Water |
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The water molecule, H2O, is composed of two hydrogen atoms and one oxygen atom. Diagrams
depicting the H2O molecule can be seen below.
depicting the H2O molecule can be seen below.
The H2O molecule is asymmetrical. The angle the O—H bonds make with each other is 104.5°. This means that because of the distribution of electrons around each atomic nucleus, the H2O molecule is polar, meaning it has a partial negative electronic charge in the vicinity of the exposed part of the oxygen atom, and a partial positive charge in the vicinity of the hydrogen atoms, see diagram (c). This drives physical and chemical activity of the water molecule.
Water exists in three physical states, solid, liquid, and vapor. Liquid water boils at or above 100°C (212°F), and eventually boils away entirely to the vapor phase. Liquid water freezes at or below 0°C (32°F) forming solid ice.
The overall gas composition contained within a perfectly sealed enclosure, as an enclosed system, which has no leaks and cannot exchange gas with the external environment, can contain water in any of the three physical states.
The state or states that may be present inside the enclosure are a function of the overall pressure of the contained gas, the number (concentration) of water molecules present in that gas, and the system temperature.
In the vapor phase H2O molecules “float” and move around freely in the space available. Molecular motion is a function of temperature. The higher the temperature, the further and faster molecules move around. In the liquid phase, as temperatures decline, H2O molecules become “condensed” together in much more dense concentrations. In that state molecules still move around, but much more slowly, while the overall physical state remains liquid. In the solid state as ice, or frost,molecular motion still occurs but is more restricted than even in the liquid state.
Dewpoint temperature determines the change of state between water being present as a vapor or as a liquid (condensate). Whether water molecules exist as liquid or vapor is a function of concentration of molecules and temperature. Generally, as temperature declines, the more molecules condense out as liquid. A nomograph defining dewpoint temperatures as a function of water vapor concentration is on p. 4. To use it, position a ruler on the pressure value on the right hand scale (at 14.7psi for typical 1 atmosphere conditions), position it at whatever the moisture concentration is known to be on the left-hand scale, and where it crosses dewpoint on the middle scale tells the temperature at or below where dew, frost, or liquid condensate will form. Conversely, if positioned at a dewpoint temperature, it will tell how much water vapor can be present before dew, frost, or liquid condensate forms.
Within the enclosed system, water molecules can be present not only as vapor, liquid, or ice, but as kind of intermediate entity in the form of adsorbate. In this form, individual “layers” of molecules can adhere to surfaces without forming discrete liquid condensate. Whether adsorbates form are a function of the surface condition of exposed substrate. Surface energy, texture, and chemical composition of the substrate surface can either attract or repel a layer or layers of adsorbate from its surroundings. The polarity of water molecules (mentioned above), with partial electrical charge distribution around the molecule, is also a factor in adsorbate formation.
Three or more monolayers of water molecules adhering to a surface, and bridging functional electrical structures, is the threshold condition for surface electrical conduction, which can lead to parametric instability of electronic devices or, depending on surface chemistry, could initiate material corrosion.
Water as liquid condensate on any surface is potentially chemically corrosive. If that occurs on an electrically conductive structure of an electronic device, the conductor may eventually lose enough material to chemical modification or complete removal as to form an electrical open, causing device failure. A contributing factor to this is not only presence of liquid condensate, but also surface cleanliness. Residual water soluble ionic contaminants, especially halide ions like chloride or alkali ions like sodium and potassium, can accelerate these kind of failure-causing reactions because of their tendency to form chemically corrosive solutions or increase the electrical conductivity of liquid condensate.
Within small enclosed microelectronic systems containing integrated circuits, MEMs devices, or optical components, to insure functional reliability and extended device lifetimes, it is essential that water vapor concentration be kept to a minimum. This is why military and aerospace specifications, e.g. Mil-Std 883, include Test Method 1018-10 “Internal Gas Analysis” for analyzing internal gas composition of hermetic devices. The maximum criterion for water vapor allowed by TM 1018 is 5,000 parts per million by volume (0.5 volume percent). It is also why Nasa invokes ASTM Method 595E “Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment” to qualify such products for space applications.
These particular test methods would not directly apply for larger electronic systems (vehicles, etc.). But the principles and needs for both moisture control and ionic cleanliness are exactly the same.
Water exists in three physical states, solid, liquid, and vapor. Liquid water boils at or above 100°C (212°F), and eventually boils away entirely to the vapor phase. Liquid water freezes at or below 0°C (32°F) forming solid ice.
The overall gas composition contained within a perfectly sealed enclosure, as an enclosed system, which has no leaks and cannot exchange gas with the external environment, can contain water in any of the three physical states.
The state or states that may be present inside the enclosure are a function of the overall pressure of the contained gas, the number (concentration) of water molecules present in that gas, and the system temperature.
In the vapor phase H2O molecules “float” and move around freely in the space available. Molecular motion is a function of temperature. The higher the temperature, the further and faster molecules move around. In the liquid phase, as temperatures decline, H2O molecules become “condensed” together in much more dense concentrations. In that state molecules still move around, but much more slowly, while the overall physical state remains liquid. In the solid state as ice, or frost,molecular motion still occurs but is more restricted than even in the liquid state.
Dewpoint temperature determines the change of state between water being present as a vapor or as a liquid (condensate). Whether water molecules exist as liquid or vapor is a function of concentration of molecules and temperature. Generally, as temperature declines, the more molecules condense out as liquid. A nomograph defining dewpoint temperatures as a function of water vapor concentration is on p. 4. To use it, position a ruler on the pressure value on the right hand scale (at 14.7psi for typical 1 atmosphere conditions), position it at whatever the moisture concentration is known to be on the left-hand scale, and where it crosses dewpoint on the middle scale tells the temperature at or below where dew, frost, or liquid condensate will form. Conversely, if positioned at a dewpoint temperature, it will tell how much water vapor can be present before dew, frost, or liquid condensate forms.
Within the enclosed system, water molecules can be present not only as vapor, liquid, or ice, but as kind of intermediate entity in the form of adsorbate. In this form, individual “layers” of molecules can adhere to surfaces without forming discrete liquid condensate. Whether adsorbates form are a function of the surface condition of exposed substrate. Surface energy, texture, and chemical composition of the substrate surface can either attract or repel a layer or layers of adsorbate from its surroundings. The polarity of water molecules (mentioned above), with partial electrical charge distribution around the molecule, is also a factor in adsorbate formation.
Three or more monolayers of water molecules adhering to a surface, and bridging functional electrical structures, is the threshold condition for surface electrical conduction, which can lead to parametric instability of electronic devices or, depending on surface chemistry, could initiate material corrosion.
Water as liquid condensate on any surface is potentially chemically corrosive. If that occurs on an electrically conductive structure of an electronic device, the conductor may eventually lose enough material to chemical modification or complete removal as to form an electrical open, causing device failure. A contributing factor to this is not only presence of liquid condensate, but also surface cleanliness. Residual water soluble ionic contaminants, especially halide ions like chloride or alkali ions like sodium and potassium, can accelerate these kind of failure-causing reactions because of their tendency to form chemically corrosive solutions or increase the electrical conductivity of liquid condensate.
Within small enclosed microelectronic systems containing integrated circuits, MEMs devices, or optical components, to insure functional reliability and extended device lifetimes, it is essential that water vapor concentration be kept to a minimum. This is why military and aerospace specifications, e.g. Mil-Std 883, include Test Method 1018-10 “Internal Gas Analysis” for analyzing internal gas composition of hermetic devices. The maximum criterion for water vapor allowed by TM 1018 is 5,000 parts per million by volume (0.5 volume percent). It is also why Nasa invokes ASTM Method 595E “Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment” to qualify such products for space applications.
These particular test methods would not directly apply for larger electronic systems (vehicles, etc.). But the principles and needs for both moisture control and ionic cleanliness are exactly the same.