What Is Low-Temperature Thermal Evaporation?
Low-temperature evaporation offers a unique alternative that minimises the energy requirements and high-temperature risk involved in evaporating organic materials.
Low-temperature evaporation uses a vacuum chamber to vaporise materials, reducing their boiling point. The temperatures in these chambers range from 50 to 600 degrees Celsius — much lower than the temperature within a traditional thermal evaporation chamber.
How Low-Temperature Evaporation Works
Many organic materials already have low boiling points, and exposing them to high temperatures during evaporation is unnecessary. By vaporising organics in a low-temperature evaporation system, these materials can avoid being exposed to unsafe levels of heat that could impact their deposition rate.
Because organic materials are extremely sensitive to temperatures, even a small temperature increase during thin-film deposition could cause the organic material to eject from the source. Low-temperature evaporation techniques allow for greater control over the deposition rate and vacuum temperatures of organic materials within physical vapour deposition.
By regulating the temperature in the vacuum system, certain qualities of the organic material can be preserved that would otherwise fail within high temperatures, such as concentration and thickness measurements. If the liquid falls below the necessary boiling temperature, a heat exchanger can recover heat from other sources to increase the temperature within the vacuum chamber.
Low-Temperature Evaporation Systems
Several low-temperature evaporation techniques offer fine control over material heating power, including:
- Two-stage systems, which provide intermediate gas cooling between two compressor steps
- Intercooler systems, which use an intermediate evaporation step to cool the discharge gas
- Cascade systems, which utilise two refrigeration circuits connected by an intermediate cascade heat exchanger
Low-temperature vapour systems typically operate in a feedback loop based on thickness measurements, which allows for a highly specialised deposition rate.
Most systems maintain normal levels of condensing pressure with low evaporation temperatures. Evaporating the material over multiple stages prevents high local pressures from increasing the discharge temperature of the material.
However, some materials with high vapour pressures may exhibit isotropic qualities through vacuum evaporation due to the high local pressures in the deposition plume.
Low-Temperature Evaporation vs. Standard Thermal Evaporation
Low-temperature evaporation stands in contrast to thermal evaporation, exhibiting these unique qualities.
Temperature
The primary difference between these two evaporation techniques is the temperature required within the chamber.
Thermal evaporation methods typically heat the source materials to temperatures of 1,000 degrees Celsius or higher, though some standard thermal evaporation systems allow for evaporation temperatures of up to 1,500 degrees [1].
Meanwhile, low-temperature evaporation supports temperatures of 600 degrees Celsius and below. These systems are ideal for organic materials that are extremely sensitive to a small temperature increase and need to maintain highly specific properties for their applications.
Design
These two evaporation systems also utilise different designs. In a low-temperature vacuum evaporation system, the heating element is separate from the material support. This design allows for more control over the material heating power.
In contrast, standard thermal evaporation uses emissions at the heated source that immediately condense onto a cold substrate. There is no separation of the heating element from the substrate materials.
Properties
A material’s evaporation method impacts its properties, requiring developers to choose the right method when forming thin films.
For example, TiN coatings produced using a low evaporation temperature of approximately 200 degrees Celsius showed an increase in hardness and residual compressive stress when compared to the same coating produced with a standard temperature method [2].
In the same study, CrN coatings deposited with low-temperature evaporation demonstrated different small-molecule compounds and chemistry than their standard evaporation counterparts.
Radiative Loss
Evaporation using lower temperatures significantly reduces radiative loss compared to conventional evaporation sources. This method is more suitable for materials that benefit from slow temperature change during organic evaporation.
Applications of Low-Temperature Evaporation
The industrial applications of low-temperature evaporation are numerous, spanning industries ranging from pharmaceutical to wastewater treatment.
Pharmaceuticals
Pharmacists have begun developing a bubble column low-temperature evaporation process for dissolving pharmaceutical APIs [3]. This method can achieve an evaporation rate of up to 6.4 grams per minute within a single stage.
Nanotechnology
Researchers have successfully used a low-temperature, catalyst-free evaporation route to evaporate ZnO nanoneedles and nanosaws [4].
Food Science
The food science industry is always looking for new methods of drying and concentrating food products to prevent spoilage. Low-temperature evaporation presents a unique, energy-efficient solution to removing liquid from foods.
Traditional drying methods are energy-intensive, but low-temperature evaporation methods can reduce water content before sending foods to the dryer, improving the efficiency of the process.
Wastewater Treatment
Reducing the volume of wastewater through drying or evaporation can minimise storage needs and disposal costs. By using vacuum evaporation, the liquid in the chamber will evaporate at lower temperatures, reducing the amount of energy necessary for evaporation.
Oil Field
Vacuum evaporation also plays a valuable role in separating water and other small-molecule compounds from oil. Oil companies can reduce operating costs by using energy-efficient evaporator systems to separate oil instead of traditional thermal evaporators.
Battery Science
Researchers have even begun using low-temperature evaporator systems within the mass spectrometry titration protocol for quantifying inactive lithium in batteries [5].
Meet the ORCA: Our Solution for Organic Evaporation
The ORCA Organic Evaporation Source from Korvus Technology is a vacuum evaporation system that uses low evaporation temperatures and high thermal conductivity to maximise stability and fine control. This user-friendly system makes low-temperature vacuum evaporation accessible to businesses across the U.K.
The ORCA supports vapour temperatures ranging between 50 and 600 degrees Celsius and uses a K-Type thermocouple that produce highly accurate temperature readings. You’ll enjoy a simplified solution to low-temperature evaporation that perfectly complements the Korvus Technology modular PVD systems.
Our low-temperature evaporation and modular PVD systems are reliable, cost-effective solutions for forming thin films across numerous industrial applications. To learn how the ORCA can benefit your company, discover more about the Hex PVD system, or better understand PVD coating, reach out to Korvus Technology today.
References
[1] Koskinen, J. (2014). Films and Coatings: Technology and Recent Development. Comprehensive Materials Processing, Accessed on 29 June 2023.
[2] Gahlin, Rickard, Bromark, Michael, Hedenqvist, Per, Hogmark, Sture, & Greger Hakansson. (1995). Properties of TiN and CrN coatings deposited at low temperature using reactive arc-evaporation. Surface and Coatings Technology, 76-77:1, 174-180.
[3] Roche, Phillip, Glennon, Brian, Jones, Roderick C., & Philip Donnellan. (2021). Low-temperature evaporation of continuous pharmaceutical process streams in a bubble column. Chemical Engineering Research and Design, 166, 74-85.
[4] Fan, H.J., Scholaz, R., Dadgar, A., Krost, A., & M. Zacharias. (2005). A low-temperature evaporation route for ZnO nanoneedles and nanosaws. Applied Physics A,, 80, 457-460.
[5] Tao, Mingming, Xiang, Yuxuan, Zhao, Danhui, Shan, Peizhao, & Yong Yang. (2022). Protocol for quantifying inactive lithium in anode-free lithium batteries by mass spectrometry titration. Communications Materials, Accessed on 29 June 2023.
