Аноден алуминий

Anodic alumina refers to an aluminum surface treated by electrochemical anodizing, producing an exceptionally durable and corrosion-resistant finish that never chips, peels or flakes – three times harder than standard aluminum and 60% lighter!

This article will delve into the fundamentals and uses of porous anodic alumina, including as a template for growing nanowires and nanotubes to produce metamaterials with unique properties.

Characteristics

Anodic porous alumina (NPA), also referred to as nanoporous anodized alumina (NAA), is an intriguing material with various applications due to its ordered and dense porous structures with pores in the range of nanometers in diameter. These pores have created new opportunities in fields like structural coloration and photonics while serving as templates for creating materials such as nanowires or nanotubes for developing metamaterials with tailored properties.

Research continues on the exact mechanisms by which these structures form, with particular interest focused on anodic alumina. It involves both chemical and electrochemical reactions, the former including direct injection of aluminum ions directly into an electrolyte solution while its electrical counterpart takes place within cracks within its oxide layer; composition of electrolyte solution and applied anodization potential are key factors when it comes to crack size.

Once cracks have formed, aluminum ions ejected into the electrolyte can interact with each other to create self-organized porous structures that depend on anodization potential and electrolyte type; their size and shape depend on factors like anodization potential as well as presence/absence of barrier layers.

As shown below, when high electric charges pass through an anoded substrate, its pores expand while their inter-pore distance decreases, leading to hexagonal arrays of pores as seen here; although similar arrays could have other shapes such as rectangular or square structures.

The morphology of pore arrays depends on their initial arrangement as pre-patterned concave pits on an anodic alumina surface and their inter-pore distances. If using graphite lattice patterns with 300nm inter-pore distances, oxide walls develop triangular shapes while honeycomb lattice patterns with 500nm inter-pore distances yield diamond-shaped pores; ultimately this shape influences light transmission and scattering properties of anodic alumina surfaces.

Applications

Since the work of Masuda and Moskovits, porous anodic alumina (AAO) has become an attractive nanofabrication platform for research in numerous fields. AAO nanotemplates have been utilized in the production of materials with specific properties in magnetism, thermoelectricity and thermoelctricity with reduced dimensions; AAO can easily be modified to produce various morphologies such as branching structures, modulated or three dimensional nanoporous structures.

AAOs’ morphological characteristics are determined by both its barrier layer and anodic potential. The barrier layer affects how quickly aluminum emits its ions into solution while anodic potential influences how quickly pores grow – their size depends on applied anodic potential, temperature, electrolyte composition and experimental conditions.

General, the larger and closer together pore diameters and interpore distances are, the faster anodic oxide will form. However, it should also be taken into consideration that their size can also depend on factors like surface chemistry that can be controlled through chemical etching or use of anodic precursors.

Further, it should be stressed that pore arrangement is determined not only by the shape of an indented pit but is also affected by initial pre-patterned concave pit arrangements on a substrate. For instance, when using FIB patterns of concaves with 300nm interpore distance on an Al substrate, triangular and rectangular oxide walls emerge after anodization (Figs 14a-14c).

At the oxide/electrolyte interface, oxalate species play an essential role in AAO formation. When combined with Al3+ ions being expelled from pores during MA conditions, these oxalate ions react with them to form water molecules which reduce resistance of barrier layers and facilitate further growth of pore matrix structures. By contrast, under HA conditions non-porous alumina formation results with higher resistance at barrier layer due to stress caused by volume expansion at metal/oxide interface.

Preparation

Production of porous alumina requires the anodization of aluminium in acidic electrolytes. This process involves oxygen ions migrating from solution onto metal surfaces and creating an insulator oxide barrier layer, due to high electrical resistance; only small current can pass through it while simultaneously acting as an insulating effect and stopping further surface evaporation.

Temperature, electrolyte composition and applied potential during anodization all play an integral part in producing different pore sizes; their structural parameters being the diameter and inter-pore distance. To create a more uniform structure of pores pulse anodization may provide.

Under this technique, anodization is interrupted after a certain time and restarted at a higher potential, thus increasing anodization time and producing thicker more porous alumina films.

Pulse anodization can also be used to produce branched or modulated pores by altering anodization conditions between MA and HA in specific sequences and changing pulse duration, producing pores with multiple diameters and high degrees of ordering.

Modifying the pH of anodizing solutions allows users to alter pore size distribution. Achieve this is possible by increasing or decreasing oxalate species concentration in an electrolyte; conversely, for smaller pores fewer species should be present in an electrolyte solution.

At another step to altering pore structures is selective etching process. This can be performed after anodization in a solution containing phosphoric acid and results in 3D porous alumina membrane with well-ordered pores even if MA conditions were used during anodization; making this method especially suitable for applications using sodium-vapor streetlights as gas containers.

Properties

Porous anodic alumina has garnered much research interest over recent decades due to its remarkable physical, chemical, and optical properties. Particularly impressive are structures comprising nanometer-scale features of this anodic material for designing optical devices like photonic crystals or lasers.

Formation of complex morphologies is determined by electrochemical reactions occurring at both metal/electrolyte interface and oxide/electrolyte interface, where an electric field generated across a barrier layer causes dissolution of oxide, release of Al3+ ions, volume expansion at metal/oxide interface, stress generation due to volume expansion at this interface and volumetric stress at metal/oxide interface depending on anodization potential, temperature, acid composition and experimental conditions.

Pulsed anodization is one of the most efficient techniques for creating highly controlled pore morphologies, enabling anodizers to adjust pore diameter and interpore distance by changing voltage settings; additionally, structural parameters of membranes formed can be adjusted by altering potential and time settings for each anodizer, leading to Morie patterns or staircase-case structures if desired.

Anodic alumina offers another advantage for functional materials development: its ability to control surface chemistry. By chemical etching or electrochemical deposition, it can create protective coatings on its surface; additionally, thermal treatment or nano-scratching may change its morphology or create protective layers on it.

Anodic Alumina provides an attractive platform for creating reduced dimensionality materials in magnetism, thermoelectricity and other fields, such as optical techniques. Furthermore, its versatility makes it a useful template for the growth of materials with various optical techniques combined properties. At InRedox we produce and provide Anodic Aluminum Oxide nanotemplates in different formats and specifications to researchers exploring science and technology opportunities based on this material; such templates may be used to investigate various applications including light guides and photonic crystals.