Cooling device and cooling preparation method

Abstract

In a cooling device and a cooling preparation method, a pressure control device (16) causes the pressure in a tank (12) to become a first pressure (negative pressure) from an initial pressure (atmospheric pressure) so that a liquid refrigerant from an external container (14) is injected into the tank (12) (S18). After a certain amount of liquid refrigerant is injected into a tank (12) (S20), a pressure control device (16) changes the pressure in the tank from a first pressure to a second pressure (atmospheric pressure) (S22). Then, a spontaneous sudden boiling occurs in the liquid refrigerant in the tank (12) over the settling period (S26). The pressure in the tank (12) temporarily and sharply increases due to spontaneous sudden boiling, and bubbles in the liquid refrigerant in the tank (12) are liquefied. Thus, the cavity is inactivated.

Description

Technical Field

[0001] The present disclosure relates to a cooling device and a cooling preparation method, and particularly, to a cooling device having a tank for storing a liquid refrigerant. Background Art

[0002] A cooling device is a device for cooling a member to be cooled. Cooling devices are used in various fields. For example, in the observation of a specimen using a transmission electron microscope, a cooling device is used to maintain the temperature of the specimen at an extremely low temperature, or to maintain the temperature of a gas molecule trap at an extremely low temperature. The gas molecule trap is used to prevent specimen contamination or to improve the vacuum degree. A cooling device generally includes a tank for storing a liquid refrigerant such as liquid nitrogen (for example, refer to Japanese Utility Model Publication No. 5-45012).

[0003] When the liquid refrigerant in the above-mentioned tank absorbs heat from the outside, the liquid refrigerant boils. More specifically, the heat from the outside is transferred to the liquid refrigerant via the inner surface of the tank. There are many minute damages, depressions, etc. on the inner surface of the tank. It is known that bubbles are likely to be repeatedly generated at such damages, depressions, etc. Such damages, depressions, etc. are called cavities.

[0004] To explain in more detail, due to the heat from the outside, the liquid refrigerant in the cavity vaporizes to generate minute bubbles. When the bubble grows into a large bubble, the bubble detaches from the cavity and floats upward. After the large bubble detaches from the cavity, minute bubbles remain in the cavity. The minute bubbles grow into large bubbles. Once bubbles are generated in the cavity, the bubbles are repeatedly generated.

[0005] The repeated generation of bubbles in the tank causes the tank to vibrate. In particular, the repeated generation of bubbles at the lower part of the tank causes the tank to vibrate significantly. The vibration of the tank has an adverse effect on the device connected to the tank. For example, when the tank is connected to a transmission electron microscope, the vibration is transmitted from the tank to the transmission electron microscope. This vibration becomes a major obstacle to specimen observation. Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] An object of the present disclosure is to reduce the vibration of a tank for storing a liquid refrigerant. Alternatively, an object of the present disclosure is to deactivate cavities on the inner surface of a tank storing a liquid refrigerant.

[0008] Solutions for Solving the Problems

[0009] The cooling device of the present disclosure is characterized by including: a tank connected to a component to be cooled via a heat conduction member; and a pressure control device including a pump that sucks the gas in the tank, making the pressure in the tank a first pressure during a first period when a liquid refrigerant is injected into the tank, and making the pressure in the tank a second pressure higher than the first pressure during a second period following the first period, so that spontaneous bumping (explosive boil) of the liquid refrigerant in the tank occurs during the second period.

[0010] The cooling method of the present disclosure is characterized by including: a first step of reducing the pressure in a tank connected to a component to be cooled via a heat conduction member from an initial pressure to a first pressure, thereby reducing the boiling point of the liquid refrigerant injected into the tank; a second step of, after the first step, increasing the pressure in the tank from the first pressure to a second pressure, thereby increasing the boiling point of the liquid refrigerant in the tank; and a third step of, after the second step, as the temperature of the liquid refrigerant in the tank rises, causing spontaneous bumping of the liquid refrigerant in the tank, so that the bubbles in the liquid refrigerant in the tank are liquefied. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a diagram showing a configuration example of the cooling device of the embodiment.

[0012] Figure 2 It is a diagram showing an installation example of the cooling device of the embodiment.

[0013] Figure 3 It is a diagram showing the generation of bubbles generated in the cavity.

[0014] Figure 4 It is a diagram showing an operation example of the cooling device of the embodiment.

[0015] Figure 5 It is a diagram showing the temperature change and pressure change during the cooling preparation process.

[0016] Figure 6 It is a diagram showing the cooling device of the comparative example. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] Hereinafter, embodiments will be described based on the drawings.

[0018] (1) Summary of the Embodiment

[0019] The cooling device of the embodiment includes a tank and a pressure control device. The tank is connected to the component to be cooled via a heat conduction member. The pressure control device has a pump that sucks the gas in the tank. The pressure control device makes the pressure in the tank become a first pressure during a first period when injecting a liquid refrigerant into the tank. In addition, the pressure control device makes the pressure in the tank become a second pressure higher than the first pressure during a second period following the first period, so that the liquid refrigerant in the tank undergoes spontaneous nucleate boiling during the second period.

[0020] According to the above configuration, it is desirable that the boiling point of the liquid refrigerant in the tank is lowered during the first period, that is, the temperature of the boiling liquid refrigerant is lowered. During the subsequent second period, the pressure in the tank is increased, so that the boiling point of the liquid refrigerant in the tank is increased. That is, the boiling of the liquid refrigerant is temporarily suppressed. After such a temporary calming process, as the temperature of the liquid refrigerant rises due to the heat flowing in from the outside, the temperature of the liquid refrigerant in the tank reaches the boiling point again. However, since the inflow of heat is relatively slow, the boiling of the liquid refrigerant does not occur immediately, and a part of the liquid refrigerant becomes in an overheat state. Then, triggered by a small vibration or the like, a part of the overheated liquid refrigerant vaporizes all at once, that is, violent boiling (nucleate boiling) occurs. This is the above-mentioned spontaneous nucleate boiling.

[0021] Spontaneous nucleate boiling causes the pressure in the tank to rise significantly in a short time. The rapid rise in pressure liquefies the bubbles in the liquid refrigerant. That is, the bubbles in the cavity disappear, and the repeated generation of bubbles in the cavity is prevented. That is, the cavity is deactivated.

[0022] After spontaneous nucleate boiling occurs, if a large amount of heat flows into the liquid refrigerant, the cavity will be reactivated. Therefore, it is desirable to form a thermal equilibrium between the tank and the component to be cooled before the last spontaneous nucleate boiling. After that, even if heat flows into the liquid refrigerant from the outside, if the amount of heat is small, or if the heat flows into the upper part of the liquid refrigerant in the tank, the vibration caused by the boiling of the liquid refrigerant is usually small, and basically no problems caused by vibration will occur. In addition, the above-mentioned first pressure and second pressure are respectively the pressures set by the pressure control device and are equivalent to the baseline. Due to the boiling (including nucleate boiling) of the liquid refrigerant, pressure peaks are superimposed on the baseline.

[0023] In the embodiment, the first pressure is a pressure lower than the atmospheric pressure. The second pressure is the atmospheric pressure. In this configuration, initially, the pressure in the tank is set to a negative pressure (negative pressure), and then the pressure in the tank returns to the atmospheric pressure. If the atmospheric pressure is used, the configuration of the pressure control device becomes simple, and the control of the pressure becomes easy. In the embodiment, at the beginning of the first period, the pressure in the tank changes from the initial pressure (atmospheric pressure) to the first pressure.

[0024] In addition, on the premise of the inflow of heat from the outside, in order to cause the liquid refrigerant to boil spontaneously, an operation of raising the boiling point of the liquid refrigerant from the first temperature to the second temperature becomes important. Therefore, the first temperature can also be determined as an arbitrary temperature, and the second temperature can be determined based on this.

[0025] The cooling device according to the embodiment includes a sensor that detects the liquid level height of the liquid refrigerant in the tank. The pressure control device determines the timing for changing the pressure in the tank to the second pressure based on the output signal of the sensor. A part of the liquid refrigerant injected into the tank is vaporized due to its boiling. It is difficult to determine the liquid level (liquid volume) of the liquid refrigerant in the tank according to the supply amount of the liquid refrigerant. According to the above configuration, the pressure in the tank can be controlled based on the actual liquid level height of the liquid refrigerant.

[0026] The cooling device according to the embodiment has: an outer container into which a liquid refrigerant is injected; and a supply pipe for transporting the liquid refrigerant in the outer container to the tank. The first pressure is a pressure lower than the atmospheric pressure. During the first period, the liquid refrigerant in the outer container is taken into the tank via the supply pipe. According to this configuration, due to the negative pressure in the tank, the liquid refrigerant is naturally taken into the tank. Therefore, the injection operation of the liquid refrigerant becomes easy. The negative pressure is a pressure lower than the atmospheric pressure.

[0027] In the embodiment, the outer container is provided at a position lower than the tank. According to this configuration, the filling operation of the liquid refrigerant into the outer container becomes easy. By avoiding working at heights, the safety of the operator is improved.

[0028] In the embodiment, a porous member is provided at the suction port of the supply pipe. According to this configuration, several advantages can be obtained. First, the porous member can function as a filter. That is, foreign matters such as ice can be prevented from entering the tank. Second, when the gas generated in the tank flows out from the porous member in a state where the porous member is exposed, the surface of the porous member will be covered by the gas. Thus, the entry of air into the supply pipe is blocked. Third, due to the porous member, the flow path resistance increases. Therefore, even if an on-off valve is not provided in the middle of the supply pipe and closed, a large pressure rise can be generated in the tank during spontaneous boiling.

[0029] In the embodiment, the pressure control device includes: a suction pipe provided between the pump and the tank; and a detector that detects the temperature at a detection position on the suction pipe. Based on the temperature detected by the detector, it is determined whether the liquid refrigerant overflows from the tank. According to this configuration, the overflow of the liquid refrigerant can be determined with a simple configuration. In the embodiment described later, the control unit functions as an overflow determination device.

[0030] In an embodiment, the pressure control device includes a relief valve that operates when the pressure in the tank reaches a specified pressure. The relief valve operates when the pressure exceeds the pressure at which the cavity can be deactivated and the pressure in the tank exceeds a specified value (upper limit value). With this configuration, safety can be improved.

[0031] The cooling preparation method of the embodiment includes a first step, a second step, and a third step. In the first step, the pressure in the tank connected to the component to be cooled via the heat conduction member is reduced from the initial pressure to the first pressure. As a result, the boiling point of the liquid refrigerant injected into the tank is lowered. In the second step, after the first step, the pressure in the tank is increased from the first pressure to the second pressure. As a result, the boiling point of the liquid refrigerant in the tank is increased. Thus, the boiling of the liquid refrigerant in the tank is temporarily suppressed. In the third step, after the second step, as the temperature of the liquid refrigerant in the tank rises, spontaneous nucleate boiling occurs in the liquid refrigerant in the tank. Thus, the bubbles in the liquid refrigerant in the tank are liquefied. That is, the cavity on the inner surface of the tank is deactivated. By suppressing the repeated generation of bubbles in the liquid refrigerant in the tank, the vibration generated in the tank can be reduced in the cooled state of the component to be cooled.

[0032] (2) Details of the embodiment

[0033] Figure 1 A configuration example of the cooling device of the embodiment is schematically shown. The cooling device of the embodiment is, for example, a device for cooling a component to be cooled included in an electron microscope. The cooling device of the embodiment can also be used for other purposes. In the embodiment, the liquid refrigerant is specifically liquid nitrogen. Other liquid refrigerants can also be used.

[0034] In Figure 1 this, the cooling device 10 includes a tank 12, an outer container 14, a pressure control device 16, a supply pipe 20, and a suction pipe 22. The tank 12 is a container for storing liquid nitrogen. The side wall of the tank 12 is made of, for example, stainless steel, and the bottom wall of the tank 12 is made of, for example, copper. The tank 12 has a vacuum insulation structure 50.

[0035] The bottom surface 12A of the tank 12 is connected to a heat conduction member 18. The heat conduction member 18 is connected to a component to be cooled (not shown). The heat conduction member 18 is made of, for example, a copper plate, a copper wire mesh, or the like. The component to be cooled is a specimen, a gas molecule trap, or the like.

[0036] A temperature sensor 44 is provided on the outer surface of the lower part of the tank 12. A temperature sensor 46 is provided on the outer surface of the upper part of the tank 12. Based on the height of the inner bottom surface of the tank 12, the temperature sensor 44 is provided at the first height, and the temperature sensor 46 is provided at the second height. The second height is a reference height for determining the timing for judging the stop of the supply of liquid nitrogen. The temperature sensor 44 is provided to detect the presence or absence of liquid nitrogen in the tank 12. The control unit 34 described later determines that the liquid level of the liquid nitrogen has reached or exceeded the second height based on the output signal of the temperature sensor 46.

[0037] The outer container 14 is a dewar for storing liquid nitrogen. The outer container 14 has a vacuum insulation structure or an insulating member 60. The outer container 14 has an opening 14A, and liquid nitrogen (refer to reference numeral 52) is injected into the outer container 14 through the opening 14A. A temperature sensor 56 is provided on the inner surface of the lower part of the outer container 14, and a temperature sensor 58 is provided on the inner surface of the upper part of the outer container 14. Specifically, based on the height of the inner bottom surface of the outer container 14, the temperature sensor 56 is provided at the third height, and the temperature sensor 58 is provided at the fourth height. The temperature sensors 56 and 58 may also be provided on the outer surface of the outer container 14.

[0038] The temperature sensor 56 is provided to determine that there is a certain amount or more of liquid nitrogen in the outer container 14. More specifically, it is used to confirm that the liquid level of the liquid nitrogen is at a position higher than the upper surface of the porous member 54 described later. The temperature sensor 58 is provided to prevent the overflow of liquid nitrogen from the outer container 14.

[0039] An insulating member is provided on the outside of the supply pipe 20, but its illustration is omitted. The supply pipe 20 is composed of a first part 20A extending in the vertical direction, a second part 20B extending in the horizontal direction, and a third part 20C extending in the vertical direction.

[0040] The lower end opening (suction port) of the first part 20A is covered by a porous member 54. The porous member 54 has a plurality of micropores, and each micropore allows the passage of liquid and gas. The porous member 54 is made of, for example, sintered metal. The porous member 54 may also be made of ceramics or the like.

[0041] The porous member 54 functions as a filter. That is, the passage of foreign substances such as ice is blocked by the porous member 54. In addition, the porous member 54 is combined with the small inner diameter of the supply pipe 20, which helps to increase the flow path resistance. After liquid nitrogen is injected into the tank 12, nitrogen gas from the tank flows out from the surface of the porous member 54 to the outside. When the amount of liquid nitrogen in the outer container 14 decreases, the porous member 54 is exposed, but since the porous member 54 is surrounded by nitrogen gas, the entry of air into the supply pipe 20 is blocked. At the same time, dew condensation and icing on the surface of the porous member 54 are prevented.

[0042] As shown in the figure, the outer container 14 is provided at a position lower than the tank 12. Specifically, the bottom wall of the outer container 14 is provided at a position lower than the bottom wall of the tank 12. It is also possible to inject liquid nitrogen into the tank 12 from a position higher than the tank 12, but in this case, the operator needs to work at a high place. According to the configuration of the embodiment, the operator only needs to work at a low place, so the burden on the operator can be reduced, and the safety of the operator can be improved.

[0043] The lower end portion 40 of the third part 20C in the supply pipe 20 enters the tank 12. This lower end portion 40 functions as a spray nozzle.

[0044] The suction pipe 22 has a first part 22A extending in the vertical direction, a second part 22B extending in the horizontal direction, a U-shaped part 24C belonging to the detection part 24, etc. An insulating member is also provided outside the suction pipe 22 as needed. The U-shaped part 24C is composed of a part extending downward, a bent part, and a part extending upward. A plurality of devices described in detail below are arranged after the U-shaped part 24C. The lower end portion 42 of the first part 22A enters the tank 12, and this lower end portion 42 functions as a suction nozzle.

[0045] The detection part 24 detects the overflow of liquid nitrogen from the tank 12. The detection part 24 has a temperature sensor 62 provided on the outer surface (bottom surface) of the bottom part of the U-shaped part 24C. When liquid nitrogen enters the U-shaped part 24C, the temperature detected by the temperature sensor 62 drops sharply. Thereby, it is determined as an overflow in the control part 34 described later. The U-shaped part 24C is made of a material with good thermal conductivity (such as copper).

[0046] Next, the pressure control device 16 will be described. The pressure control device 16 has a pressure reducing valve 26, a flow rate adjusting valve 28, an opening / closing valve (solenoid valve) 30, a vacuum pump 32, and also has a control part 34 for controlling their operations.

[0047] The pressure reducing valve 26 is a safety valve. The pressure reducing valve 26 is normally in a closed state and automatically opens when the pressure in the suction pipe 22, i.e., the pressure in the tank 12, reaches a specified pressure (upper limit value). The specified pressure is, for example, 0.03 MPa (0.3 atmospheres). The pressure reducing valve 26 can also be provided at other positions as long as its function can be exerted. The flow rate adjusting valve 28 is a valve for adjusting the flow rate of the gas flowing through the suction pipe 22. In the embodiment, the flow rate adjusting valve 28 functions during the suction process.

[0048] The vacuum pump 32 is a suction device for sucking the gas in the tank 12 to make the pressure in the tank 12 lower than the atmospheric pressure. The vacuum pump can be operated only during suction, or the vacuum pump 32 can be operated all the time. A buffer tank can also be provided in front of the vacuum pump 32. Various pumps can be used as the vacuum pump 32. A pump for evacuating the inside of the column of a transmission electron microscope can also be used as the vacuum pump 32.

[0049] The control unit 34 is composed of a computer, a microcomputer, etc. Specifically, the control unit 34 is composed of a processor that executes a program. The operation of the on-off valve 30 and the operation of the vacuum pump 32 are controlled by the control unit 34. The operation of the flow rate adjusting valve 28 can also be controlled by the control unit 34. Output signals from the temperature sensors 44, 46, 56, 58, 62 are input to the control unit 34. The control unit 34 controls the operation of the cooling device 10 based on these output signals.

[0050] Specifically, the control unit 34 determines the presence of liquid nitrogen in the tank 12 based on the output signal of the temperature sensor 44. In addition, the control unit 34 determines that the liquid nitrogen level has reached the second height based on the output signal of the temperature sensor 46. As described later, the control unit 34 changes the pressure in the tank from the first pressure (negative pressure) to the second pressure (atmospheric pressure) at a timing after a specified time has elapsed since the timing when it is determined that the liquid level has reached. The control unit 34 determines the overflow of liquid nitrogen based on the output signal of the temperature sensor 62. In the case of liquid nitrogen overflow, an alarm is output.

[0051] The control unit 34 determines that a certain amount or more of liquid nitrogen has entered the outer container 14 based on the output signal of the temperature sensor 56. The control unit 34 supplies liquid nitrogen from the outer container 14 to the tank 12 only when a certain amount of liquid nitrogen has entered the outer container 14. The control unit 34 determines that the liquid nitrogen level has reached the fourth height (upper limit of the liquid level height) based on the output signal of the temperature sensor 58. In this case, an alarm is output.

[0052] The control unit 34 is connected to an operation panel 36 as an input device and is also connected to a display 38. The operation panel 36 includes a plurality of switches, a plurality of buttons, etc. The display 38 is composed of, for example, a liquid crystal display.

[0053] The supply pipe 20 is relatively long and has a relatively small inner diameter. Further, a porous member 54 is provided at the suction port of the supply pipe 20. Therefore, the flow path resistance in the supply pipe 20 is large. An on-off valve 30 is provided on the suction pipe 22. When the on-off valve 30 is in the closed state, the inside of the tank 12 is close to a closed space. In this state, due to spontaneous ebullition described later, a rapid pressure rise occurs in the tank 12 in a short time. Conversely, the length and inner diameter of the supply pipe 20, the porosity of the porous member 54, etc. are determined in such a way that such a pressure rise is generated.

[0054] If moisture enters the tank 12 and ice is generated in the tank, it is likely to cause relatively large bubbles for this reason. Therefore, a heater (not shown) is provided outside the bottom of the tank 12. Before supplying the liquid refrigerant, if necessary, by energizing the heater, the moisture in the tank 12 can be released to the outside as water vapor. A heater (not shown) is also provided outside the upper part of the outer container 14. When frost or ice adheres to the upper part of the outer container 14, by energizing the heater, the frost and ice adhering to the upper part of the outer container 14 can be removed.

[0055] Figure 2 An installation example of the cooling device according to the embodiment is shown. The transmission electron microscope 70 has a lens barrel 72 that extends in the vertical direction (up and down direction). Inside the lens barrel 72, an electron gun, a focusing lens, an objective lens, a specimen chamber, etc. are provided. A specimen 78 to be observed is held by a specimen holder 80. Near the specimen 78, a gas molecule trap 82 is provided. The gas molecule trap 82 is provided to capture contaminants to prevent specimen contamination and to increase the vacuum degree in the specimen chamber.

[0056] The object to be cooled by the cooling device 10 is the gas molecule trap 82. The tank 12 is connected to the gas molecule trap via a heat conduction member 18. The gas molecule trap 82 is fixed in the specimen chamber via a member with low heat conductivity (such as glass).

[0057] The transmission electron microscope 70 has a base (horizontal plate) 74 supported by a base 76. An outer container 14 is provided on the base 74. The supply pipe provided between the outer container 14 and the tank 12 has a portion that extends relatively long in the vertical direction. In Figure 2 The illustration of the pressure control device is omitted.

[0058] Figure 3Shows the function of the cavity. (A) to (G) show the growth process of the bubble in stages. There is a very small depression (cavity) 102 on the inner surface 100 of the tank. Small bubbles 104 are generated in the cavity. The bubble gradually grows (refer to reference numerals 106, 108, 110, 112). As shown in (G), the grown bubble 114 detaches from the cavity and floats upward. At this time, small bubbles 116 remain in the cavity. This bubble 116 becomes the source of the next large bubble. Once a bubble is generated in the cavity, bubbles will be repeatedly generated.

[0059] Figure 4 The operation example of the cooling device according to the embodiment is shown in a flowchart. Figure 5 Shows the change in the temperature of the liquid refrigerant and the change in the pressure inside the tank during the cooling preparation process. First, the operation example will be described, and then the temperature change and pressure change will be described in detail.

[0060] In the initial state, the inside of the tank is in an empty state, or an indefinite amount of liquid refrigerant remains in the tank. The pressure inside the tank in the initial state (initial pressure) is the atmospheric pressure. Hereinafter, for the sake of convenience of explanation, it is assumed that the tank is in an empty state in the initial state.

[0061] In Figure 4 S10 shows the cooling preparation process, and S28 shows the cooling process. First, in S12, an operator injects a specified amount of liquid nitrogen into the external container. When the liquid level height of the liquid nitrogen exceeds the upper limit height (or when a specified time has elapsed since the time of the excess), an alarm (sound, light, image, etc.) is output in S14. After injecting a specified amount of liquid nitrogen into the external container, in S16, the operator operates the supply start button inside the operation panel.

[0062] In the state where the vacuum pump is operating, when the supply start button is operated, the control unit causes the on-off valve to open. Then, the pressure inside the tank changes from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). The first pressure can be adjusted by changing the operation amount of the flow rate adjustment valve. In addition, when the liquid level height of the liquid nitrogen in the external container is lower than the first liquid level height (lower limit), the supply of liquid nitrogen does not start. When the same situation occurs during the supply of liquid nitrogen, the supply of liquid nitrogen is stopped.

[0063] When the pressure in the tank changes from atmospheric pressure to the first pressure, the liquid refrigerant in the outer container starts to move towards the tank. In a state where the supply pipe and the tank are not sufficiently cooled, first, liquid nitrogen in the supply pipe violently vaporizes. Then, even when liquid nitrogen enters the tank, the liquid nitrogen will violently vaporize. When the tank contains air, the nitrogen gas generated by vaporization displaces the air, and the air in the tank is replaced by nitrogen gas. As a result, the water vapor contained in the air is discharged to the outside. When the pressure in the tank exceeds the specified pressure due to the vaporization of liquid nitrogen, the pressure reducing valve automatically operates. When the supply pipe and the tank are cooled to a certain extent, liquid nitrogen starts to accumulate in the tank.

[0064] In S20, it is detected that the liquid level of liquid nitrogen in the tank has reached the second height (specified height). In S22, at a timing after a specified time has elapsed since the timing when the reaching is detected, the control unit stops the supply of liquid nitrogen. Specifically, the on-off valve is closed. As a result, the pressure in the tank changes from the first pressure to the second pressure (atmospheric pressure).

[0065] Then, after a certain stabilization period, in S26, spontaneous nucleate boiling occurs multiple times in the liquid refrigerant in the tank. The mechanism of spontaneous nucleate boiling will be described in detail later. Due to spontaneous nucleate boiling, typically the relatively large spontaneous nucleate boiling that occurs last, the pressure in the tank temporarily rises sharply, and the bubbles contained in the liquid refrigerant liquefy. In the experiment, it was confirmed that the bubbles liquefy due to a pressure increase of 0.005 MPa (0.05 atm) or more. As a result, the cavities existing on the inner surface of the tank are deactivated. The repeated generation of bubbles caused by the cavities no longer occurs, or the repeated generation of bubbles decreases.

[0066] In S28, the component to be cooled is stably cooled by the cooling device. In addition, in order to deal with the situation where the liquid level at the second height is not detected due to a defect in the temperature sensor provided at the second height, a detection unit for detecting overflow is provided. It is also possible to determine that the supply is incorrect in S24 and stop the supply of the liquid refrigerant when the elapsed time since the output signal of the temperature sensor provided at the second height is lower than the threshold exceeds a certain time. Alternatively, it is possible to determine that the supply is incorrect in S24 and stop the supply of the liquid refrigerant when the liquid level is not detected at the second height even after a specified time has elapsed since the start of the supply of liquid nitrogen.

[0067] In addition, when an indefinite amount of liquid refrigerant remains in the tank in the initial state, spontaneous nucleate boiling can also occur by performing the above control.

[0068] Figure 5The upper part shows the temperature change during the cooling preparation process. The horizontal axis is the time axis, and the vertical axis is the temperature axis. Reference numeral 84 shows the temperature detected by the temperature sensor (lower temperature sensor) set at the first height. Reference numeral 86 shows the temperature detected by the temperature sensor (upper temperature sensor) set at the second height. Ta shows the boiling point of the liquid refrigerant under atmospheric pressure.

[0069] Figure 5 The lower part shows the change in the pressure inside the tank during the cooling preparation process. The horizontal axis is the time axis, and the vertical axis is the pressure axis. D1 shows the injection period (the first period), and D2 shows the subsequent period (the second period) following the injection period D1. pa shows the atmospheric pressure.

[0070] In a state where the vacuum pump is operating, the on-off valve is opened at time t0. Then, the pressure inside the tank changes from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). Thereby, the supply of the liquid refrigerant to the tank is started. That is, in a state where the atmospheric pressure is applied to the liquid refrigerant in the external container, when the pressure inside the tank becomes the first pressure lower than the atmospheric pressure, due to the pressure difference, the liquid refrigerant in the external container starts to move into the tank via the supply pipe. The pressure inside the tank is maintained at the first pressure. The gas attraction force, the weight of the liquid nitrogen in the supply pipe 20, and the flow path resistance of the supply pipe 20 contribute to the formation and maintenance of the first pressure.

[0071] In a state where the supply pipe and the tank are not sufficiently cooled, first, the liquid refrigerant in the pipe violently boils, and then the liquid refrigerant in the tank violently boils. After the liquid nitrogen or nitrogen gas from the external container starts to enter the tank, the temperature detected by the lower temperature sensor drops sharply, and slightly later than this, the temperature detected by the upper temperature sensor drops sharply.

[0072] During the period t1, as described above, violent boiling occurs in the tank (refer to reference numeral 88). During the period t1, in the process of the tank (especially the bottom wall) and the heat conduction member reaching an extremely low temperature, the heat conductivity increases sharply, so that the heat inflow into the tank increases sharply, and as a result, the initial boiling 90 occurs. When the heat capacity of the member to be cooled is relatively large and it is not sufficiently cooled, the heat inflow becomes large, and sometimes film boiling occurs exceeding nucleate boiling. When liquid nitrogen is supplied to a tank that is at room temperature and in an empty state, the initial sudden boiling 90 is likely to occur. In contrast, when liquid nitrogen remains in the tank and the tank is cooled, such an initial sudden boiling 90 is less likely to occur.

[0073] After storing a large amount of liquid nitrogen in the tank, especially after it becomes full, when initial flashing occurs, the liquid nitrogen will overflow from the tank and the liquid nitrogen will be wasted. Therefore, the flow rate etc. is adjusted so that initial flashing occurs before the tank becomes full.

[0074] After initial flashing 90 occurs, the amount of heat flowing into the liquid nitrogen in the tank is relatively large, and the boiling (nucleate boiling) of the liquid nitrogen will continue.

[0075] At time t2, following the lower temperature sensor, the detected temperature of the upper temperature sensor is also lower than the boiling point Ta of the liquid refrigerant under atmospheric pressure. The detected temperatures of the two temperature sensors respectively show the actual boiling points of the liquid refrigerant at each height. At time t2, when the liquid level of the liquid refrigerant is detected by the upper temperature sensor, at time t3 after a predetermined time has elapsed from this detection timing, the on-off valve is closed, and the pressure in the tank is switched from the first pressure (negative pressure) to the second pressure (atmospheric pressure). As a result, the supply of the liquid refrigerant to the tank is stopped. The bubbles in the liquid refrigerant liquefy when the pressure rises.

[0076] In addition, overall, the detected temperature of the upper temperature sensor is higher than that of the lower temperature sensor because when the stirring of the liquid nitrogen in the tank is not very large, the colder liquid nitrogen is heavier than the warmer liquid nitrogen.

[0077] During period t4, the inflow of heat from the outside continues, but since the temperature of the liquid refrigerant in the tank is lower than the boiling point of the liquid refrigerant under atmospheric pressure, the liquid refrigerant basically does not boil and becomes quiescent. However, due to the heat flowing in from the outside, the temperature of the liquid nitrogen gradually rises, and its temperature approaches the boiling point of the liquid nitrogen under atmospheric pressure. At time t5, the temperature detected by the upper temperature sensor reaches the boiling point of the liquid nitrogen under atmospheric pressure.

[0078] Then, at time t5, at the inner surface of the upper part of the tank, the temperature of the liquid nitrogen reaches the boiling point. However, since the inflow of heat is relatively stable, the liquid nitrogen does not boil immediately, and a part of the liquid nitrogen becomes superheated. In such a situation, triggered by a small vibration etc., a part of the superheated liquid nitrogen vaporizes all at once. Specifically, in the illustrated example, from period t6 to period t8, multiple spontaneous flashes 92 occurred. At time t7, the temperature detected by the lower temperature sensor reached the boiling point of the liquid nitrogen under atmospheric pressure.

[0079] At time t9, the final spontaneous boiling-over 94 occurred. This occurred after the temperature detected by the lower temperature sensor reached the boiling point of liquid nitrogen at atmospheric pressure. The final spontaneous boiling-over 94 was a relatively large boiling-over. After the final spontaneous boiling-over 94 occurred, no spontaneous boiling-over occurred. Each spontaneous boiling-over, especially the final large spontaneous boiling-over 94, causes a large pressure rise in the tank for a short time. Due to each spontaneous boiling-over, especially the final large spontaneous boiling-over 94, the bubbles in each cavity on the inner surface of the tank are liquefied. That is, each cavity is deactivated. Figure 5 Pb in Figure 5 indicates the pressure threshold. When a pressure rise exceeding pb occurs, each cavity is effectively deactivated. As described above, pb is 0.005 MPa (0.05 atm), but its value can vary according to various conditions. For example, pb can be set to 0.003 MPa (0.03 atm), or pb can be set to 0.007 MPa (0.07 atm).

[0080] In order to deactivate each cavity by the final large spontaneous boiling-over 94 and maintain this state, it is desirable that the thermal equilibrium between the tank and the cooled member occurs before the final spontaneous boiling-over 94. For example, when the thermal equilibrium with the cooled member occurs at the time point 45 minutes after the start of the liquid nitrogen supply, various conditions should be determined in such a way that the final spontaneous boiling-over 94 occurs at a time later than 45 minutes. The flow rate can also be adjusted by a flow rate adjustment valve to meet such conditions. For example, when the flow rate is increased, that is, when the exhaust speed is increased, the flow path resistance of the supply pipe becomes larger, whereby the first pressure becomes lower and the temperature of the liquid nitrogen in the tank is further lowered. In this case, after the supply of liquid nitrogen is stopped, the time when the temperature of the liquid nitrogen reaches the boiling point of liquid nitrogen at atmospheric pressure can be postponed. However, if the temperature of the liquid nitrogen is arbitrarily lowered, the liquid nitrogen may freeze when the pressure in the tank is returned to atmospheric pressure. It is desirable to adjust the temperature of the liquid nitrogen so as not to cause such a problem.

[0081] After period t8, the inflow of heat into the tank continues and the boiling of the liquid nitrogen in the tank continues. However, after the thermal equilibrium between the tank and the cooled member occurs, the heat flowing into the upper part of the tank becomes dominant. Therefore, the liquid nitrogen boils mainly in the upper part of the tank, specifically in the part near the liquid level on the inner surface of the tank. The vibration caused by the bubbles generated near the liquid level is smaller than the vibration caused by the bubbles generated in the lower part of the tank, and the mechanical influence on the tank is small. In addition, since the evaporation of the liquid refrigerant at the liquid level takes away heat, the boiling stop state may also continue to some extent.

[0082] In the experiment, taking the start time of the liquid nitrogen supply as a reference, thermal equilibrium occurred after 45 minutes, and then the final spontaneous boiling occurred after 65 minutes. Then, a low vibration state was generated. This low vibration state was maintained for a long time until the liquid nitrogen in the tank disappeared.

[0083] Figure 6 A comparative example is shown. The cooling device of the comparative example is connected to a transmission electron microscope. The cooling device has a tank 120. When injecting liquid nitrogen into the tank 120, a funnel 124 is used. The connection part 122 between the funnel 124 and the tank 120 is connected via a bent pipe. In this state, liquid nitrogen is injected into the funnel 124. When using the cooling device of the comparative example, the injection operation of liquid nitrogen must be carried out at a high place. In addition, the operation burden is also large.

[0084] In contrast, according to the cooling device and the cooling preparation method of the embodiment, the injection operation does not need to be carried out at a high place, so the safety of the operator is improved. The burden during the operation can also be reduced. However, the inactivation of the cavity caused by spontaneous boiling may occur regardless of the installation position of the external container. In the embodiment, the cooling device is connected to the cooled member in the transmission electron microscope, but the cooling device can also be connected to the cooled member in other devices. It is also possible to reduce the pressure in the tank during or after the injection of liquid nitrogen, and then increase the pressure in the tank, thereby causing the liquid refrigerant to boil spontaneously.

Claims

1. A cooling device (10), characterized in that: Include: a tank (12) connected to a cooled member via a heat conducting member (18); and A pressure control device (16) includes a pump (32) for sucking gas from the tank (12), causing the pressure in the tank (12) to become a first pressure during a first period of injecting liquid refrigerant into the tank (12), and causing the pressure in the tank (12) to become a second pressure higher than the first pressure during a second period following the first period, thereby causing the liquid refrigerant in the tank (12) to spontaneously boil during the second period.

2. The cooling device (10) according to claim 1, characterized in that: The first pressure is a pressure lower than the atmospheric pressure.

3. The cooling device (10) according to claim 2, characterized in that: The second pressure is atmospheric pressure.

4. The cooling device (10) according to claim 1, characterized in that: comprising a sensor (46) for detecting the liquid level of the liquid refrigerant in the tank (12), The pressure control device (16) determines the timing for changing the pressure in the tank (12) to the second pressure based on the output signal of the sensor (46).

5. The cooling device (10) according to claim 1, characterized in that: Include: an outer container (14) filled with liquid refrigerant; and a supply pipe (20) for conveying the liquid refrigerant in the external container (14) to the tank (12), The first pressure is a pressure lower than atmospheric pressure, During the first period, the liquid refrigerant in the external container (14) is taken into the tank (12) via the supply pipe (20).

6. The cooling device (10) according to claim 5, characterized in that The outer container (14) is disposed at a lower position than the tank (12).

7. The cooling device (10) according to claim 5, characterized in that A porous member (54) is provided at the suction port of the supply pipe (20).

8. The cooling device (10) according to claim 1, characterized in that The pressure control device (16) comprises: a suction pipe (22) provided between the pump (32) and the tank (12); and a detector (62) for detecting the temperature at a detection position on the suction pipe (22), Based on the temperature detected by the detector (62), overflow of liquid refrigerant from the tank (12) is determined.

9. The cooling device (10) according to claim 1, characterized in that: The pressure control device (16) includes a pressure reducing valve (26) that operates when the pressure in the tank (12) reaches a predetermined pressure.

10. A cooling preparation method, characterized in that: Include: A first step (S18) of reducing the pressure in a tank (12) connected to a cooled member via a heat conducting member (18) from an initial pressure to a first pressure, thereby reducing the boiling point of a liquid refrigerant injected into the tank (12); A second step (S22), after the first step (S18), of increasing the pressure in the tank (12) from the first pressure to a second pressure, thereby increasing the boiling point of the liquid refrigerant in the tank (12); and In the third step (S26), after the second step, as the temperature of the liquid refrigerant in the tank (12) rises, the liquid refrigerant in the tank (12) undergoes spontaneous sudden boiling, thereby liquefying the bubbles in the liquid refrigerant in the tank (12).

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