Products > Fujihokka Alloy > Frequently Asked Questions

This section offers brief explanations on common queries raised by users on Fujihokka, its applications and benefits.

"Fujihokka is Radiant Emission!"

1. Question : What is Fujihokka?

1.i. Definition of Fujihokka

Fujihokka is a surface treatment process developed and patented by Fujikura Ltd of Japan. Conventional anodising techniques are used to form the primary film but the subsequent unique film modifications give Fujihokka its exceptionally high average emissivity of 90% (recorded range of 88~94%).

As the base aluminium alloy is needed solely for the purpose of forming the layer of aluminium oxide (a ceramic) on its surface, Fujihokka treatment is largely a surface effect and does not affect or alter the properties of the base alloy.

Fujihokka treatments are used primarily to improve thermal performance of aluminium alloys. However, in addition to high emissivity, there are many ways this process can be altered to derive various chemical and physical properties.

Fujihokka is a generic term for a range of materials, all having excellent emissivity but which physical and chemical properties vary according to the base alloy used. There are many options available using suitable base alloys but treatment parameters are usually selected to suit particular applications or customer requirements.

However, the primary Fujihokka materials would be the Premium Grade (PG). PG is suitable for most engineering applications that involve heating, drying and cooling.

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2. Question : How does Fujihokka work?

2.i. Thermal Equilibrium by Convection, Conduction and Radiation

Thermal energy is transferred either by conduction, convection or radiation. Conduction is generally most efficient followed by radiation with convection the least efficient.

When a material is placed in a hotter environment, its temperature will initially be lower than the surrounding environment, i.e. it is not at thermal equilibrium. The introduction of the cooler object into the hotter environment will create a convection (airflow) process where heat is transferred from the hotter environment (through air) to the object. This is relatively inefficient.

Once absorbed by the material, thermal energy will spread throughout its mass by conduction. Hence, the thermal conductivity of the material will determine how fast heat energy is being distributed throughout its mass. When the material has attained the same temperature as the environment, it will be in thermal equilibrium i.e. no heat energy is transferred.

However, just because a material has good thermal conductivity e.g. aluminium or copper, it does not mean that it has good emissivity (ability to absorb & emit thermal energy efficiently) as well. Hence, the above explanation does not account for the effect of radiant energy.

If the material has a high emissivity, (surface effect), it can absorb, or emit, radiative energy very efficiently. This would imply that thermal equilibrium with its environment could be achieved much quicker i.e. thermal energy is transferred more efficiently.

The best way to illustrate this is to consider three equally sized sheets; steel, untreated aluminium and Fujihokka treated aluminium, all having the same surface temperature of e.g. 200 C. The emissivities of steel and the untreated aluminium are relatively low (10-15%), when compared to Fujihokka (90%).

As radiative thermal energy (Q) is directly proportionately to emissivity (refer to the Stefan-Boltzman equation), Fujihokka will be radiating much more thermal energy than the untreated aluminium and steel, even though all three materials are at the same surface temperature.

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2.ii. Far Infrared Radiation

All materials above absolute zero emit far infrared radiation, which is the vital component for all heating, drying and cooling processes. As materials cool they emit IR radiation (the principle of how IR cameras function). Far infrared radiation consists of electromagnetic waves within the 2~1000 micron wavelength spectrum.

These waves are strong enough to excite the molecular vibration of materials and the lattice vibration of a crystal. The waves in the 2~30 micron region are the most important in heating, cooling and drying applications.

The inherent frequency of a material is dependent on its chemical structure. When this frequency synchronises with that of far infrared radiation, the far infrared radiation is absorbed causing a rise in temperature of the material. For example, materials such as water, epoxy and acrylic resins all absorb energy within the 2.5~16 micron region.

Since energy level is inversely proportional to wavelength, the shorter the wavelength, the higher the energy level. Hence, if the right quality of thermal energy can be applied, i.e. at infrared radiation below 30 micron wavelength, heating, drying or curing processes will become more efficient.

However, inorganic materials have lower infrared absorption compared to organic materials, such as oil and soot, which absorbs infrared strongly. It is also not recommended that the Fujihokka surface be painted as this will adversely affect its thermal performance. This is because most paints absorb strongly in the far infrared region.

For cosmetic purposes, Fujihokka treatment can be modified such that the surface is in colours (gold, bronze, natural etc.) other than black. However, a slight reduction in emissivity (~3%) may be experienced.

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2.iii. Reflectivity & Emissivity

The surface finish of Fujihokka is pre-determined by the amount of etching the aluminium is given during the treatment process. As the Fujihokka surface is (usually) black and highly emissive, the reflectivity is very low (~10%). Heavy etching will produce a matt finish, which will have even lower reflectivity.

If the surface of a material is very reflective (i.e. a mirror), there is little contribution from radiant energy because it will absorb only a small amount of the heat and reflect the rest. However a reflective surface will only reflect the low-grade thermal energy that is incident upon it.

A totally non-reflective surface that does not reflect any of the incident radiation it is exposed to will be a black body, the perfect absorber and emitter of electromagnetic radiant energy. For Fujihokka, because it operates best below 10 micron, it is able to absorb and emit high-grade energy without increasing the source load.

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2.iv. Measuring Emissivity

There are two methods to measure emissivity of a material. One is to derive from empirical measurement of surface reflectivity and the other is to measure the actual far infrared thermal emission.

The method using surface reflectivity is based on the assumption that, at a given temperature and assuming no energy is absorbed, the emissivity of a material is based on the degree of reflectivity. For example, the material is irradiated with infrared and the reflected infrared measured. The assumption is that any infrared energy not reflected must be radiated. Hence a material having reflectivity of 2% (i.e. a matt black paint) will have an emissivity of 98%.

Although this method is relatively simple and inexpensive, it does not show the true reality in terms of material emissivity. During the initial stages when Fujikura Europe was establishing manufacture of Fujihokka in the UK, the reflectivity method was used to check the emissivity of the treated material, showing an emissivity value of more than 90%. However, when this material was sent to Japan for testing on emissivity, the actual emissivity values, based on actual infrared emission measurements, was only ~73%. This simple reflectivity method therefore tends to over estimate emissivity.

The reflectivity method also tends to support the theory that emissivity improves with how black and how non-reflective a surface is. However, one of the best high emissivity paints tested by Fujikura is actually white in colour. This contradicts the conventional wisdom since white paint would have higher reflectivity than black paint and hence should have a lower emissivity.

In order to measure the true emissivity of a material, a more accurate method is to use an FTIR spectrophotometer modified for emission measurements. Comparison is made with an actual National Physical Laboratory accredited black body source (not a computer generated black body curve). Samples are heated, to +/- 1 C accuracy, and the actual radiant far infrared emission measured. Dividing this value by the black body emission at the same temperature gives the emissivity of the material.

In addition, it is not sufficient to quote emissivity at a given temperature. The relevant wavelengths must also be quoted as the intensity of the radiant energy is inversely proportionate to wavelength. It is relative easy to achieve high emissivity above 10 micron wavelength but not so easy below 10 micron, which is where Fujihokka radiates strongly and most materials absorb strongly.

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2.v. Fujihokka Oven Lining

To show how Fujihokka can help improve thermal efficiency, one can line an existing convection oven with one-side treated PG sheet materials over the existing stainless steel walls. The Fujihokka treated side is to be lined facing inwards so as to radiate thermal energy inwards.

When the oven is turned on, the Fujihokka lining will heat up very quickly due to the high emissivity of the Fujihokka treatment (efficient emission and absorption of thermal energy) and high thermal conductivity of the aluminium substrate. Hence, the desired operating temperature can be achieved quicker than before i.e. reduces heat-up time.

When lining the oven with Fujihokka, no attempt is made to achieve an excellent thermal contact between the Fujihokka sheet and the existing steel wall. Hence, the conduction of thermal energy from the aluminium substrate into the steel wall is adversely affected both by the poor contact and the relatively poor thermal conductivity of the steel.

It is true that eventually, the steel wall will come into thermal equilibrium with the Fujihokka sheet. When it does, heat can only be lost from the outer steel wall by natural convection and radiation, which are both inefficient due to the slow airflow and low emissivity of steel respectively. Hence, the Fujihokka lining creates an 'insulation effect' and helps to reduce heat loss from the oven.

Fujihokka will always strive to maintain the same temperature as its surroundings. Hence, when the oven temperature drops below that of the Fujihokka lining, far infrared electromagnetic waves are radiated back into the oven at light velocities until thermal equilibrium is attained. This effect is far quicker than any conductive heat loss from the aluminium substrate to the steel wall.

Hence, Fujihokka has become a second heat source which radiates thermal energy from within its mass back into oven, rather than retaining it, once there are any temperature differentials. As this reaction happens almost instantaneously, Fujihokka responds to temperature changes faster than the oven's thermostatic controls can. This will result in the oven's heaters running less frequently and directly reduce energy consumption.

Fujihokka does not merely reflect the thermal energy that is incident upon its surface. It instead absorbs whatever low-grade energy (>10 micron wavelength) from convection and starts to radiate high-grade energy (<10 micron wavelength) when it becomes hotter than its environment. As most materials absorb infrared energy very efficient below 10 micron wavelength, there are distinct possibilities that Fujihokka oven lining will help to reduce process time, meaning that the components actually experience less heat than with conventional convection ovens. It is not uncommon for ovens used, for example, in resin/paint curing applications to be operated at a lower temperature after Fujihokka lining, yet still maintaining or indeed improving product quality.

In summary, by lining an oven with Fujihokka sheet material, one can expect to reap benefits from the reduction in heat-up time, lower energy consumption and possible reduction in process time.

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3. Question : What are the other advantages of Fujihokka?

3.i. Scratch/Wear Resistance

Sheet material is usually supplied with a polyethylene (PE) protective sheet on the surface. This sheet has low tack so that it remains in place during handling etc. However, this can be easily stripped off later.

Scratches that do occur will not seriously affect performance, either mechanically or thermally. However, a number of things can be done to alter the hardness/wear resistance of the Fujihokka coating.

Making the coating thicker will give a longer service life under certain operational conditions but the process conditions can also be altered to produce a harder wearing coating. The 'standard' Fujihokka process is used to maximise thermal performance but not to maximise wear resistance. If the customer requires high wear resistance, it can be done but depending on the alloy used, a very slight (~3%) drop in emissivity may be experienced.

The best Vickers hardness that Fujihokka treatment can achieve is ~600, which is about the hardest an anodised coating can be. The highest-grade chrome plated treatments have Vickers values of ~2000.

However, hardness is often not a true measure of durability. The durability of Fujihokka can be significantly improved (~3 times) by impregnation with a low-friction material. This impregnation reduces the coefficient of friction of the surface by ~60% so abrasive materials are more likely to 'slip' off the surface than erode it.

Fujihokka and other similar anodised coatings have been used to improve wear/impact resistance of aluminium components in aggressive applications such as engine parts.

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3.ii. Corrosion Resistance

The 'standard' Premium Grade Fujihokka material is able to withstand pH values of 5~9. However, this range can be expanded to pH values of 3~11 by impregnating the Fujihokka surface with Telfon (PTFE).

However, Telfon is inert and extremely non-polar, it is able to resist adhesion from a wide range of materials i.e. it is hard to bond Telfon to many materials. Hence, the wear resistance of conventionally applied Teflon coatings is not optimal as it is usually just sprayed on and baked.

With Fujihokka, the wear resistance is much improved because Teflon is not just sitting on the surface of the material but is impregnated deep into the pores of the anodised surface.

However, it should also be noted that the corrosiveness of chemicals will double with every 10 C increase in temperature. Hence, a chemical, which may be non-corrosive at low temperatures, can become very corrosive at elevated temperatures.

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3.iii. Peel Resistance

Fujihokka surface treatment is NOT a paint or lacquer, which are prone to peeling. This is because there is only an adhesive bond (i.e. physical bond) between them and the aluminium substrate.

The Fujihokka treatment involves forming a special aluminium oxide (alumite) film on the surface of the aluminium, which is chemically bonded to the aluminium. As such, there is no adhesive interface that can break down with time. Furthermore, even if the Fujihokka surface is damaged, there will be no peeling of the alumite film as a result of the damage.

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3.iv. Heat Resistance

The Fujihokka coating is a technically a ceramic and has a melting point of ~1900 C. However, it is not recommended that it be used continuously above 450 C in a static (ie heat energy cannot be readily emitted) environment because aluminium alloys soften at this temperature. Despite this, the high melting point of the Fujihokka coating will allow it to withstand heat bursts.

In a dynamic environment (ie where heat energy can be readily emitted), Fujihokka materials may be used above the theoretical melting temperatures so long as the component has sufficient thermal mass to hold the heat energy before it is dissipated.

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3.v. Exposure to Environment

Although there are no industrial standards for high emissivity products, the Fujihokka process adopts the British Standard 3987, which defines the quality of coloured anodic films for architectural applications. However, this is also relevant to many other applications. In generally, this standard requires the anodic films to withstand exposure to an outside environment for 25 years.

In this BS specification, there is a method of determining the seal effectiveness of the film. Generally, with the highest quality anodic films, the pores are sealed as the final part of the anodising process. How well the pores are sealed affects the performance properties of the anodic film.

Sealing is done by immersing the anodic film into slightly alkaline solution at 96 C. During this hydration process, an impervious layer will form over the anodic film and plug the pores.

The seal value can be easily measured by a simple conductivity test. A small adhesive masked rubber ring is attached to the anodised surface to form a reservior. Potassium sulphate solution is then poured into this reservior.

A conductivity meter is used to measure the conductivity through the anodised layer in micro siemens. The seal value is calculated by multipling the conductivity reading by the film thickness in micron. The BS specification calls for seal values no greater than 500. If anodic films are to be exposed to an outside environment or to any aggressive environment, it is vital that the surface film is adequately sealed.

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4. Question : What are the properties of Fujihokka base alloys?

4.i. Suitable Types of Aluminium Alloys

Most commercially available aluminium alloys can be used as starting materials for PG Fujihokka treatment. The material properties of the substrate alloy will NOT be affected by Fujihokka treatment BUT the surface properties (emissivity, wear & corrosion resistance) will be greatly improved.

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4.ii Hardness of Aluminium Base Alloys

All aluminium alloys start life in the molten form and they are commercially available in various state of hardness; quarter hard, half hard, full hard etc.

Regardless of the state of hardness an aluminium alloy is in, it will harden further with age (age hardening) and/or mechanical deformation (work hardening).

Hence, an aluminium alloy in a half-hard state may be required for the component manufacturing process so that it would be flexible enough to withstand deformation. During the manufacturing process, this alloy would be hardened further through deformations and the final result may be an aluminium component, which has attained the fully hard state.

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4.iii. Deformation Under Pressure

As Fujihokka surface treatment does not affect the bulk material properties of the underlying aluminium alloy, deformity would relate to the properties of the alloy used for Fujihokka treatment. For most practical purposes, proof stress in compression can be taken as the same as proof stress in tension.

Regarding physical properties at elevated temperatures, tensile strength and hence compression strength will decrease with temperature. This will again depend on the base alloy properties.

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4.iv. Specific Gravity

The Fujihokka surface layer is an alumite (aluminium oxide) compound having different properties to the base material.

However, the specific gravities are roughly equivalent because although alumite has a specific gravity of ~4, as opposed to 2.7 for aluminium; an anodised layer contains many pores, which effectively reduces the specific gravity to ~2.7.

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5. Question : How does Fujihokka compare with other materials?

5.i. Fujihokka verses Copper

Copper has an excellent thermal conductivity compared to aluminium, which only has ~60% thermal conductivity of copper. However, the emissivity of copper, which is less that 50%, is inferior to that of Fujihokka.

If copper is used for heat exchange tubing, the thermal energy is transferred primarily by convection/conduction, not radiation. However, with Fujihokka, radiative energy is also a major contributor, making for more efficient energy transfer.

The end result is that a Fujihokka based heat exchanger will be more efficient and it may be possible to downsize the exchanger and yet still maintain the desired energy transfer.

The specific gravity of aluminium is a quarter that of copper, which means that aluminium is of lighter weight than copper and that material costs can be reduced as metals are sold by weight.

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5.ii. Fujihokka verses Steel

In the case of a heat exchange using water as the cooling element, a steel tube has poor thermal conductivity (24.5 W/M/K) and poor emissivity (10~15%). It therefore tends to retain heat rather than transferring it. In event of rusting, emissivity of steel may be increased to ~45% due to the iron oxide formed on the steel surface - but this is generally an unsatisfactory situation.

However, Fujihokka will be more efficient because the aluminium substrate has a much higher thermal conductivity (240 W/M/K) and the Fujihokka treatment boosts the emissivity to 90%.

This will enable thermal energy to flow freely into the water, which absorbs energy strongly at 2.5 micron due to the predominance of OH groups. Fujihokka, remarkably, will also have a greater corrosion and abrasion resistance than stainless steel.

The specific gravity of aluminium is one third that of steel, which means that aluminium is of lighter weight than steel and that material costs can be reduced as metals are sold based on weight.

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5.iii. Fujihokka verses High Emissivity Paints

Suppliers of high emissivity paints often quote emissivity vaules in excess of 90%. However, these values are probably derived from empirical measurement of surface reflectivity rather than actual far infrared thermal emission (see Reflectivity & Emissivity and Measuring Emissivity).

High emissivity paints are also not suitable for 'clean' environments. They contain solvents, which can be trapped within the paint layer. Paints adhere to substrate materials so surface preparation (cleaning, priming etc.) is paramount.

Such physical bonds do tend to break down with time, which is accelerated by temperature effects. This can lead to paints cracking and peeling with time. In addition, paints can oxidise with time resulting in a gradual drop in performance.

In contrast, Fujihokka is an aluminium oxide surface layer which is chemically bonded to the substrate aluminium i.e. it will not crack, peel or deteriorate in any way with time. Fujihokka is eminently suitable for clean environments and has been tested under high vacuum conditions. The results showed that Fujihokka does not release volatile materials.

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5.iv. Fujihokka verses Standard Anodising

Although standard anodising techniques can offer similar advantages as Fujihokka with regard to wear & corrosion resistance, it cannot match Fujihokka's high emissivity (~60% vs 90%), especially below the 10 micron wavelength.

* NB : There are many tests which measure hardness under different experimental conditions (e.g. indenters made in different sizes, shapes, materials, and applied with different loads) and deduce their data using different formulae. As a result, there is no direct analytic conversion between hardness measures. Instead one must correlate test results across the multiple hardness tests.

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6. Fujihokka Application Data

This section outlines the necessary information required to process any Fujihokka enquiries.

1. General Details of Application

a) Information on current materials used (e.g. aluminium alloy type)
b) Nature of application e.g. cooling, heating or drying

2. Operating Conditions

a) Specify the maximum or minimum operating temperature and pressure.
b) Specify and identify what is in contact with Fujihokka material e.g. gas, liquid etc.
c) Is abrasion/corrosion resistance important?
d) Is the Fujihokka material to be coated e.g. non-stick treatment?

3. Material Specifications :

a) Dimensional information and engineering drawing or samples of required parts.
b) For sheet materials, please specify length x width x thickness (mm) required and one or two side treated.
c) For tube materials, please specify length x internal diameter x wall thickness (mm).
d) Possibility of supplying a sample of existing component (aluminium based) for PG treatment.
e) Is Fujihokka material to be welded, where (masking) and to what materials?

4. Commercial Information

a) Total quantity
b) Call-off
c) Delivery dates
d) Target price

5. Alternative Offer

Rather than just supplying Fujihokka sheet or tube materials, engineering fabrication services, together with Fujihokka treatment, for finished items can also be offered.

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