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发布日期:2021年05月14日
What is Solidification?

The change of matter from the liquid state to the solid-state at a particular temperature is called solidification or freezing.

This occurs when the temperature of a liquid is lowered below its freezing point. Although most materials' freezing point and melting point are the same temperatures, this is not the case for all substances, so freezing point and melting point are not interchangeable terms.

Solidification is always an exothermic process that means heat is released when a liquid changes into a solid. The only known exception to this rule is the solidification of low-temperature helium. Energy must be added to helium-3 and helium-4 for freezing to take place.

For example, agar chemical used in the food and the laboratory displays a hysteresis in its melting and freezing point. It melts at 85°C (185°F) and solidifies from 32°C to 40°C (89.6°F to 104°F).

Solidification is based on casting technology and an important feature of some other processes, including crystal growth, welding, surface alloying, ingot production, materials purification and refining. It is an important process in metals technology and a part of ceramics and polymers' technologies.

What is Solidification

In solidification, a solid phase is nucleated and grows with a crystalline structure. Where a solid crystalline phase does not nucleate in the cooling process, and glassy structures are formed. Several examples of solidification may be found in everyday life, such as:

  • Freezing of water to form ice
  • Formation of snow
  • Congealing of bacon grease as it cools
  • Solidification of melted candle wax
  • Lava hardening into solid rock

Crystallization

Most liquids freeze by crystallization, the formation of crystalline solid from the uniform liquid. This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the whole system's temperature remains nearly equal to the melting point due to slow removal of heat when in contact with air is a poor heat conductor.

Because of the latent heat of fusion, the freezing is greatly slowed, and the temperature will not drop anymore once the freezing starts but will continue dropping once it finishes. Crystallization consists of two major events, nucleation and crystal growth.

  1. Nucleation is the step wherein the molecules start to gather into clusters on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure.
  2. Crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. The thermodynamics of freezing and melting is a classical discipline within physical chemistry, which nowadays develops in conjunction with computer simulations.

Supercooling

Crystallization of pure liquids usually begins at a lower temperature than the melting point due to the high activation energy of homogeneous nucleation.

The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is expended to form this interface, based on the surface energy of each phase.

If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei.

In the presence of irregularities on the surface of the containing vessel, solid or gaseous impurities, pre-formed solid crystals may occur. Some energy is released by the partial destruction of the previous interface, raising the supercooling point near or equal to the melting point.

For example, the melting point of water at 1 atmosphere of pressure is very close to 0 °C (32 °F, 273.15 K). In the presence of nucleating substances, the freezing point of water is close to the melting point, but in the absence of nucleators, water can supercool to -40 °C (-40 °F, 233 K) before freezing. Under high pressure, water will supercool to as low as -70 °C (-94 °F, 203 K) before freezing.

Exothermicity

Freezing is almost an exothermic process that means as liquid changes into solid, heat and pressure are released. This is often seen as counter-intuitive since the material's temperature does not rise during freezing, except if the liquid were supercooled. But this can be understood since heat must be continually removed from the freezing liquid, or the freezing process will stop.

The energy released upon freezing is a latent heat and is known as the enthalpy of fusion and is the same as the energy required to melt the same amount of the solid.

For example, low-temperature helium is the only known exception to the general rule. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be added to these substances to freeze them.

Vitrification

Certain materials, such as glass and glycerol, may harden without crystallizing; these are called amorphous solids.

Amorphous materials and some polymers do not have a freezing point, as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in their viscoelastic properties over a range of temperatures.

Such materials are characterized by a glass transition at a glass transition temperature, roughly defined as the material's density vs. temperature graph's knee point. Because vitrification is a non-equilibrium process, it does not qualify as freezing, which requires equilibrium between the crystalline and liquid state.

Freezing of Living Organisms

Many living organisms can tolerate prolonged periods at temperatures below the freezing point of water. Most living organisms accumulate cryoprotectants such as anti-nucleating proteins, polyols, and glucose to protect themselves against frost damage by sharp ice crystals.

Most plants, in particular, can safely reach temperatures of -4 °C to -12 °C. Certain bacteria, notably pseudomonas syringae, produce specialized proteins that serve as potent ice nucleators. They use to force ice formation on the surface of various fruits and plants at about -2 °C.

The freezing causes injuries in the epithelia and makes the nutrients in the underlying plant tissues available to the bacteria.

1. Bacteria

There are three species of bacteria,

  • Carnobacterium pleistocenium.
  • Chryseobacterium greenlandensis.
  • Herminiimonas glaciei.

These have reportedly been revived after surviving for thousands of years frozen in ice.

2. Plants

Many plants undergo a hardening process, which allows them to survive temperatures below 0 °C for weeks to months.

3. Animals

The nematode haemonchus contortus can survive 44 weeks frozen at liquid nitrogen temperatures.

Other nematodes that survive at temperatures below 0 °C include

  • Trichostrongylus colubriformis
  • Panagrolaimus davidi

Many species of reptiles and amphibians survive freezing. Human gametes and 2-, 4- and 8-cell embryos can survive freezing and are viable for up to 10 years, a process known as cryopreservation.

Experimental attempts to freeze human beings for later revival are known as cryonics.

Food Preservation

Freezing is a common food preservation method that slows both food decay and the growth of microorganisms.

Besides the effect of lower temperatures on reaction rates, freezing makes water less available for bacteria growth.

1. The quality of frozen food depends on

  • Rate of freezing (°C/hr)
  • Ambient storage (freezing medium) temperature (Ta)
  • The constancy of temperature (cycling of temp. is not good)

2. Factors affecting the rate of freezing (°C/hr)

  • Convective heat transfer coefficient (h)
  • Ambient storage (freezing medium) temperature (Ta)

3. Advantages of rapid freezing

  • Smaller ice crystals are formed.
  • Thus, less structural damage to the product.
  • Prevents concentration (of sugars, fats etc.)

4. Freezing time

  • Time is taken to freeze the majority (~95%) of the product.
  • A product is never completely frozen (~5?10% unfrozen)

Purpose of freezing of foods

The freezing of food is to slow down rates of detrimental reactions by lowering temperature and water activity, such as:

  • Microbial spoilage
  • Enzyme activity
  • Nutrient loss
  • Sensorial changes
  • Prolongs shelf life beyond that of refrigerated foods

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