Preferential alignment

The preferential alignment is a criterion of an orientation of a molecule or atom. The preferential alignment can be related to the formation of the crystal structure of an amorphous structure.

Polymeric masses with high atomic distances can either be in an oriented or non oriented state. These higher distances (up to 1000 Å) form great regions, where the molecular chains may be preferentially oriented, something which can happen independent to the existence or not of crystallinity.

The crystallization of a material is significantly influenced by preferential alignment, which refers to the directional tendency of atoms or molecules to arrange themselves during solidification. This alignment is governed by factors such as temperature, pressure, composition, and external influences like magnetic or electric fields [Hoong CC]. The key characteristics of material crystallization based on different preferential alignments include:

1. Random Crystallization (No Preferential Alignment) Atoms or molecules arrange themselves without a dominant directional influence. Forms polycrystalline structures with random grain orientations. Common in metals that undergo rapid cooling or solidification.

2. Uniaxial Alignment (One Preferred Direction) Crystals grow preferentially along a single axis due to factors like temperature gradient or applied stress. Results in columnar grain structures, commonly seen in directional solidification processes (e.g., single-crystal turbine blades). Mechanical properties (e.g., strength, ductility) may differ along and across the alignment direction.

3. Biaxial or Planar Alignment (Two Preferred Directions) Atoms tend to align along two dominant directions, often influenced by strain, pressure, or epitaxial growth conditions. Results in textured materials, where grains have preferred orientations but are not fully single-crystal. Found in thin films and layered materials where deposition conditions control crystallization.

4. Polycrystalline Preferred Orientation (Texture Formation) Some grains align preferentially while others remain randomly oriented. Leads to anisotropic properties, meaning mechanical, electrical, or thermal behavior varies with direction. Common in rolled metals (e.g., steel sheets) and ceramics.

5. Epitaxial Growth (Highly Controlled Crystallization) Atoms deposit layer by layer on a pre-existing crystalline substrate with matching lattice orientation. Produces single-crystal thin films with superior electrical and optical properties. Used in semiconductor manufacturing (e.g., silicon wafers, GaN LEDs).

6. Magnetic or Electric Field-Induced Alignment Certain materials, especially ferroelectric or ferromagnetic, align based on applied external fields. Leads to engineered anisotropy, useful in memory storage, piezoelectric devices, and sensors. Each of these preferential alignments affects the final microstructure, mechanical strength, electrical conductivity, and thermal properties of the material.