Introduction Modified atmospheres in laboratory and industrial furnaces
Creating a modified atmosphere in a laboratory or industrial furnace / oven involves changing the composition of the atmosphere within a sealed vessel in order to achieve the ideal conditions for a specific process.
There are several different types of modified atmosphere, the properties of which determine their suitability for an application. Most modified atmospheres fall into one of three categories, inert, reactive, or vacuum.
The following is an introduction into the different types of modified atmosphere, how they can be created, Carbolite Gero solutions for the process as well as typical applications and frequently asked questions.
Carbolite Gero products are commonly used in air, but with additional equipment, some products are able to contain a modified atmosphere. Because air contains oxygen, heating a sample in air may cause it to oxidise which is not always desirable for some applications.
| N2 | nitrogen | 78.08% |
| O2 | oxygen | 20.95% |
| Ar | argon | 0.93% |
| CO2 | carbon dioxide | 0.038% |
| other gases | 0.002% |
Heat treating materials within a modified atmosphere ensures a controlled working environment, increased repeatability, and more consistent results.
Depending on the type of material processed and environment required, modified atmospheres can be used to either protect samples from oxidation during heat treatment, or actively promote reactions. Inert gases, such as argon (Ar) or nitrogen (N2), and reducing gases such as hydrogen (H2), are used to prevent oxidation, whilst oxidising gases, such as oxygen (O2) or nitrous oxide (N2O), are used to promote oxidation.
The choice of atmosphere is entirely dependent on the requirements of the heat treatment process.
Carbolite Gero generally uses nitrogen or argon to create inert atmospheres inside products.
Nitrogen is typically referred to as inert when it is to be used at temperatures below 1800°C. It displaces oxygen, so is ideal for use in applications where oxidation is undesirable.
Nitrogen is not a "noble" gas and, under specific conditions, can react with oxygen to form gases such as nitric oxide (NO) and nitrogen dioxide (NO2). These are collectively referred to as NOx gases (the "x" referring to the number of oxygen atoms present in the compound).
Where an inert atmosphere is required, nitrogen is a cheaper alternative to argon, provided that the material being heat treated (or any subsequent by-products) will not react with it.
Argon is a completely inert “noble” gas and will not react with any material it comes into contact with. It displaces oxygen, so is ideal for use in applications where oxidation is undesirable.
While more expensive than nitrogen, argon has the benefit of being able to be used at temperatures above 1800°C without any risk of reaction.
Carbolite Gero furnaces can be adapted for use with various reactive gases, such as hydrogen (H2), carbon monoxide (CO), ammonia (NH3), methane (CH4), etc. Of these, the most frequently used is hydrogen.
Hydrogen has only one electron, which makes it highly reactive. Subsequently it can be used as a reducing gas to react with and break down other materials, e.g. reacting with and removing oxides from metals.
It has an autoignition temperature of around 500°C (932°F), subsequently, it is important to ensure that adequate safety precautions are taken during use. Prior to introducing hydrogen into a vessel, the air must first be removed; this is usually achieved by purging with an inert gas. The vessel must then be heated above the autoignition temperature to ensure the hydrogen is burnt in a controlled manner.
For lower-temperature processes where the properties of hydrogen are required, a less reactive forming gas can be used. One typical forming gas is a nitrogen and hydrogen mixture containing a maximum of 5% hydrogen. At such low concentrations, the hydrogen is not typically explosive.
When working with gases that contain more than 5% hydrogen, a gas safety system is required to protect against explosions.
When working with reactive gases, it is important to be aware of both the lower explosion limit (LEL) and the upper explosion limit (UEL) for the gas in question. The LEL is the minimum concentration of gas or vapour that will cause a flash or catch fire when exposed to an ignition source, whilst the UEL is the maximum concentration of gas capable of igniting. Concentrations above the upper explosion limit are considered too rich and will not be able to burn.
Explosion range of Hydrogen
In laboratory and industrial furnaces / ovens there are two main methods of creating a modified atmosphere within a sealed vessel, “purging” or “evacuation and backfilling”. Both methods result in very low oxygen levels, however “evacuation and backfilling” typically results in a much purer atmosphere. The process of creating a modified atmosphere is known as “atmospheric exchange”.
Purging involves flowing inert gas into a sealed vessel to displace the oxygen and remove it from the vessel. Any water present on the surface of the vessel (adsorbed water) will not be removed by the process of purging. This process results in a modified atmosphere that is acceptable for many processes. It may be necessary to have two different gas flow rates; a high flow rate for the initial purge to get the oxygen levels as low as possible, followed by a lower flow rate during processing to maintain the desired gas concentration levels within the vessel. Carbolite Gero's HTMA oven range uses this principle.
The “evacuation and backfilling” method involves two stages. The initial stage requires the use of a vacuum pump to evacuate the vessel and extract as much air and adsorbed water as possible. This is followed by a period of “backfilling”, where a flow of inert gas is introduced to displace any residual elements or compounds.
This process can be repeated as many times as necessary to achieve the desired atmosphere within the vessel. Provided that the vessel is gas tight, this method is a fast way of achieving a purer modified atmosphere. The evacuation and backfilling method is ideal if the parts being heat treated are porous, as the vacuum pump removes any air that would otherwise remain trapped if using the purging method alone.
Evacuation and backfilling should only be performed when the vessel is at ambient temperature. Operation at high temperatures risks damaging the vacuum pump.
A – Purging vessel with N2 at 40 litres per hour (10x furnace volume per hour)
B – Purging vessel with N2 at 400 litres per hour (100x furnace volume per hour)
C – Evacuation and backfilling of vessel
In addition to inert and reactive modified atmosphere, it is also possible to heat treat samples completely under vacuum in a furnace / oven, without introducing a gas into the sealed vessel. The use of a vacuum pump has the additional benefit of extracting unwanted air and molecules from porous samples.
It is important to note that unless specifically designed for the purpose, vessels should not be evacuated with a vacuum pump whilst hot. The change in atmospheric pressure, coupled with the reduction in material strength caused by temperature changes can cause vessels, particularly those with a rectangular design, to rupture.
There are different levels of vacuum achievable, depending on the pump type used:
Pressure (mbar) | Type | |
| Rough vacuum | 1000 - 1 | Rotary vane pump |
| Fine vacuum | 1 - 10-3 | Roots pump |
| High vacuum | 10-3 - 10-7 | Turbomolecular pump |
| Ultra high vacuum | < 10-7 | Turbomolecular pump |
Other pumps (oil diffusion pump, cryo-pump, ion getter pump etc.) are available on request.
Note: Pumps which have no pumping speed in the rough and fine vacuum range, such as turbomolecular pump and oil diffusion pump, need to be used in combination with a pre-pump, such as a rotary vane pump.
Rotary vane pump
Roots pump
Turbomolecular pump
Specially designed cylindrical retorts allow for the use of vacuum at high temperatures, however due to the increased strain, the larger the retort, the lower the maximum operating temperature.
For more information on available vacuum furnace solutions, please see Carbolite Gero’s GPCMA and GLO furnace ranges.
Carbolite Gero retort with vacuum
Whereas most vessels for working in modified atmosphere are placed within a furnace chamber with the heating elements and insulation on the outside of the retort, “cold wall” vacuum furnaces contain both the heating elements and insulation inside the vessel itself. The positioning of the insulation has the effect of ensuring that the outer wall of the vessel remains cool, helping to protect the structural integrity of the vessel and therefore allowing the furnace to be operated under vacuum at high temperatures. These specialised furnaces are also available with water-cooling systems to further ensure that the vessel maintains a cool outer surface.
Evacuating a vessel results in the reduction of atoms and molecules inside. However, a perfect vacuum is unachievable so the number of particles can never be completely reduced to zero. In a vacuum of <10-7 mbar there are still <109 particles per cubic cm.
The table below shows the number of particles in 1cm3. The mean free path length (λ) is the average distance over which a particle can travel as a result of a collision with another particle. The greater the distance, the fewer particles are likely to be present. The λ value is dependent on the vacuum pressure.
| Rough vacuum | Fine vacuum | High vacuum | Ultra high vacuum | |
| Pressure (mbar) | 1000-1 | 1 – 10-3 | 10-3 – 10-7 | < 10-7 |
| Number of particles per cm3 | 1019 – 1016 | 1016 – 1013 | 1013 – 109 | <109 |
| Mean free path length (λ) | < 100 µm | 100 µm – 100 mm | 100 mm – 1 km | > 1 km |
The following table shows the different units for pressure. The SI unit is the pascal (Pa).
| Pa | bar | mbar | Torr (mm Hg) | atm | at | |
| 1 Pa | 1 | 10-5 | 10-2 | 7.5 x 10-3 | 9.87 x 10-6 | 1.02 x 10-5 |
| 1 bar | 105 | 1 | 10-3 | 750 | 0.987 | 1.02 |
| 1 mbar | 102 | 10-3 | 1 | 0.75 | 0.987 x 10-3 | 1.02 x 10-3 |
| 1 Torr | 133 | 1.33 x 10-3 | 1.33 | 1 | 1.32 x 10-3 | 1.36 x 10-3 |
| 1 atm (phys) | 101330 | 1.0133 | 1013.3 | 760 | 1 | 1.033 |
| 1 at (techn) | 98100 | 0.981 | 981 | 736 | 0.986 | 1 |
To maintain a modified atmosphere, a sealed vessel is required. This could comprise a work tube with specialised end seals for use with tube furnaces, or a retort, typically used in chamber furnaces.
Carbolite Gero offer standard gas packages and associated equipment to assist in creating and maintaining modified atmospheres within our products, as well as a range of products specially designed for modified atmosphere applications.
Optional modified atmosphere equipment and accessories allow for greater operational flexibility, as products can be used for multiple applications involving different gases, vacuum, or no modified atmosphere.
Carbolite Gero offers a range of options to enable modified atmosphere in the standard tube furnace range. These options included special work tube packages, inert gas packages, vacuum pump packages as well a hydrogen safety system.
In chamber furnaces, a retort is typically used to maintain a modified atmosphere. Optional equipment and accessories allow for greater operational flexibility, as products can be used for multiple applications involving different gases, vacuum, or no modified atmosphere.
In addition, there are dedicated chamber furnaces and ovens that are fully equipped for controlled atmosphere operation as standard.
The vacuum furnace range offered by Carbolite Gero includes vacuum chamber furnaces, vacuum hood furnaces, bottom loading furnaces, laboratory vacuum furnaces, and vacuum tube furnaces. Each furnace can be used with either a reactive gas or an inert gas. The majority of products in our vacuum furnace range are available with either metal, graphite, or ceramic insulation. On request, graphite models can be configured to safely operate at up to 3000°C.
These are some of many applications that require a modified atmosphere in a laboratory or industrial furnace / oven.
Pyrolysis is the method of decomposing materials at high temperatures within an inert atmosphere. An inert atmosphere is necessary as the materials are likely to combust when heated in the presence of oxygen.
Pyrolysis is frequently used to achieve the carbonisation of organic materials, converting them into a carbon/carbon-rich state. When materials are carbonised, they can have vastly different properties, and there are many fields of research into how the beneficial properties of carbonised materials can be harnessed.
University of York & Biorenewables Research Centre use pyrolysis to convert recycled starch into materials for use in battery technology.
3D printing is one additive manufacturing technique that can be used to create intricate metal structures that would otherwise be impossible to produce via traditional methods.
Typically, the metal source material must be in powder form and may be mixed with a binder material to help hold the resulting structure together. This binder must then be removed either chemically or via heat treatment.
The heat treatment must take place under a modified, oxygen-free atmosphere as exposure to air will cause the metal to oxidise, potentially ruining a part that is relatively expensive to produce.
Either an inert or reducing atmosphere can be used to protect the metal part form oxidation.
3D printed metal part before and after heat treatment in an inert atmosphere with a Carbolite Gero furnace.
With the advent of commercially available electric vehicles, there has been an increase in demand for battery technology, which has in turn put additional pressures on potentially finite resources, namely valuable metals such as lithium, cobalt, nickel, and copper. To be able to meet demand, the recycling of existing dead batteries is necessary in order to reclaim these metals for future use.
One such reclamation method involves breaking the old batteries into small chunks and heating them in an inert atmosphere within a rotary tube furnace to vaporise and remove the plastic. The inert atmosphere is necessary to prevent the plastic from burning, as it could cause potentially toxic fumes, and contaminate the metal with carbon. Vaporising the plastic ensures that the metal can be easily and cleanly extracted.
The most effective way to join different materials so they are vacuum-tight is to subject them to a soldering and brazing process within a high vacuum environment. Two dissimilar materials are connected using a metallic material, known as solder or brazing filler. The complete process requires a high or ultra-high vacuum environment, and maximum temperatures of 1100°C. The vacuum atmosphere prevents oxidation and allows the use of flux-free solder material.
Soldering of electronic compound under normal (left) and under high vacuum (right) conditions. You can spot the bubbles in the solder joint in the left image.
Hard metals are used to make wood working tools, rotating tools, window- or glass cutting tools etc. Small saw blade tips predominantly consist of tungsten carbide (WC), however small amounts of cobalt (Co) and titanium (Ti) may be included.
Metal powder is mixed with polymeric binder (Paraffin) and pressed into shape. Debinding and sintering of the pressed shapes can then be performed within a vacuum environment inside a graphite furnace.
During the debinding process, it is important to maintain a controlled gas flow to protect the furnace construction.
The sintering process requires very precise temperature control in order to preserve the small grain size of the carbides. Because of this, temperatures cannot be allowed to exceed 1450°C.
By applying a defined partial pressure atmosphere during the sintering process, cobalt diffuses towards the surface of the saw blades. This diffusion process negates the requirement to perform a further sputtering process, but requires a high precision of atmosphere control within the furnace. Millions of tungsten carbide tool tips are produced globally every day.
Creating a modified atmosphere involves changing the composition of the atmosphere within a sealed vessel in order to achieve the ideal conditions for a specific process. There are several different types of modified atmosphere, the properties of which determine their suitability for an application. Most modified atmospheres fall into one of three categories, inert, reactive, or vacuum.
Inert atmospheres are ideal for processes that involve samples which may become damaged by exposure to oxygen. They typically require the use of argon (Ar), or nitrogen (N2), which is classed as inert when below 1800°C. These gases displace oxygen, and do not react with sample materials, creating a protective atmosphere during heat treatment.
The term “reactive” is used to describe a range of atmospheres that are used to catalyse or support chemical reactions within a sample during processing. Reactive atmospheres are typically used to either promote oxidation reactions, which result in the formation of oxide compounds (iron oxide, carbon dioxide etc.), or reduction reactions, which remove oxide compounds from a sample. Examples of reactive atmospheres include the use of oxidising gas (O2 / N2O) and reducing gas (H2).
A vacuum atmosphere is required when it is necessary to have a complete absence of oxygen, or any other elements or compounds, within an environment. There are different levels of vacuum pressure that can be achieved through using different types of vacuum pumps; these levels include rough, fine, high, and ultra-high. The level of vacuum required is dependent on the application.
There are two main methods of creating a modified atmosphere within a sealed vessel, “purging” or “evacuation and backfilling”. Both methods result in very low oxygen levels, however “evacuation and backfilling” typically results in a much purer atmosphere. The process of creating a modified atmosphere is known as “atmospheric exchange”.
Purging involves flowing inert gas into a sealed vessel to displace and remove oxygen. Often, two different gas flow rates are used; a high flow rate for the initial purge to reduce oxygen levels, followed by a lower flow rate during processing to maintain the desired gas concentration levels and reduce overall gas consumption. Purging achieves a useable atmosphere in a shorter time frame due to the initial high gas flow rate.
The “evacuation and backfilling” method involves two stages. The initial stage involves using a vacuum pump to extract as much oxygen and unwanted particles as possible from both the vessel, and from within any porous samples inside. The evacuation stage is followed by a period of “backfilling”, where a flow of inert gas is introduced to displace any residual particles. This process can be repeated as many times as necessary.
There are four types of vacuum pump that are commonly used: rotary vane pumps, roots pumps, oil diffusion pumps, and turbomolecular pumps. Each pump is capable of achieving vacuum pressures within a particular range, and the choice of pump is dependent on the requirements of the application process. Carbolite Gero offer standard rotary vane and turbomolecular vacuum pump packages, which can achieve vacuum levels of 5x10-2 mbar, and 1x10-5 mbar respectively.
The accepted definition of a vacuum is a reduced number of (gas) molecules and atoms within a sealed volume (vessel) at a constant temperature, when compared to ambient conditions. If a vacuum is applied to a sealed vessel, the number of particles inside is reduced, however a perfect vacuum will never be achievable as even in ultra-high vacuum conditions there are still billions of particles within one cm3.
The pressure (P) is defined as the quotient of the force (F) acting perpendicularly on a surface, and the area (A) of this surface, so "P=F/A". The SI unit of pressure is “pascal” with the unit symbol Pa, however pressure can also be stated in other units, such as bar, mbar etc.
Whether it is a standard product with modified atmosphere capability or a fully customised system, Carbolite Gero has manufactured thousands of furnaces over the years and realised projects around the globe.
Contact us for a free consultation and talk to a product specialist to find the most suitable solution for your application needs!
