An ultrahigh-energy microengine device and a method of manufacturing the same

Abstract

The application discloses a kind of based on hydrogen combustion and micro needle electrode ignition ultra-high energy density micro engine device and preparation method thereof, belong to micro electro mechanical system and micro energy technology field.The device mainly includes top cylinder shell, bottom cylinder shell, piston, top plug-in unit and bottom plug-in unit.It is in that a pair of opposite micro needle electrode in micro combustion chamber ignites pre-mixed hydrogen / oxygen, generates high-temperature high-pressure gas to push piston to carry out two-stroke reciprocating motion, to convert chemical energy directly into mechanical energy.The device adopts unique combustion chamber flow channel design, combined with the flow divider on piston, effectively optimizes scavenging process, reduces fuel short circuit, improves gas exchange efficiency.

Description

Technical Field

[0001] This invention relates to the field of micro-energy and micro-power systems, specifically to an ultra-high energy micro-engine device and its preparation method. Background Technology

[0002] The rapid development of the Internet of Things (IoT), portable electronic devices, implantable biomedical devices, microrobots, and distributed wireless sensor networks poses a severe challenge to the continuous, stable, and long-term power supply for these devices. Traditional chemical batteries (such as lithium-ion batteries) suffer from limited energy density (typically below 500 Wh / kg), long charging times, limited cycle life, self-discharge, and environmental pollution after disposal. Environmental energy harvesting technologies such as solar energy and vibration energy are limited by the intermittency of energy sources and low energy flux density, making it difficult to meet the demands of high power consumption or continuous operation. Therefore, developing a micro power source with energy density far exceeding existing batteries, sustainable operation, rapid refueling, and environmental friendliness has become a current research hotspot and frontier.

[0003] Against this backdrop, power microelectromechanical systems (MEMS) have emerged. Power MEMS aims to miniaturize macroscopic power devices (such as internal combustion engines, fuel cells, and turbines) using MEMS technology, thereby achieving efficient energy conversion and output at the micrometer to millimeter scale. Among them, combustion-based micro heat engines, especially micro internal combustion engines, have attracted much attention because they can theoretically inherit the high energy density advantage of macroscopic internal combustion engines (the energy density of hydrocarbon fuels can be an order of magnitude higher than that of lithium batteries). Hydrogen, as a clean fuel, has water as its main combustion product, resulting in significant environmental benefits. It also has a wide combustion limit and a high flame propagation speed, making it very suitable for achieving rapid and stable combustion in microscale combustion chambers.

[0004] However, scaling down the internal combustion engine principle to millimeters or even smaller scales presents numerous challenges: (1) Ignition difficulties: The reduced scale leads to an increased surface area to volume ratio, resulting in increased heat loss. Traditional spark plug ignition is difficult to reliably ignite the air-fuel mixture under small gaps and limited energy. (2) Unstable combustion: The flame quenching effect is significant at the microscale, resulting in low combustion efficiency and a tendency for incomplete combustion. (3) Friction and sealing: The increased frictional loss of small moving parts places extremely high demands on the sealing between the piston and cylinder. Manufacturing and integration: Complex three-dimensional motion structures and gas and electrical circuits require integrated manufacturing and reliable packaging at micron precision.

[0005] Existing research on micro internal combustion engines mainly falls into two categories: volumetric and dynamic. Volumetric engines have a relatively simple structure, are easy to implement in a two-stroke cycle, avoid complex valve mechanisms, and are more suitable for miniaturization. However, existing micro internal combustion engine designs still have shortcomings in ignition reliability, scavenging efficiency, thermal management, and long-term operational stability. Especially for free-piston structures, designing an efficient scavenging system to prevent fresh charge from being directly short-circuited out, and achieving reliable, low-energy ignition, are key to improving their performance.

[0006] Microneedle electrode ignition is an ignition technique suitable for microscale combustion. By machining the electrode into a pointed shape, a strong electric field can be generated near the tip at a relatively low voltage, inducing partial discharge (corona discharge or spark discharge), thereby reliably igniting the gas-fuel mixture. This method has low energy consumption, precise ignition positioning, and is easily integrated with MEMS processes.

[0007] In summary, there is an urgent need for an ultra-high energy density micro-engine device and method that combines efficient microscale ignition technology, optimized combustion chamber and flow channel design, and can achieve high-precision manufacturing and packaging through advanced microfabrication technology, in order to break through the bottleneck of existing micro-energy technology. Summary of the Invention

[0008] Purpose of the invention The primary objective of this invention is to overcome the technical defects of existing micro energy devices, such as low energy density, short driving time, and insufficient environmental friendliness, as well as the unreliable ignition, low scavenging efficiency, and complex manufacturing of existing micro internal combustion engines. The invention aims to provide a micro engine device based on hydrogen combustion, using microneedle electrode ignition, with ultra-high energy density, compact structure, stable operation, and environmental friendliness.

[0009] Another objective of this invention is to provide a method for fabricating the aforementioned micro-engine device. This method is based on mature SOI micromachining technology and gold-silicon eutectic bonding technology, which enables high-precision, high-strength, and high-airtightness integrated manufacturing of micron-level complex three-dimensional motion structures, flow channel systems, and electrode systems, thereby ensuring the performance and reliability of the device.

[0010] Technical solution To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides an ultra-high energy density micro-engine device.

[0011] At the heart of this device is a two-stroke free-piston micro internal combustion unit. Its main components include: a top cylinder housing, a bottom cylinder housing, a piston, a top insert unit, and a bottom insert unit. These five core components are assembled into a single unit using precision micromachining and bonding processes.

[0012] Combustion and Ignition System: A pair of microneedle electrodes are mounted opposite each other on the top and bottom cylinder housings. The tips of these electrodes extend into the combustion chamber formed by the two housings. When a high-voltage pulse is applied to the electrodes, an electric spark is generated between the tips to ignite the premixed hydrogen / oxygen filling the combustion chamber. The microneedle electrode design reduces the ignition voltage, improving the reliability and accuracy of ignition within a confined microscale space.

[0013] Intake, Exhaust, and Scavenging System: The combustion chamber wall is equipped with intake and exhaust channels. The intake channel connects to a sophisticated external fuel supply system, while the exhaust channel discharges the combusted exhaust gases (mainly water vapor) to the outside. The piston reciprocates within the cylinder, and its trajectory determines the opening and closing timing of the intake and exhaust ports, thus achieving a two-stroke cycle. A key innovation of this invention lies in the unique design of the combustion chamber flow channel and the piston. A flow divider with a specific geometry is designed at the bottom of the piston. When the piston moves near bottom dead center, the flow divider changes the flow direction of the fresh air-fuel mixture flowing in from the intake channel, causing it to move upwards, forming a "high-injection" flow pattern, increasing the residence time in the combustion chamber. Subsequently, the flow pattern transitions to a "low-injection" flow pattern, flowing along the shortest path to the exhaust port. This flow pattern sequence effectively reduces the "short-circuit" phenomenon where the fresh air-fuel mixture escapes directly from the exhaust port without combustion, significantly improving scavenging efficiency. Specific optimized geometric parameters are given in the embodiments.

[0014] Power output and recovery mechanism: A miniature helical spring is integrated inside the piston. During the combustion expansion stroke, the piston moves downward against the spring force; at the end of expansion, the elastic potential energy stored in the spring is released, pushing the piston back to top dead center, completing the compression stroke and preparing for the next ignition. This design eliminates the need for a separate crank-connecting rod mechanism, simplifying the structure and reducing friction.

[0015] Monitoring and observation structure: A transparent observation window (which can be sealed with glass or transparent polymer) is provided on the side wall of the device casing, allowing direct observation of the piston's dynamic movement via a high-speed camera, facilitating performance research and fault diagnosis. Simultaneously, the device integrates a pressure measurement channel, connecting the combustion chamber to an external pressure measurement port. This port can be connected to a miniature pressure sensor to monitor changes in combustion pressure in real time, providing crucial data for cycle analysis and control.

[0016] Materials and Packaging: The main structural components are made of monocrystalline silicon, leveraging its excellent strength, hardness, fatigue properties, and perfect compatibility with MEMS processes. All components are sealed together using gold-silicon eutectic bonding, a technology that creates a high-strength, highly airtight metallurgical interface, ensuring the reliable sealing of the micro-combustion chamber under high temperature and pressure. The top and bottom insert units are auxiliary structures and may contain electrical interfaces, auxiliary supports, or adapter structures for connecting to external systems.

[0017] Miniaturized size: The entire device is extremely small in size, with a typical design of about 22.4 mm in length, about 26 mm in width, and only 8 mm in thickness. The combustion chamber volume is less than 100 cubic millimeters, truly realizing the miniaturization of the power unit.

[0018] This invention provides a method for fabricating the above-mentioned ultra-high energy density micro-engine device, comprising: SOI wafer preparation: A standard SOI wafer with a top silicon and substrate silicon thickness of 500μm and a 3μm buried oxide layer was selected as the base material. The buried oxide layer serves as an ideal etching stop layer in subsequent deep etching.

[0019] Double-sided lithography and mask fabrication: Photoresist is spin-coated on both sides of the SOI wafer, and precise alignment and exposure are performed using a double-sided lithography machine to form patterns of the top cylinder housing, bottom cylinder housing, and piston structure, respectively. Subsequently, a chromium layer is deposited and stripped to form a durable chromium mask.

[0020] ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching): This technique, particularly the Bosch process, uses an inductively coupled plasma reactive ion etching technology with a chromium mask as a protective layer to perform anisotropic deep etching on silicon. This step requires double-sided etching, penetrating a 500μm silicon layer until it is blocked by the buried oxide layer. In a single process, all three-dimensional silicon structures, including the combustion chamber, various channels, observation windows, piston shape, and internal microsprings, can be etched simultaneously.

[0021] Structural release and cleaning: Wet chemical etching is used to remove the exposed buried oxide layer, releasing the separate top cylinder housing, bottom cylinder housing, and piston components. Residual chromium masking is also removed.

[0022] Microneedle electrode integration can be achieved in two ways: Post-assembly: The microneedle electrode array is fabricated separately on another substrate, and then transferred to a designated cavity in the cylinder housing using precision alignment and bonding techniques. Integrated manufacturing: On the silicon structure of the cylinder housing, conductive regions are formed by localized heavy doping through ion implantation, and then metal leads and reinforced needle tips are formed through photolithography and evaporation.

[0023] Bonding surface metallization: To achieve gold-silicon eutectic bonding, a gold layer needs to be deposited in the bonding ring region of the two cylinder housings. A thin steel mask, prepared by laser cutting, is precisely aligned onto the bonding surface, and then a gold film of a certain thickness is deposited by vacuum evaporation or sputtering. This method avoids complex photolithography steps and is suitable for small-batch or prototype manufacturing.

[0024] Precision alignment and eutectic bonding: Under a microscope, the piston is carefully placed into the chamber of the bottom cylinder housing, and then the top cylinder housing is precisely aligned with the bottom cylinder housing. The assembly is placed in a bonding machine, and uniform pressure is applied in a vacuum or nitrogen environment, and the temperature is raised above the gold-silicon eutectic temperature. At this temperature, interdiffusion occurs at the gold-silicon interface, forming a low-melting-point eutectic liquid phase, which then cools and solidifies, forming a strong, hermetically sealed metallurgical bond. This step completes the sealing of the combustion chamber and the assembly of the entire moving structure in one step.

[0025] Plug-in unit packaging: Finally, the pre-processed top and bottom plug-in units (which may be made of plastic, metal or silicon) are fixed to both ends of the core engine unit by epoxy resin adhesive or mechanical snap-fit, providing protection, electrical connection and mechanical interface.

[0026] Beneficial effects Compared with the prior art, the technical solution provided by the present invention has the following significant advantages: 1. Directly utilizing the chemical energy of hydrogen combustion, its energy density is far higher than that of the best lithium-ion batteries, providing dozens of times the range of batteries of the same weight. The combustion product is mainly water, achieving true zero carbon emissions and resulting in extremely outstanding environmental benefits.

[0027] 2. Employing microneedle electrode spark ignition, optimized for microscale combustion, it requires low ignition energy and generates a stable electric spark within tiny gaps, successfully solving the ignition challenge of micro-combustion chambers. The optimized combustion chamber and piston splitter design significantly improves scavenging efficiency, reduces fuel waste, and ensures more complete combustion, resulting in a higher theoretical thermodynamic cycle efficiency.

[0028] 3. Utilizing the free-piston two-stroke principle, it eliminates the complex valve mechanism and crankshaft, resulting in fewer moving parts and an extremely compact structure. An integrated micro-spring serves as the return mechanism, ensuring a simple and reliable operating principle. The high stiffness and low coefficient of thermal expansion of silicon material help maintain dimensional stability during thermal cycling and reduce leakage.

[0029] 4. The built-in observation window and pressure measurement channel provide a direct means for studying microscale combustion dynamics, piston kinematics and device failure mechanisms, which is conducive to the continuous optimization and online monitoring of the device.

[0030] 5. The entire fabrication process is based on a mature MEMS process platform, exhibiting excellent process compatibility and repeatability. Gold-silicon eutectic bonding ensures the mechanical strength and sealing of the final product, meeting the stringent requirements of the high-temperature and high-pressure operating conditions of micro internal combustion engines. This method lays the technological foundation for the mass production of micro power systems.

[0031] 6. Compared to traditional charging which takes several hours, "energy replenishment" can be completed in a short time by replacing or replenishing the micro hydrogen storage tank, greatly improving the convenience of use. It is especially suitable for remote or long-term deployment scenarios where frequent charging is not possible. Attached Figure Description

[0032] Figure 1 This is an exploded structural diagram of the ultra-high energy density micro-engine device provided in an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram illustrating the two-stroke working principle of the micro-engine device in an embodiment of the present invention.

[0034] Figure 3 This is a two-dimensional schematic diagram of the specific structure of the piston in an embodiment of the present invention.

[0035] Figure 4 This is a schematic diagram of the geometric parameters and streamlines of the two-dimensional flow model of the combustion chamber in an embodiment of the present invention.

[0036] Figure 5 This is a diagram showing the key geometric dimensions of the micro-engine device in this embodiment of the invention.

[0037] Figure 6 This is a process flow diagram of the fabrication process of the core component of the micro-engine device provided in the embodiments of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of protection of the invention.

[0039] Example 1: Detailed Explanation of Device Structure The ultra-high energy density micro-engine device 100 of this embodiment is mainly composed of the following five parts stacked sequentially: bottom plug-in unit 106, bottom cylinder housing 102, piston 104, top cylinder housing 101, and top plug-in unit 105.

[0040] The top cylinder housing 101 and bottom cylinder housing 102 are fixed components constituting the core combustion chamber 103. They are made of single-crystal silicon through micromachining, exhibiting extremely high dimensional accuracy and surface finish. A microneedle electrode 107 is embedded on the opposing inner surfaces of each housing. The metal tips of the electrodes are precisely aligned and extend into the central region of the combustion chamber 103; the gap between them can be adjusted during assembly, typically 50 micrometers. Electrode leads extend from the side of the housing and connect to an external ignition circuit. During ignition, a short pulse of several kilovolts is applied to the electrodes, generating an electric spark in the gap between the needle tips.

[0041] The combustion chamber 103 has three main openings on its side: Intake passage 108: Connects to an external fuel mixing and supply unit. This unit typically includes a high-purity hydrogen cylinder, an oxygen cylinder, a mass flow controller, a static mixer, etc., and is capable of precisely mixing and supplying gas at a stoichiometric ratio (hydrogen:oxygen = 2:1) or a slightly fuel-rich ratio.

[0042] Exhaust passage 109: The exhaust gases after combustion are discharged through this passage. A miniature muffler or condenser can be connected externally.

[0043] Pressure measurement channel 111: A narrow channel connecting one end to the combustion chamber and the other end to a miniature pressure measurement port 112. This port can be connected to a piezoresistive or capacitive miniature pressure sensor to convert the pressure signal into an electrical signal for recording by the data acquisition system.

[0044] A rectangular observation window 110 is provided on one of the side walls of the device. This window is formed into a through-hole during silicon etching, and then sealed with a transparent glass or sapphire sheet by anodic bonding or adhesive bonding, which maintains the seal while allowing optical observation.

[0045] The piston 104 is the only moving part in the device. It is also integrally fabricated from single-crystal silicon and mainly comprises: Sealing ring 1041: The upper part of the piston, its diameter precisely matches the inner diameter of the combustion chamber 103 (the gap is on the order of micrometers), and it serves to seal the combustion gases. Its surface can be treated with a silicon carbide or diamond-like carbon film coating to reduce the coefficient of friction and wear.

[0046] Splitter 1042: A boss structure extending from the lower end of the sealing ring, the cross-sectional shape of which is optimized by computational fluid dynamics. When the piston moves to near the bottom dead center, the splitter is directly opposite the outlet of the intake passage 108, which can deflect the horizontally flowing airflow upward, causing the fresh mixture to fill the upper space of the combustion chamber.

[0047] Integrated microspring 1043: A cantilever beam or helical elastic structure manufactured inside the piston body using a deep etching process. In this embodiment, it is a planar micro-helical spring with a total length L_spring = 10 mm, a spring wire width W_spring = 240 μm, and 7 turns N. One end of the spring is anchored inside the piston, and the other end is connected to the moving part of the piston (or the piston itself is the movable part of the spring). It provides the necessary restoring force.

[0048] The top insert unit 105 and bottom insert unit 106 can be made of polyimide, stainless steel, or ceramic. Their primary functions are: to protect the brittle silicon core structure; to provide a mechanical interface for connection to external test benches or application systems; and to accommodate and lead out electrical interfaces. They are typically connected to the core portion using epoxy resin adhesive or screws.

[0049] Scavenging / Compression Stroke: Under the restoring force of the integrated spring 1043, the piston moves from bottom dead center to top dead center. First, the exhaust port is closed, then the intake port is closed. The piston moves upward to compress the fresh hydrogen-oxygen mixture that has been previously drawn into the combustion chamber. At the same time, due to the upward movement of the piston, a negative pressure is generated in the crankcase, preparing for the intake of the next cycle.

[0050] Ignition and Expansion Stroke: As the piston approaches top dead center, the air-fuel mixture is compressed to a predetermined pressure. The ignition circuit is triggered, and the microneedle electrode 107 generates a spark, igniting the air-fuel mixture. Combustion is completed in a very short time, producing high-temperature, high-pressure gas that pushes the piston to its lower dead center at high speed, performing work. This work can be output through the piston rod or directly generated by electromagnetic induction.

[0051] Exhaust and Intake Strokes: As the piston descends, it first opens the exhaust port 109, allowing hot exhaust gases to escape. As the piston continues downward, the intake port 108 opens, and fresh air-fuel mixture begins to flow into the combustion chamber under pressure differential, helping to scavenge residual exhaust gases. The diffuser 1042 at the bottom of the piston plays a crucial role in this stage, optimizing scavenging flow. After reaching bottom dead center, the piston begins a new upward stroke under spring force.

[0052] Example 2: Combustion Chamber Flow Channel Optimization Design One of the key factors in improving the performance of this invention is the coordinated design of the intake, exhaust, and piston splitter to maximize scavenging efficiency.

[0053] A simplified two-dimensional model of the combustion chamber cross-section when the piston is at bottom dead center. Key dimensions are defined as follows: A = 1 mm: The vertical distance from the upper edge of the intake passage outlet to the top surface of the piston.

[0054] B = 0.5 mm: Vertical height of the air intake passage outlet.

[0055] C = 1 mm: Vertical distance from the lower edge of the exhaust passage inlet to the top surface of the piston.

[0056] D = 1 mm: Vertical height of the exhaust channel inlet.

[0057] The profile of the piston splitter 1042 is specially designed to effectively block the airflow ejected horizontally from the intake port at this time.

[0058] The transient flow field was simulated using computational fluid dynamics software. High-injection stage: Fresh air-fuel mixture is injected into the intake port at a certain velocity, impacting the inclined surface of the piston distributor. The main kinetic energy of the airflow is converted into upward motion, forming an upward-climbing jet. This forces fresh gas to fill the upper part of the combustion chamber, where the residual exhaust gas concentration is high, which is beneficial for scavenging.

[0059] Low-injection phase: As airflow continues to flow in and combustion chamber pressure changes, along with the establishment of the flow field behind the diffuser, the main direction of the airflow gradually changes. The gas tends to seek the path of least resistance, i.e., flowing directly from the intake port to the already opened, lower-pressure exhaust port. At this time, the streamline is shorter.

[0060] By optimizing dimensions A, B, C, and D, as well as the splitter angle, the intensity, duration, and switching timing of "high injection" and "low injection" can be controlled. The goal is to achieve "high injection" as much as possible in the early stages of scavenging to displace exhaust gas, and to allow "low injection" in the later stages of scavenging to quickly refresh the gas in the cylinder, while minimizing the "short circuit" caused by the overlap of the two. Simulation results show that the design of this embodiment can reduce fuel short circuit losses and improve scavenging efficiency, significantly outperforming a simple opposed intake and exhaust port design.

[0061] Overall dimensions and performance parameters of the device The detailed external dimensions and key dimensions of the combustion chamber of the micro-engine device 100 in this embodiment are shown.

[0062] Overall device dimensions: Length L_device = 22.4 mm, Width W_device = 26 mm, Thickness (height) H_device = 8 mm.

[0063] Combustion chamber: diameter D_cylinder ≈ 4 mm, piston stroke S ≈ 2 mm. Based on this, the stroke volume V_s ≈ π can be calculated. (D_cylinder / 2)^2 S ≈ 25 mm³ (0.025 cc). This is an extremely small displacement.

[0064] Example 4: Detailed preparation method Step 1: Substrate preparation and cleaning Prepare several 4-inch or 6-inch SOI wafers. Wafer specifications: top silicon thickness 500 μm ± 5 μm, buried oxide layer thickness 3 μm ± 0.5 μm, substrate silicon thickness 500 μm ± 10 μm. Thoroughly clean the wafers using a standard RCA cleaning process to remove organic, metallic, and particulate contaminants, obtaining a hydrophilic clean surface.

[0065] Step 2: Double-sided photolithography and chromium mask fabrication Front-side photolithography: A positive photoresist (such as AZ 4620) with a thickness of approximately 10 μm is spin-coated onto the device layer (top silicon) surface of the SOI wafer. Front-side exposure is performed on a double-sided photolithography machine using a specialized double-sided photomask (the mask pattern corresponds to the structure of the top cylinder housing 101 and piston 104). After development, a chromium layer approximately 300 nm thick is deposited by electron beam evaporation in the photoresist opening area. Subsequently, a lift-off is performed to obtain the chromium mask pattern on the front side.

[0066] Backside photolithography: The wafer is flipped, and similar photolithography, chromium evaporation, and lift-off processes are repeated on the substrate surface. The mask pattern used corresponds to the structure of the bottom cylinder housing 102. The key is that the front and back patterns must be strictly aligned, with an alignment accuracy better than 5 μm.

[0067] Step 3: ICP-RIE Deep Silicon Etching The SOI wafer with double-sided chromium masks is placed in an ICP-RIE device. Etching is performed using Bosch process parameters: Etching gas: SF6, flow rate 200 sccm, used for isotropic silicon etching.

[0068] Passivation gas: C4F8, flow rate 100 sccm, used for depositing passivation polymer on the sidewalls.

[0069] Cycle time: 5-8 seconds for etching step, 3-5 seconds for passivation step, adjusted according to etching rate.

[0070] RF power: ICP power 2000 W, RF bias power 100 W.

[0071] Pressure: 10 mTorr.

[0072] Etching is performed simultaneously from both sides of the wafer. Due to the high anisotropy of the Bosch process, etching proceeds vertically downwards, with the sidewalls almost vertical. After penetrating 500 μm of silicon, the etching automatically stops on the buried oxide layer. This step simultaneously defines the complete three-dimensional shape of the cylinder housing's chambers, channels, windows, and piston, including its internal microspring 1043 structure.

[0073] Step 4: Remove the sacrificial layer and mask to release the component. The etched wafer was immersed in a 49% hydrofluoric acid solution to etch away the exposed buried oxide layer of silicon dioxide. This process released the piston structure and separated the top and bottom cylinder housings from the wafer. Subsequently, all residual chromium masking was removed using a chromium etching solution. The wafer was then thoroughly rinsed with deionized water and critically dried to prevent structural adhesion.

[0074] Step 5: Fabrication and integration of microneedle electrodes were performed separately on the top and bottom cylinder housing components: Ion implantation: Photolithography is performed in the areas where electrode tips and leads need to be formed, followed by high-dose boron ion implantation to form p++ heavily doped regions and reduce contact resistance.

[0075] Metallization: Photolithography defines the electrode pattern. Electron beam evaporation deposits the adhesion layer and the host layer. After lift-off, the metal electrode is formed.

[0076] Needle sharpening: Anisotropic wet etching or localized ICP etching can be used to form a sharp cone on the silicon tip of the electrode, and the metal layer on top then forms a micro-needle tip. The tip curvature radius can be achieved at the sub-micron level.

[0077] Step 6: Metallization of bonding surfaces Using a 100 μm thick stainless steel sheet, an ultrafast laser cutter was used to create a perforated mask according to the precise shape of the bonding rings on the top and bottom cylinder housings. Under a microscope, the mask was precisely positioned and temporarily fixed to the bonding surface of the cylinder housing component. The component was then placed in an electron beam evaporation stage, where a gold layer approximately 2 μm thick was deposited. After removing the steel mask, a precise gold film pattern was left in the bonding ring area.

[0078] Step 7: Gold-silicon eutectic bonding In a cleanroom environment, perform the following operations: Surface activation: The metallized top cylinder housing 101, bottom cylinder housing 102 and piston 104 are placed in an oxygen plasma cleaner for 1-2 minutes to remove organic contaminants and improve surface activity.

[0079] Pre-alignment: On a precision bonding alignment stage equipped with a microscopic vision system, the bottom cylinder housing 102 is first fixed. The piston 104 is picked up with a vacuum pen and carefully placed into the combustion chamber of the bottom housing, ensuring that its movement is free from interference. Then, the top cylinder housing 101 is picked up and aligned face-to-face with the bottom housing. Using pre-designed alignment marks on the housing, an overlay accuracy better than 2 μm is achieved.

[0080] Bonding: The aligned multilayer structure is transferred to a vacuum bonding machine. The chamber is evacuated to <10⁻³ Pa. A uniform mechanical pressure of approximately 3 MPa is applied. The temperature is increased to 380°C at a rate of 10°C / min and held at this temperature for 60 minutes. During this period, gold and silicon interdiffusion occurs at the contact interface, forming an Au-Si eutectic liquid phase layer approximately 3-5 μm thick. The liquid phase layer fills the microscopic unevenness, achieving close contact.

[0081] Cooling: Cool to room temperature at a controlled rate. The eutectic liquid phase solidifies, forming a strong metallurgical bond layer. This bond layer can withstand high temperatures and pressures while maintaining excellent airtightness.

[0082] Step 8: Final Packaging and Testing The bonded core engine unit is removed from the bonding machine. Conductive epoxy resin is used to connect the wire bonding points of the microneedle electrodes to the micropads pre-placed on the top insert unit 105. Then, the top insert unit 105 and the bottom insert unit 106 are bonded to the top and bottom of the core unit using high-strength, high-temperature resistant epoxy resin. After curing, the complete microengine device 100 is obtained.

[0083] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A micro-engine device with ultra-high energy density, characterized in that, include: Top cylinder housing (101), bottom cylinder housing (102), piston (104) disposed in combustion chamber (103) formed between the top cylinder housing (101) and the bottom cylinder housing (102), top insert unit (105) and bottom insert unit (106). A pair of microneedle electrodes (107) are disposed opposite to each other on the top cylinder housing (101) and the bottom cylinder housing (102) for generating a spark in the combustion chamber (103) to ignite the fuel; The combustion chamber (103) is connected to the external fuel supply system through the intake passage (108) and to the external environment through the exhaust passage (109); the piston (104) can reciprocate in the combustion chamber (103) under the drive of the combustion gas, and periodically open or close the ports of the intake passage (108) and the exhaust passage (109); The device housing has an observation window (110) on its side for directly observing the movement of the piston (104); The device is also provided with a pressure measurement channel (111), one end of which is connected to the combustion chamber (103) and the other end terminates at the pressure measurement port (112), which is used to connect a pressure sensor to monitor the combustion pressure.

2. The ultra-high energy density micro-engine device according to claim 1, characterized in that, The piston (104) is a one-piece micro-machined structure. Its main body includes a sealing ring (1041) that fits with the inner wall of the combustion chamber (103) and a flow divider (1042) extending downward from the sealing ring (1041). The geometry of the flow divider (1042) is configured such that when the piston (104) is near the bottom dead center, it can guide the fuel gas flowing in from the intake passage (108) upward and delay its direct flow to the exhaust passage (109), thereby optimizing the scavenging efficiency.

3. The ultra-high energy density micro-engine device according to claim 2, characterized in that, The piston (104) also includes a micro-spring structure (1043) integrated therein, the micro-spring structure (1043) having a total length of 10 mm, a single-turn spring width of 240 μm, and a number of turns of 7, which is used to provide restoring force after the expansion stroke and assist the piston (104) to return to the top dead center to complete the compression stroke.

4. The ultra-high energy density micro-engine device according to claim 1, characterized in that, The opening positions, shapes, and sizes of the intake passage (108) and exhaust passage (109) on the wall of the combustion chamber (103) are optimized by hydrodynamics. Specifically, in the two-dimensional projection model of the combustion chamber (103), when the piston (104) is at bottom dead center, the distance from the upper edge of the intake passage opening to the piston top surface is defined as A=1mm, the vertical height of the intake passage opening is B=0.5mm, the distance from the lower edge of the exhaust passage opening to the piston top surface is C=1mm, and the vertical height of the exhaust passage opening is D=1mm. This geometric configuration causes the fuel to flow in with a "high injection" flow state, which then changes to a "low injection" flow state, effectively reducing the short-circuit loss of fresh charge.

5. The ultra-high energy density micro-engine device according to claim 1, characterized in that, The tip spacing of the microneedle electrode (107) is adjustable, ranging from 10 to 100 micrometers, to adapt to the hydrogen ignition requirements under different mixing ratios and pressures; the electrode material is a high-temperature resistant and oxidation-resistant metal or alloy, preferably a platinum-iridium alloy or tungsten.

6. The ultra-high energy density micro-engine device according to claim 1, characterized in that, The main structural material of the top cylinder housing (101), bottom cylinder housing (102) and piston (104) is single crystal silicon, which utilizes its excellent mechanical properties and micromachining compatibility; the components are connected by gold-silicon eutectic bonding to achieve airtight sealing and mechanical connection.

7. The ultra-high energy density micro-engine device according to claim 1, characterized in that, The overall external dimensions of the device are: length 22.4 mm, width 26 mm, and thickness 8 mm; the equivalent diameter of the combustion chamber (103) is between 3 and 5 mm, and the stroke volume is less than 100 cubic millimeters, achieving a high degree of miniaturization.

8. A method for preparing an ultra-high energy density micro-engine device as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Prepare an SOI wafer, which includes a top silicon layer, a buried oxide layer and a substrate layer. The thickness of the top silicon layer and the substrate layer is 500 μm, and the thickness of the buried oxide layer is 3 μm. S2: A chromium mask layer is deposited and patterned on the surface of the top silicon layer and the substrate layer of the SOI wafer to form an etching mask corresponding to the three-dimensional structure of the top cylinder housing (101), the bottom cylinder housing (102) and the piston (104); S3: Using inductively coupled plasma reactive ion etching technology, the SOI wafer is subjected to double-sided deep etching using the chromium mask formed in step S2 as a mask until the buried oxide layer is stopped, thereby forming the preliminary main structure of the top cylinder housing (101), the bottom cylinder housing (102) and the independent piston (104) structure respectively; wherein, the etched structure includes the combustion chamber (103), the intake channel (108), the exhaust channel (109), the pressure measurement channel (111), the observation window (110) and the sealing ring (1041) and the distributor (1042) on the piston (104). S4: Remove the residual chromium mask and the buried oxide layer that serves as an etch stop layer to release the independent silicon structure components; S5: Fabrication of microneedle electrodes (107): A metal electrode pattern is formed on another silicon wafer or glass substrate by photolithography, evaporation / sputtering and lift-off processes, and a needle tip structure is formed by deep etching or micromachining. Then, it is precisely aligned and bonded to the designated positions of the top cylinder housing (101) and the bottom cylinder housing (102); or, an integrated microneedle electrode is formed directly on the silicon structure of the top / bottom cylinder housing by local doping and metallization processes. S6: Piston micro-spring integration: When etching to form the piston body in step S3, a mask is designed simultaneously, and the micro-spring structure (1043) is integrally processed inside the piston (104) or at the connection by controlling the etching parameters. S7: Surface metallization and bonding preparation: Using a precision metal mask prepared by laser cutting, gold layers are selectively vapor-deposited or sputtered on the bonding surfaces of the top cylinder housing (101) and the bottom cylinder housing (102) to form a bonding ring; S8: Component assembly and bonding: The piston (104) is placed in the combustion chamber (103) of the bottom cylinder housing (102), and then the top cylinder housing (101) and the bottom cylinder housing (102) are aligned and bonded. Under vacuum or inert atmosphere protection, a certain pressure and temperature are applied to perform gold-silicon eutectic bonding to form a sealed micro engine core unit. S9: Plug-in unit integration: The pre-prepared top plug-in unit (105) and bottom plug-in unit (106) are fixed to the upper and lower ends of the already bonded core unit by means of mechanical clamps, adhesive bonding or secondary bonding, to complete the packaging of the entire micro-engine device.

9. The preparation method according to claim 8, characterized in that, The inductively coupled plasma reactive ion etching described in step S3 uses the Bosch process, alternating etching and passivation cycles to obtain silicon microstructures with high aspect ratios and steep sidewalls.

10. The preparation method according to claim 8, characterized in that, The conditions for gold-silicon eutectic bonding in step S8 are: temperature between 350℃ and 400℃, pressure of 1-5MPa, and bonding time of 30-60 minutes, to ensure that the bonding interface has high airtightness and sufficient mechanical strength.

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