgassing cycle, the DC power supply was turned off to keep the Pd target at its open-circuit potential, while XRD looped scans were continuously performed for 18 h.

In situ XRD spectra clearly showed the transition from α-Pd to β-Pd phase during the electrochemical loading (Extended Data Fig. 3), which occurred about 20 min after turning on the electrochemical cell. The resulting lattice constant of a β-Pd phase gives the H/Pd ratio using equation (1) in ref. ²⁷, which is correlated to the D/Pd ratio of the Pd target in the Thunderbird Reactor. The electrochemical loading of more than 30 min yielded a single β-Pd phase. The H/Pd ratio achieved was typically around 0.7, indicating the formation of PdD with a high D/Pd ratio during the electrochemical loading in the Thunderbird Reactor.

We chose 200 mA as the current because it equates to a current density of 133 mA cm⁻², as we have previously reported that 100–150 mA cm⁻² is an optimal current density in our membrane reactor^54,55. XRD measurements of a Pd target loaded at 50 mA (Extended Data Fig. 4) indicated that the α to β transformation was too slow for the timescale of our experiments. The subsequent natural outgassing and vacuum cycles revealed that the β-Pd phase formed during the electrochemical loading cycle persisted over 18 h, both in ambient air and in vacuum (Extended Data Figs. 5 and 6). The outgassing rate did not show a notable difference between ambient air and under vacuum. The H/Pd ratio calculated from the lattice constant of the β-Pd(111) peak for the initial 60 min of the natural outgassing and vacuum cycle was higher than 0.6 for both cases, indicating that a β-Pd phase with a high H/Pd ratio was stable over the timescale of our fusion experiments with electrochemical loading cycled on and off.