A method for testing the work function of a rare earth boride crystal

Abstract

The application discloses a method for testing the work function of rare earth boride crystals, and belongs to the technical field of thermionic emission cathodes. ‑4 The method is performed in a vacuum diode with a vacuum degree better than 1*10 ‑4 Pa. The crystal to be tested is processed into a micro rod with a diameter of about 0.2 mm and embedded in a graphite sleeve, and finally installed on a filament holder clamped by pyrolytic graphite. The anode is composed of a metal cylinder and a grid, and is used for attracting and collecting emitted electrons. The crystal is heated to the working temperature by adjusting the heating current, the acceleration voltage is adjusted step by step, and the emission current is recorded to obtain an I-V curve (or J-V curve); the curve is processed by using a double logarithmic deviation method to obtain the zero-field emission current, and then the work function is calculated according to the Richardson-Dushman formula. The micro crystal and the graphite wrapping structure are adopted, the total emission beam current and the heating power are effectively reduced, the anode outgassing and ion back bombardment are reduced, the side emission is shielded, and the accuracy of the work function test is significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of thermionic emission cathode technology, specifically relating to a method for testing the work function of rare earth boride crystals. Background Technology

[0002] Thermionic emission cathode materials are widely used in high-end equipment such as electron beam additive manufacturing, electron beam exposure machines, scanning electron microscopes, and electron beam lithography systems. Their emission performance directly affects the beam current density, brightness, and lifespan of the equipment. Among many cathode materials, rare-earth boride crystals (such as lanthanum hexaboride and cerium hexaboride) are considered ideal electron source materials due to their low work function, excellent thermal emission capability, good resistance to poisoning, and long lifespan. The work function is a key parameter for measuring the emission capability of a cathode material; the lower the value, the easier it is for the material to emit electrons at the same temperature, thus providing a larger beam current or higher brightness.

[0003] For practical applications of thermionic cathode materials, the focus is more on their work function at high operating temperatures, rather than their surface barrier value at very low temperatures. Therefore, it is necessary to test the relationship between the thermionic current density and accelerating voltage using a vacuum diode or electron gun system at temperatures close to the actual operating temperature (typically 1400℃ to 1600℃), and then calculate the work function based on the Richardson-Dushmann formula. Within this temperature range, the emission current density of rare-earth boride cathodes is typically around 5 A·cm⁻¹. -2 Up to 30 A·cm -2 Between these conditions, the emission current is extremely sensitive to changes in the vacuum environment and the ion backbombardment effect.

[0004] The test methods reported in the existing literature generally have several shortcomings. First, many tests use millimeter-sized crystal samples, resulting in a large emission area and a total emission current of hundreds of milliamperes. When a high-current electron beam bombards the anode (or receiver), it generates significant Joule heating. The increased anode temperature releases adsorbed gas, causing a rapid decrease in vacuum. Simultaneously, high-energy electrons impacting the anode surface induce desorption and secondary electron effects. These factors all interfere with the cathode surface state, leading to significant fluctuations in the emission current and requiring a long stabilization time to obtain reliable readings. Second, high-current bombardment also produces strong ion backbombardment. Gas molecules released from the anode are ionized and accelerated back to the cathode surface under the influence of the electric field, causing sputtering and surface damage to the cathode material, thereby altering the emission performance and causing the test results to deviate from the true value. Furthermore, it is difficult to completely suppress electron emission from the crystal's side surface in existing methods. Side-emitted electrons also reach the anode and are included in the total current, resulting in the measured current containing non-side-face emission components, which in turn affects the accuracy of the work function calculation.

[0005] To address the aforementioned issues, researchers have attempted to confine the emission region using shielding or focusing electrodes, but the shielding effect is either unsatisfactory or results in interactions with the crystal. Furthermore, these methods often employ large-sized crystals with indirect heating, which not only leads to complex structures and high power consumption but also causes uneven temperature distribution within the crystal, increasing temperature measurement errors. Therefore, there is an urgent need for a testing method that can effectively suppress side emission, reduce anode bombardment power, and improve the stability of the testing environment to ensure that the work function data accurately reflects the thermal emission performance of the crystal end face. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for testing the work function of rare earth boride crystals. The crystal to be tested is processed into a fine rod and embedded in a graphite sleeve to shield side emission. At the same time, the total emission current is reduced to mitigate anodic outgassing and ion backbombardment, thereby achieving accurate and stable testing of the work function under high-temperature operating conditions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for testing the work function of rare earth boride crystals, comprising:

[0009] The rare earth boride crystal to be tested was processed into a fine rod.

[0010] The micro rod is embedded in a graphite sleeve, and the graphite sleeve is installed on a pyrolytic graphite clamping filament holder to form a cathode assembly.

[0011] The cathode and anode assemblies are mounted in a vacuum diode, wherein the vacuum degree of the vacuum diode is better than 1×10⁻⁶. -4 Pa;

[0012] Heating current is passed through the pins of the filament holder to heat the graphite sleeve and the micro rod to the working temperature;

[0013] An accelerating voltage with step-varying values ​​is applied between the anode assembly and the cathode assembly, and the emission current corresponding to each accelerating voltage is recorded to obtain a current-voltage curve.

[0014] The zero-field emission current is obtained by processing the current-voltage curve using the double logarithmic deviation method; the work function is calculated using the zero-field emission current and the operating temperature according to the Richardson-Dushman formula.

[0015] Furthermore, the diameter of the micro rod is approximately 0.2 millimeters.

[0016] Furthermore, the roundness of the micro rod is no greater than 3 micrometers, and the surface roughness of the emitting end face of the micro rod after polishing is within 10 nanometers; the fitting gap between the graphite sleeve and the micro rod is no greater than 2 micrometers.

[0017] Furthermore, when the pyrolytic graphite clamping filament holder clamps the graphite sleeve, the C-plane of the pyrolytic graphite is used as the clamping surface.

[0018] Furthermore, the anode assembly consists of a metal cylinder and a grid.

[0019] Furthermore, the grid is located near the cathode assembly to attract electrons, and the metal cylinder is disposed on the side of the grid away from the cathode assembly to receive electrons passing through the grid.

[0020] Furthermore, the metal cylinder has a cup-shaped structure with multiple vent holes on its side.

[0021] Furthermore, the metal cylinder and the grid are cleaned to remove oil stains before use, and then baked in a vacuum furnace at approximately 900°C to remove gas.

[0022] Furthermore, the sidewall of the vacuum diode is provided with an observation window, through which the temperature of the graphite sleeve is read using an optical pyrometer, and the temperature of the graphite sleeve is used as the working temperature of the micro rod.

[0023] Furthermore, the operating temperature is 1400°C to 1600°C.

[0024] In a second aspect, the present invention provides an electronic device, comprising: one or more processors; and a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned method for testing the work function of a rare earth boride crystal.

[0025] Thirdly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned method for testing the work function of a rare-earth boride crystal.

[0026] The beneficial effects of this invention are as follows:

[0027] The testing accuracy is significantly improved. By processing the crystal under test into a fine rod and tightly embedding it in a graphite sleeve, the graphite sleeve effectively shields the sides of the crystal, completely suppressing side electron emission. This ensures that the current collected at the anode comes entirely from the polished end face of the crystal, eliminating the interference of non-end face emission on the IV curve measurement, and making the work function calculation result truly reflect the thermal emission performance of the crystal end face.

[0028] The stability of the testing environment has been significantly improved. By using a microcrystal with a diameter of approximately 0.2 mm, the emission area has been significantly reduced, and the total emission current has been reduced to the milliampere or even microampere level. The electron beam bombardment power experienced by the anode is extremely low, avoiding excessive temperature rise and gas desorption of the anode. This effectively suppresses vacuum degradation and ion back-bombardment, resulting in smaller emission current fluctuations, shorter stabilization time, and higher measurement repeatability and reliability.

[0029] It boasts high heating efficiency and accurate temperature measurement. Employing a pyrolytic graphite clamping method, it selects the C-plane with high resistivity as the clamping surface, improving electrothermal conversion efficiency and reducing heating power consumption. Simultaneously, an observation window is provided on the side of the diode, allowing direct reading of the graphite sleeve temperature using an optical pyrometer. Since the graphite sleeve is in close contact with the crystal, the measured temperature is the actual operating temperature of the crystal, avoiding errors caused by indirect temperature measurement.

[0030] It has a wide range of applications. This method is applicable to a variety of rare earth boride crystal materials (such as lanthanum hexaboride, cerium hexaboride, etc.) and can be extended to the work function testing of other high-temperature thermal emission cathode materials, demonstrating good versatility and application prospects. Attached Figure Description

[0031] Figure 1 This is a system schematic diagram of a method for testing the work function of rare earth boride crystals according to the present invention.

[0032] Figure 2 A cathode comprising a crystal, a graphite sleeve, a pyrolytic graphite block, and a filament holder is provided as an embodiment of the present invention;

[0033] Figure 3 This is a schematic diagram of the complete structure of a testing system provided in an embodiment of the present invention;

[0034] Figure 4 This is a JV test curve according to one embodiment of the present invention. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] like Figure 1 As shown, this invention proposes a method for testing the work function of rare-earth boride crystals. By processing the crystal into microrods at the hundred-micrometer level, the total emission current is reduced, avoiding the adverse effects of excessive anode heating and vacuum degradation on crystal emission. Furthermore, encasing the crystal in a graphite sleeve suppresses side emission, ensuring that the measured current originates entirely from the emitting end face, thus improving testing accuracy. The method includes:

[0037] The rare earth boride crystal to be tested was processed into a fine rod.

[0038] The micro rod is embedded in a graphite sleeve, and the graphite sleeve is installed on a pyrolytic graphite clamping filament holder to form a cathode assembly.

[0039] The cathode and anode assemblies are mounted in a vacuum diode, wherein the vacuum degree of the vacuum diode is better than 1×10⁻⁶. -4 Pa;

[0040] Heating current is passed through the pins of the filament holder to heat the graphite sleeve and the micro rod to the working temperature;

[0041] An accelerating voltage with stepwise variation is applied between the anode assembly and the cathode assembly, and the emission current corresponding to each accelerating voltage is recorded to obtain a current-voltage curve (IV curve).

[0042] The zero-field emission current is obtained by processing the current-voltage curve using the double logarithmic deviation method; the work function is calculated using the zero-field emission current and the operating temperature according to the Richardson-Dushman formula.

[0043] Specifically, such as Figures 1-2 As shown, the rare earth boride crystal 100 to be tested is processed into a fine cylindrical rod (the diameter of the fine rod is approximately 0.2 mm), embedded in a graphite sleeve 200, and then clamped by two pieces of pyrolytic graphite 300. The entire assembly is then mounted onto a filament holder 400 to form an emitting cathode assembly. The anode assembly consists of a metal cylinder 600 and a grid 500 covering the front opening of the metal cylinder 600. The grid 500 is close to the cathode assembly, attracting electrons. A small portion of the emitted electrons is intercepted by the grid 500, while the majority strike the bottom of the metal cylinder 600. This design ensures that, at a lower accelerating voltage, the anode bombardment area is far from the cathode end. By adjusting the heating current flowing through the pins of the filament holder 400, the graphite sleeve 200 and the rare earth boride crystal 100 under test are heated to the required operating temperature; an accelerating voltage is applied between the anode assembly and the cathode assembly to make the rare earth boride crystal 100 under test emit a beam, and by changing the accelerating voltage, the IV (or JV) curve of the crystal at that temperature can be obtained.

[0044] To ensure the accuracy of the crystal emission area calculation, the roundness of the processed rare-earth boride crystal 100 is better than 3 μm. To reduce the limitation of electron overflow caused by surface defects of the rare-earth boride crystal 100, the emission end face is polished to a roughness within 10 nm. To suppress lateral electron emission from the rare-earth boride crystal 100, the mating gap between the graphite sleeve 200 and the rare-earth boride crystal 100 is no greater than 2 μm. If oil and air are adsorbed on the surfaces of the metal cylinder 600 and the grid 500, they will contaminate the crystal surface of the rare-earth boride crystal 100 under electron bombardment or cause a sharp drop in vacuum, resulting in a weakened emission beam. Therefore, the grid 500 and the metal cylinder 600 need to be cleaned to remove oil and then thoroughly baked in a vacuum furnace at approximately 900°C to remove gas. To improve the heating efficiency of the cathode assembly, the C-plane with higher resistivity is selected for the clamping surface of the pyrolytic graphite 300.

[0045] In an optional embodiment of the present invention, the vacuum diode is a CF35 metal six-way diode 700, with its lower port serving as the evacuation port directly connected to an oil-free vacuum pump; its front port is connected to an observation window for easy observation of the cathode temperature; its upper port is connected to a vacuum gauge for detecting the vacuum level; and its left and right ports are respectively equipped with an anode assembly and a cathode assembly, such as... Figure 3 As shown.

[0046] According to an embodiment of the present invention, the metal cylinder 600 and the grid 500 are made of stainless steel and are connected by bolts. Four vent holes are symmetrically opened on the side of the metal cylinder 600. An M10 threaded hole is located at the rear of the metal cylinder 600, which is screwed to a CF35 vacuum electrode 601 with an M10 screw. The anode potential is introduced by a high-voltage power supply through this electrode.

[0047] According to an embodiment of the present invention, the two pins of the cathode assembly are inserted into a copper plate 604 with a crown spring, and the copper plate 604 is fixed to a hexagonal stud 603 on the CF35 dual electrode 602. The hexagonal stud 603 is screwed into the screw of the electrode 602 by its internal thread, and the copper plate 604 has a mounting hole, which is fixed by screwing a bolt into the internal thread of the hexagonal stud 603.

[0048] In one specific embodiment of the present invention, the gap between the grid 500 and the cathode assembly is approximately 4 mm, the graphite sleeve has a diameter of 0.51 mm and a length of 2 mm, the rare earth boride crystal 100 to be tested is a 100-oriented LaB6 single crystal with an emission surface diameter of 167.2 μm (measured using a tool microscope or electron microscope), the operating temperature is 1500 ℃, the accelerating voltage is 50-800 V, a positive voltage is applied to the anode assembly, one of the cathode electrode rods is grounded, the accelerating voltage is adjusted in 50 V steps, and the corresponding emission current I1 is recorded. The JV curve is obtained after double logarithmic processing as shown below. Figure 4 As shown, the current density at the inflection point (i.e., the zero-field emission current density) is 10.185 A·cm.-2 According to the Richardson-Dushmann formula:

[0049] J0=A0·T 2 ·exp(-Φ / kT)

[0050] Where J0 is the zero-field emission current density of the material; A0 is the Richardson constant, A0 = 120 A·cm. -2 ·K -2 T – Material temperature, in K; Φ – Effective work function of the material, in eV; k – Boltzmann constant, k = 8.617 × 10⁻⁶ -5 eV / K. The work function at 1500 °C is 2.66 eV, which is consistent with the theoretical value, indicating that the method of the present invention is relatively accurate and reliable in testing the work function of rare earth boride crystals.

[0051] In a second aspect, the present invention provides an electronic device, comprising: one or more processors; and a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned method for testing the work function of a rare earth boride crystal.

[0052] Thirdly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned method for testing the work function of a rare-earth boride crystal.

[0053] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for testing the work function of rare earth boride crystals, characterized in that, include: The rare earth boride crystal to be tested was processed into a fine rod. The micro rod is embedded in a graphite sleeve, and the graphite sleeve is installed on a pyrolytic graphite clamping filament holder to form a cathode assembly. The cathode and anode components are mounted in a vacuum diode, wherein the vacuum level of the vacuum diode is much better than 1×10⁻⁶. -4 Pa; Heating current is passed through the pins of the filament holder to heat the graphite sleeve and the micro rod to the working temperature; An accelerating voltage with step-varying values ​​is applied between the anode assembly and the cathode assembly, and the emission current corresponding to each accelerating voltage is recorded to obtain a current-voltage curve. The zero-field emission current was obtained by processing the current-voltage curve using the double logarithmic deviation method. The work function is calculated using the zero-field emission current and the operating temperature, according to Richardson's formula.

2. The method for testing the work function of a rare earth boride crystal according to claim 1, characterized in that, The diameter of the micro rod is approximately 0.2 mm.

3. The method for testing the work function of a rare earth boride crystal according to claim 2, characterized in that, The roundness of the micro rod is no greater than 3 micrometers, and the surface roughness of the emitting end face of the micro rod after polishing is within 10 nanometers; the fitting gap between the graphite sleeve and the micro rod is no greater than 2 micrometers.

4. The method for testing the work function of a rare earth boride crystal according to claim 1, characterized in that, When the pyrolytic graphite clamping filament holder clamps the graphite sleeve, the C-plane of the pyrolytic graphite is used as the clamping surface.

5. The method for testing the work function of a rare earth boride crystal according to claim 1, characterized in that, The anode assembly consists of a metal cylinder and a grid.

6. The method for testing the work function of a rare earth boride crystal according to claim 5, characterized in that, The grid is located near the cathode assembly to attract electrons, and the metal cylinder is located on the side of the grid away from the cathode assembly to receive electrons passing through the grid.

7. The method for testing the work function of a rare earth boride crystal according to claim 6, characterized in that, The metal cylinder has a cup-shaped structure with multiple vent holes on its side.

8. The method for testing the work function of a rare earth boride crystal according to claim 5, characterized in that, The metal cylinder and the grid are cleaned to remove oil stains before use, and then baked in a vacuum furnace at about 900°C to remove gas.

9. The method for testing the work function of a rare earth boride crystal according to claim 1, characterized in that, The vacuum diode has an observation window on its sidewall. The temperature of the graphite sleeve is read through the observation window using an optical pyrometer. The temperature of the graphite sleeve is used as the working temperature of the micro rod.

10. The method for testing the work function of a rare earth boride crystal according to claim 9, characterized in that, The operating temperature is 1400℃ to 1600℃.

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