The Bridgman effect is a phenomenon that seems to occur when an electric current is passed through an anisotropic crystal. It is also known as the Internal Peltier effect.
Anisotropy is the property of a material that allows it to change or acquire different properties in different directions. It is just the opposite of isotropy. It can also be defined as a difference in structure when measured along different axes. The difference can be observed in a material’s physical or mechanical properties. An anisotropic crystal shows a difference in absorbance, refractive index, conductivity, and tensile strength.
Discovery of Bridgman effect
The effect was named after Percy Williams Bridgman. He was an American physicist who received the Nobel Prize in 1946 in Physics for his work on the physics of high pressures. He also wrote an effective article on the scientific methods and other aspects of the philosophy of science.
Both, the Bridgman effect and the Bridgman–Stockbarger technique are named after him. In the Bridgman effect, there is absorption or liberation of heat energy. This absorption or liberation is due to the non-uniformity in current distribution when current is passed through the material.
The Bridgman effect may be observable in geology also. It describes the slip-stick behavior of materials under very high pressure. The stick-slip phenomenon is the sudden jerking motion that is seen to occur when two objects are sliding over each other.
The Bridgman–Stockbarger method
The Bridgman–Stockbarger technique, is named after two physicists.
- Harvard physicist Percy Williams Bridgman, and
- MIT physicist Donald C. Stockbarger.
The Bridgman Stockbarger method includes two similar but slightly different techniques. The technique is primarily used for growing single crystal ingots. An ingot is a block of steel, gold, silver, or any other metal, which is rectangular or elliptical in shape. This method can also be used for solidifying polycrystalline ingots.
The methods involve heating a crystalline material above its melting point. After this, the material is slowly cooled from one end of the container, where a seed crystal is located. The process can be carried out in a horizontal or vertical orientation. It usually involves a rotating Stick structure to stir the melt.
The polycrystalline ingots can also be produced from a feedstock consisting of rods, chunks, or other irregularly shaped pieces. These pieces are melted once and allowed to re-solidify. The resultant microstructure of the ingots thus obtained is the characteristic of directionally solidified metals.
About Bridgman method
The Bridgman method is a popular way of making semiconductors. Certain semiconductor crystals such as gallium arsenide are made by this method. The traditional method of making gallium arsenide was the Czochralski method which is more difficult, hence the Bridgman method is useful here.
The process can reliably produce single-crystal ingots. But this has a drawback as well. This method does not necessarily result in uniform properties throughout the crystal.
There is a slight difference between the Bridgman technique and Stockbarger technique. The difference is so precise that sometimes it becomes difficult to identify.
Both methods utilize a temperature gradient- The Bridgman technique utilizes an uncontrolled temperature gradient produced at the end of the furnace. The Stockbarger technique has a proper shelf that separates two coupled furnaces. The temperatures of the two furnaces are above and below the freezing point.
The Bridgman crystal growth method is considered one of the simplest melt-based techniques. It has been used extensively for the growth of lead iodide crystals in the case of detector applications.
Every method has advantages as well as some disadvantages. The main advantage of the Bridgman method is its simplicity and its ease of implementation. The primary disadvantage of the Bridgman method is that the lead iodide crystal remains in contact with the surface of the ampoule. An ampoule is a small sealed glass capsule that contains the liquid in a measured quantity.
Contact with this can lead to the introduction of impurities. If the growing crystal sticks to the surface of the crucible, many thermal strains may be introduced after the crystal cools down. Despite of these disadvantages, fairly large single crystals of lead iodide have been grown by this method.