This implies the need for shielding the end with helium and liquid nitrogen, which in turn leads to complications for an explosive pellet discharge. The analysis of these problems suggested alternate experiments using nitrogen instead of oxygen.
Nitrogen does not have the fire hazards of oxygen and has similar freezing temperatures and molecular forces, so that its strength and behavior should be similar to that of oxygen. By crimping the end of a stainless steel tube and pushing a rubber stopper down to the end, a freezing cup was created. Liquid nitrogen LN 2 was then poured into the tube and allowed to solidify at the bottom on the order of an hour.
A solid nitrogen test consisted of 1 quickly withdrawing the tube from the dewar, 2 inverting it over a LN 2 cooled block, 3 Waiting a few seconds until the heat transfer to the tube melted the side of the solid nitrogen plug and it dropped out of the tube, 4 quickly manipulating the plug to a test position, and 5 performing a short mechanical or visual test on the plug.
The plug survived about 5 seconds before total evaporation. Clear, condensation-free, visual access was obtained by taking advantage of the density stability of the cold gas that prevented water vapor from interfering with inspection through the open top of the apparatus. The most interesting insight gained by manipulating these plugs is that they are clearly plastic at higher temperatures.
In the case of a cylinder placed between two supports over a LN 2 bath, it would initially maintain its position in response to a downward force at the center and then bend into a U shape.
A similar plastic behavior was seen in response to impact with a hammer. The cylinder would squash but never shatter or splatter. This was very surprising behavior that implied that the solid nitrogen is a plastic material of moderate strength.
These observations confirmed the behavior of the solid during the more sophisticated shear tests. The solid can be clear or white, depending on the condensation history - the white color arises from included voids. A cryogenic mechanical testing apparatus 8 was used to measure the shear forces needed to move a metal rod embedded in solid oxygen. A Sintech Model computer-integrated mechanical strength testing system was used in combination with a Janis 10CNDT cryostat capable of operation between room temperature and 1.
Figure 1 shows a schematic of the assembled apparatus. The cryostat is a standard model with vacuum insulation between a liquid nitrogen dewar surrounding a liquid helium dewar, in turn surrounding a gaseous helium test volume that contains the mechanical testing apparatus.
The primary design difficulties for this experiment were to: 1 develop an apparatus that could be easily assembled yet support significant mechanical shearing forces, 2 fit the apparatus within the small cryostat test volume 5 cm diameter, 12 cm long , 3 provide enough heat capacity and thermal conductivity to liquefy then solidify the oxygen in a reasonable time, 4 distribute the loading on the solid to give accurate shear measurements, 5 provide entry and exit for the oxygen gas through the cryostat's thermal barriers and make the flow rate high enough to fill the cup relatively quickly, 6 provide a precisely metered oxygen supply source, 7 contain the oxygen so it only condenses in the cup and not on the colder surfaces of the dewar, and 8 provide temperature monitoring and temperature control to be able to perform experiments at different temperatures.
Procedures also had to be developed to feed the oxygen gas, liquefy it, solidify it, and then achieve the test temperature. Some of the lessons learned were: 1 The gas feed tube is easily blocked if its temperature is below the oxygen solidification temperature, 2 The oxygen is density stable in a cold helium bath and the gas diffusion rates are very slow, so gaseous oxygen can easily be kept in an open cup without significant losses to condensation on farther away colder surfaces; a vacuum container is not needed, 3 To minimize solidification time it is necessary to have enough heat capacity in the supporting structure to solidify the oxygen than to rely on convective and conductive heat transfer from cold external helium gas, 4 Thermal equilibration times for the solid oxygen can be long if the solid dimensions approach 1 cm, making experiments long, 5 The assembly, thermal, structural, space, liquid containment, shearing geometry, and motion constraints on the shearing apparatus lead to a very complicated design, 6 Proper condensation of the oxygen is crucial.
Oxygen condensation on parts of the cryostat where it was not wanted prevents accurate filling of the shearing volume and can lead to an explosion in the vacuum pump during pumpdown. Condensation in the shearing apparatus might also cause additional solid contact that would confuse the shear measurement.
An important aspect of the solidification of oxygen is the supply of thermal energy needed to solidify the oxygen from the gas. Cooling through solid oxygen is very slow, so the volume of oxygen to be used was minimized by a metal insert in the solidification cup to keep the solidification time as short as possible. The insert also absorbed much of the heat of condensation. On a volume basis the specific heat of available materials at 67 K does not vary with material, so the most convenient material - aluminum - was chosen.
Care was taken to assure that shearing did not take place against the aluminum. The oxygen needed to fill the cup was estimated to result in a K warm up of the structure, such that if solidification is begun with the shearing apparatus at 60 K, the structure remains cold enough to keep the oxygen from boiling 90 K , after which the liquid could be solidified by thermal conduction.
The minimum temperature of the structure was 55 K to prevent blockage of the gas inlet tube by solid formation this happened quickly in practice at lower temperatures. The overall shearing apparatus Fig. The configuration of the cryogenic shearing fixture is also shown in Fig. This fixture consists of an inner central tension fixture designed to assure strictly axial loading of the puller rod in the solid, and an outer stress-transmitting shell ending in the oxygen cup.
Brass, copper, and aluminum are the materials of choice for use with oxygen. The copper cup was sized to accommodate a wide range of shearing diameters, depending on the actual shear strength of the oxygen. Sealing was minimal, using only as-machined metal to metal contact. The shearing plate was shaped to provide a shearing edge and to assure that overfilling the cavity with oxygen will not affect the shear results.
The central rod was a commercial high strength stainless steel 1. Measurements versus temperature were done to explore the temperature dependence of the shear strength. Oxygen for solidification was supplied from a This tank was filled from a standard, high purity oxygen cylinder and was warmed to room temperature after gas transfer to eliminate the effects of expansion cooling during gas transfer.
The tank was connected to the cryostat with a 0. Since the tube end was in the bottom of the condensation cup any gas reaching the end liquefied as it bubbled up though the liquid oxygen already in the cup. The liquefaction, solidification, and shearing processes were all performed under one atmosphere of helium. Oxygen was bled into the cryostat for 5 - 10 minutes while the temperature of the copper cup was monitored to be sure it was much colder than 90 K, keeping the oxygen vapor pressure low.
In practice the transfer was begun at a structure temperature of approximately 60 K; during condensation this temperature never rose above 70 K. Before assembling the apparatus the volume in the copper cup was measured by filling with water and weighing, as was the volume of the gas tank.
For each puller geometry repeat tests and tests at different temperatures were performed by shearing the solid, warming to liquefy, replacing the puller, solidifying again, and shearing again. After a series of tests the apparatus was allowed to warm up to room temperature after boiling off the cryogenic liquids, and the oxygen was vented to the room.
The specimen chamber that contained the oxygen was always first flushed with helium if evacuation was necessary. After the oxygen testing apparatus was assembled and inserted into the cryostat, the cryostat was first cooled to K with liquid nitrogen.
Liquid helium was then transferred into the cryostat. When the temperature of the copper oxygen container reached about 65 K, oxygen gas was bled into this container through the tygon tubing and liquefied. A precise gas volume of oxygen was added so that the volume of the desired liquid oxygen would fill the oxygen container to a liquid level about a millimeter above the bottom surface of the brass shear disk G in Fig.
The condensed oxygen is shown as item E in the figure. In the case of the cylindrical bolt and the shear rods, 18 ml. When the radially slotted shear rod was used, 20 ml. A pressure drop of 78 kPa in the 20 liter storage tank resulted in a transfer of During liquefaction of oxygen, the temperature was maintained between 62 and 68 K by bleeding appropriate amounts of liquid helium from the cryostat reservoir through a capillary tube into the testing chamber.
This temperature was selected to be as far as possible below the vaporization temperature of Thus, 3, joules were required to cool room temperature oxygen to 18 ml. It was estimated that the rise in temperature of the metal containing the liquid oxygen based on its heat capacity would have been about After liquefying the oxygen, the test chamber was cooled to the desired testing temperature and held at that temperature for 15 minutes to ensure that the solid oxygen was at the same temperature as the copper oxygen container, as measured by the silicon diode sensor.
The elapsed time between a phase change and a test was typically on the order of 30 minutes. A mechanical test was then performed. After shear testing the solid oxygen at a strain rate of 0. Then the oxygen test apparatus was recooled for an additional test. Solid oxygen shear tests were performed using 1 a stainless steel bolt with standard coarse threads, 2 a smooth stainless steel smooth rod, 3 a teflon TFE coated stainless steel smooth rod, and 4 a shear rod with multiple grooves.
A stainless steel bolt was used for preliminary tests to assess the type of deformation that would occur during shear tests of solid oxygen so that further experiments could be designed more accurately.
This is why the air is so dry in very cold places. Then carbon dioxide would freeze, and then nitrogen. The last gases to freeze would be oxygen and argon. At absolute zero, all the atoms in the different gas molecules would merge into one atom see the question this week on absolute zero.
For scientists living at the South Pole in the dead of winter it can get as cold as minus 80 degrees Celsius. This makes it very hard for them to breath outside without a special air supply. At such cold temperatures, the carbon dioxide freezes and drops out of the air.
Remember that it is the amount of carbon dioxide in our blood, not the amount of oxygen, that triggers our brain to take a breath. The scientists would pass out simply by "forgetting" to breathe. Yes, you can freeze air, and yes, each ingredient of air will freeze at a different temperature , so that if you were to take a jar of air and slowly make it colder and colder, each different ingredient would freeze into a different layer, just as you suggest.
Water, for example which is often present in air as humidity , freezes at 32 degrees Fahrenheit, or 0 degrees Celsius. Molecules in the liquid escape as a gas at the same rate at which gas molecules stick to the liquid, or form droplets and become part of the liquid phase.
Figure 4. Equilibrium between liquid and gas at two different boiling points inside a closed container. Because there are more molecules in the gas, the gas pressure is higher and the rate at which gas molecules condense and enter the liquid is faster. As a result the gas and liquid are in equilibrium at this higher temperature. This temperature is the boiling point at that pressure, so they should exist in equilibrium. The gas surrounding an open pot is not pure water: it is mixed with air.
What about water at This temperature and pressure correspond to the liquid region, yet an open glass of water at this temperature will completely evaporate.
Again, the gas around it is air and not pure water vapor, so that the reduced evaporation rate is greater than the condensation rate of water from dry air. If the glass is sealed, then the liquid phase remains. We call the gas phase a vapor when it exists, as it does for water at The ice and liquid water are in thermal equilibrium, so that the temperature stays at the freezing temperature as long as ice remains in the liquid. Once all of the ice melts, the water temperature will start to rise.
Vapor pressure is defined as the pressure at which a gas coexists with its solid or liquid phase. Vapor pressure is created by faster molecules that break away from the liquid or solid and enter the gas phase. The vapor pressure of a substance depends on both the substance and its temperature—an increase in temperature increases the vapor pressure.
Partial pressure is defined as the pressure a gas would create if it occupied the total volume available. In a mixture of gases, the total pressure is the sum of partial pressures of the component gases , assuming ideal gas behavior and no chemical reactions between the components. Thus water evaporates and ice sublimates when their vapor pressures exceed the partial pressure of water vapor in the surrounding mixture of gases.
If their vapor pressures are less than the partial pressure of water vapor in the surrounding gas, liquid droplets or ice crystals frost form. Is energy transfer involved in a phase change? If so, will energy have to be supplied to change phase from solid to liquid and liquid to gas? What about gas to liquid and liquid to solid? Why do they spray the orange trees with water in Florida when the temperatures are near or just below freezing? Yes, energy transfer is involved in a phase change.
We know that atoms and molecules in solids and liquids are bound to each other because we know that force is required to separate them. So in a phase change from solid to liquid and liquid to gas, a force must be exerted, perhaps by collision, to separate atoms and molecules.
Force exerted through a distance is work, and energy is needed to do work to go from solid to liquid and liquid to gas.
This is intuitively consistent with the need for energy to melt ice or boil water. The converse is also true. Going from gas to liquid or liquid to solid involves atoms and molecules pushing together, doing work and releasing energy. Heat, cool, and compress atoms and molecules and watch as they change between solid, liquid, and gas phases. Figure 5. The phase diagram for carbon dioxide. The axes are nonlinear, and the graph is not to scale. Dry ice is solid carbon dioxide and has a sublimation temperature of — Skip to main content.
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