Ice Physics. Sveinung Løset, prof. Department of Civil and Transport Engineering, Marine Civil Engineering Group, NTNU UNIS, Arctic Technology

Transcription

1 Ice Physics Sveinung Løset, prof. Department of Civil and Transport Engineering, Marine Civil Engineering Group, NTNU UNIS, Arctic Technology

2 The water molecule When oxygen makes 2 covalent bonds as in water, H 2 O, it is surrounded by four electron pairs. R OO = 2.97 Å,, R OH = 0.96 Å 2 bonding pairs and 2 lone pairs, in roughly tetrahedral arrangement (104.5 )

3 Hydrogen bonding Hydrogen bond a chemical bonding which arises when a H atom lies between 2 of the highly electronegative atoms F, O or N Accounts for the tetrahedral bonding of the molecules in ice In the H bond the H nucleus remains covalently bonded to one of the O atoms The strength of the hydrogen bond is intermediate between that of a covalent bond and the residual van der Waals interaction -> > a melting point mid-way between those of a covalent crystal like diamond and a rare gas like neon

4 Molecular structure

5 Continuous molecular structure

6 Defects in ice Defects Type Remarks Point defects Line defects Plane defects Vacancy Interstitial H 3 O + OH - D-defect L-defect Electron excitation Impurity molecule Dislocation Stacking fault Water molecule missing from ice structure Water molecule not at a structure position Water molecule with additional proton Water molecule with proton missing O-O bond with two protons on or near it. O-O bond with no proton on it Place where an electron is excited from its ground state Molecule other than H 2 O on a H 2 O site Boundary line of a region where part of the crystal has been displaced relative to another part A plane on which the stacking sequence is not what it should be in the structure

7 Defects in ice

8 Water -> Ice Triple point 3 phases are in equilibrium: T = K, p = Pa H 2 O expands on freezing Other examples: Silicone, germanium Max density at T = 3.98 C The crystals reveal the hexagonal symmetry of the crystal lattice of ice The c-axis c axis [0001] is the hexagonal axis

9 Development of ice cover Growth sequence

10 Growth sequence of ice crystals (a) (b)

11 Growing of isolated crystals Initial discs, size 1 mm Stellar ice crystals

12 Pancake ice

13 Phase relation of sea ice, S = 34.3 ppt

14 Sea ice crystal structure

15 Structure of first-year sea ice

16 Thin section showing cellular substructure Spacing between brine layers, ~ 0.6 mm

17 Types and stages of level ice Type New ice Nilas Pancake ice Young ice Subdivision Recently formed ice. > Frazil ice: Fine spicules or plates of ice, suspended in water. > Grease ice: A later stage of freezing than frazil ice when the crystals have coagulated. Grease ice reflects little light, giving the sea a matt appearance. > Slush: Snow which is saturated and mixed with water on land or ice surfaces. > Shuga: An accumulation of spongy white ice lumps, a few centimetres across. They are formed from grease ice or slush. A thin elastic crust of ice (< 10 cm thick), easily bending on waves and swell. > Dark nilas (< 5 cm thick). > Light nilas (> 5 cm thick). Predominantly circular pieces of ice from m in diameter, and up to about 10 cm in thickness, with raised rims due to the pieces striking against one another. It may be formed on a slight swell from grease ice, slush or shuga or as a result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, of grey ice. Ice in the transition stage between nilas and first-year ice, cm thick. > Grey ice young ice cm thick. Less elastic than nilas and breaks on swell. Usually rafts under pressure. > Grey-white ice young ice cm thick. Under pressure more likely to ridge than to raft. First-year ice Sea ice of not more than one winter s growth, developing from young ice, thickness 0.3 m 2 m. > Thin first-year ice ( m thick) > Medium first-year ice: first-year ice m thick. > Thick first-year ice: first-year ice over 1.2 m thick. Old ice Sea ice that has survived at least one summer s melt. Most topographic features are smoother than on first-year ice. > Second year ice: Old ice which has survived only one summer s melt. > Multi-year ice: Old ice up to 3 m or more thick which has survived at least two summer s melt.

18 Types of ice surface features Type Level ice Deformed ice Rafted ice Ridge Rubble Stamukha Hummock Hummocked ice Subdivision Sea ice which is unaffected by deformation. A general term for ice which has been squeezed together and in places forced upwards or downwards. Type of deformed ice formed by one piece of ice overriding another. A line of ice formed by pressure or shear: > New ridge: Ridge with sharp peaks and slopes of sides usually about 40 > Weathered ridge: Ridge with peaks slightly rounded and slope of sides usually Individual fragments are not discernible. > Very weathered ridge: Ridge with peaks very rounded, slope of sides usually > Aged ridge: Ridge which has undergone considerable weathering. > Consolidated ridge: A ridge in which the upper parts of the ridge has frozen together. Ice piles haphazardly one piece over another in the form of ridges or walls. Grounded ridge. A hillock of broken ice which has been forced upwards by pressure. May be fresh of weathered. The submerged volume of broken ice under the hummock, forced downwards by pressure, is termed a bummock. Sea ice piled haphazardly one piece over another to from an uneven surface. When weathered, has the appearance of smooth hillocks.

19 Physical and thermal properties Ice/snow Parameter Value Freshwater ice Density (kg/m 3 ) Porosity (%) Latent heat of fusion (kj/kg) Specific heat capacity (kj/kg C) Thermal conductivity (W/m C) Snow Density (kg/m 3 ) Latent heat of fusion (kj/kg) Specific heat capacity (kj/kg C) Thermal conductivity (W/m C)

20 Effects of temperature 1-E 2a,b- σ T 3a,b- σ c

21 Uniaxial strength vs. temperature

22 Tensile strength vs. temperature (V)

23 Tensile strength vs. temperature (H)

24 Salinity vs. depth 0 Salinity, g/l Depth, cm

25 Porosity η V b = V + V b a = S ( ) T V a = 1 ρ ρ 0

26 Brine volume vs. depth 0 Brine volume, % Depth, cm Core 1 Core 2

27 Uniaxial compressive strength vs. brine porosity

28 Uniaxial tensile strength vs. brine porosity

29 Columnar sea ice σ c = ε η ( MPa) H loaded σ c = ε η ( MPa) V loaded

30 Granular sea ice σ c = ε η ( MPa)

31 Flexural strength σ f = 1.76 exp 18.6 η ( MPa) FL h σ = = f Bh /12 2 6FL b b 3 2 Bh

32 Modulus of elasticity in bending 4FL 3 E f = Bh 3 Δ

33 Tensile strength (V and H) σ σ tv th η = ( MPa) η = ( MPa)

34 Shear strength (V and H) η τ = ( MPa) 39