Figure 5 depicts the oxidation reaction, the mechanism inherent in the protective Aluminum oxide layer. This outer Aluminum oxide layer protects the Aluminum nitride core from earth’s atmosphere. [8]

Aluminum oxide and Aluminum hydroxide are listed among the least toxic substances to exist. In fact, both have medicinal and pharmaceutical value in many forms. For example: Aluminum hydroxide is an active ingredient in over the counter antacid tablets, and Aluminum oxide is used for ceramic medical implants, artificial bones, and sterile labware. [13]

Like many other ceramics, only highly concentrated exposure or ingestion of powdered Aluminum oxide materials have demonstrated toxicity or caused other acute symptoms. Due to the crystal size of this oxide layer, no surface flaking occurs over time. Surface flaking of oxides due to their enlarged crystal size can be seen with materials such as iron. As the iron molecules react with readily available oxygen molecules, iron oxide forms. Iron oxide’s crystal structure takes up too much space on the surface of the iron, causing the iron oxide molecules to become disrupted and flake. This flaking leaves fresh iron exposed to the atmosphere. The exposed iron again oxides and the process repeats until all the iron has been oxidized. Once the protective Aluminum oxide layer is formed and thickened on an AlN substrate via heat-treating, the AlN surface is effectively impermeable to O2, N2, and H2O. This means it is not possible for “aluminum dust” or “free aluminum particles” to be formed as suggested by other public parties within the community.

Surface oxidation is inherent when talking about AlN and its presence poses no risk to consumers. The reaction is beneficial, as it is one of the ways we can protect AlN from hydrolysis and environmental exposure. In the unlikely event of a consumer ingesting such an oxide, it would be an imperceptible quantity and totally harmless, as these compounds are sold over the counter as active ingredients in many antacids.

4.3 Reactions with acids

Aluminum oxide can react with some strong acids and bases. The term for this reaction is amphoterism, as in Aluminum nitride is amphoteric. Al2O3 reacts with Hydrofluoric acid as a base, and Sodium hydroxide as an acid. The chemical reaction with Hydrochloric acid is as follows:

Al2O3+6HCl→2AlCl3+3H2O

In this chemical reaction we see the formation of Aluminum chloride or AlCl3. Unlike Aluminum oxide or Aluminum hydroxide, AlCl3 is soluble in water. However, AlCl3 will only stay dissolved in the solution as long as HCl is present. When either the acid is neutralized, or all the water evaporates, Aluminum oxide will precipitate out of the solution.

It is important to note that this does not happen with all acids; the chlorine in Hydrochloric acid is directly responsible for this reaction. Acids are highly caustic, and some can even dissolve quartz, ruby, and sapphire materials.

AlN ceramics should not be exposed to any strong acids. Acids pose risk to the insert and consumers - but can easily be avoided by following product instructions.

4.4 Reactions with bases

Aluminum oxide can also react with some bases. One hydrolysis study specifically tested the effects of bases on hydrolysis. [9] Below is their base-assisted hydrolysis reaction:

AlN + NaOH + 3H2O -> NaAl(OH)4(aq) + NH3, AlN + OH- + 3H2O -> Al(OH) 4 + NH3

This experiment found over 80% conversion in 1.5M NaOH by 1500 seconds (0.4 hours), as opposed to the 47 hours it took with water vapor, alone. It’s important to note that there was minimal increase in reaction speed above 1M NaOH. [10] This second reaction is also of note because it describes how the protective layer on treated AlN can be dissolved. It is written as follows:

Al2O3 + 2NaOH -> 2NaAlO2 + H2O

Each of these reactions are important for consumers because they show that NaOH dissolves treated AlN. The second reaction shows how NaOH will break down the protective layer on an AlN insert, exposing the raw AlN underneath. Once the untreated AlN is exposed to the NaOH, the previously stated reactions begin. Constant submersive exposure of NaOH to AlN would lead to the complete degradation of an AlN insert.

Do not expose AlN inserts to any bases or basic cleaners. Bases could pose risk to the integrity of the insert and health of consumers, but can easily be avoided by following the products instructions.

4.5 Physical & thermal reactions, phenomena, and stresses

Heat energy can burn, melt, or boil a material, but can also cause damage due to thermomechanical stress caused by uneven thermal expansion. Concentrates are vaporized at high enough temperatures that many hard materials can be affected by such thermal stress. AlN has significantly higher thermal conductivity, lower specific heat, lower density, and lower thermal expansion than ruby or sapphire, and this level of thermal diffusivity and thermal expansion helps mitigate virtually any risk of thermal shock during normal operation of the product, including heating the insert directly with flame. High thermal diffusivity reduces the magnitude of any thermal gradient within the object, and is a key performance indicator in vaporization surface materials. AlN ceramics are stable in air up to 2516*F, due to the protective oxide layer that is formed as low as room temperature. After 2516*F, bulk oxidation could occur due to the level of excitement of the molecules, however, this would be extremely unlikely to occur in this application without deliberate and severe overheating. Hoaxes from uninformed competitors have declared that the flame of commonly used torches exceeds this temperature, and thus degrades the ceramics - this is factually inaccurate because temperature is a function of energy present in a substance - By placing the ceramic under the flame, it does not automatically become heated to the extreme temperature of the flame, and instead must absorb sufficient energy to reach this temperature over time; this is simple thermodynamic principle.



Particle exposure from aluminum nitride is unlikely due to scraping with a dabber or other common tool, as virtually all metal tools will instead be abraded by the harder ceramic, causing metal marking. While unsightly, metal marking is not harmful. It can be avoided by using caution, being gentle, and avoiding scraping action when using any metal forceps or tweezers to handle the ceramic product.

AlN ceramic heats up very evenly due to its extreme thermal diffusivity, and does not expand much - as such, AlN will never crack from using the wrong dabber, vaporizing different types of concentrates, or accidentally heating up one side of the banger or insert too quickly. Synthetic corundum (sapphire) inserts however, are susceptible to large internal thermal differentials caused by their material properties that are arguably mediocre in regards to thermal energy dissipation. These large thermal energy differentials cause physical expansion differentials through the component (thermomechanical stress), leading to cracking, shattering, chipping, etc. AlN ceramics can safely be blowtorched directly without fear of these thermomechanical stresses leading to damage. Aluminum nitride, Aluminum hydroxide, and Aluminum oxide are not soluble in water. Their respective approximate melting points are as follows: 2200 *C, 2403 *C, and 2072 *C. Typical, and even the most atypical physical or thermal user interactions with these three aluminum-based molecules will not cause them to vaporize. Unless heated in excess of 2516*F, no detrimental reactions will occur and cause damage to an AlN insert or consumers.

5. Conclusion

In this paper, we set out to examine the safety and stability of AlN by researching and following each found reaction to its conclusion. We discovered that AlN ceramics will not react with any elements commonly found in normal operating conditions, and that it poses no greater risk than other ceramics such as aluminum oxide, etc. Regardless of the possible negative reactions AlN can experience in extreme situations, the research supports the conclusion that FadeSpace S-Tier inserts are safe, and pose zero specific risks to consumers. The available research repeatedly demonstrates that hydrolysis is avoidable, the oxide layer of the material poses no risk, the reactions with acids and bases are easily counteracted by avoiding exposure, and there is no clear risk of any hazardous mechanical, thermal, or thermomechanical phenomena. We therefore conclude: if consumers do not expose AlN ceramic to anything other than air, isopropyl alcohol, water, concentrates, and heating devices / short periods of flame exposure,

an AlN ceramic piece will remain stable and safe for years.

Bibliography:

Li J, Nakamura M, Shirai T, Matsumaru K, Ishizaki C, Ishizaki K. Mechanism and Kinetics of Aluminum Nitride Powder Degradation in Moist Air. Journal of the American Ceramic Society. 2005 [accessed 2020 Feb 14];89(3):937–943. doi:10.1111/j.1551-2916.2005.00767.x

https://www.azom.com/article.aspx?ArticleID=3060

This paper is an excellent addition to AlN research, showing that Aluminum nitride powder will undergo hydrolysis in the presence of water vapor regardless of the production method. However, as the temperature approached and then passed 100 *C the rate of hydrolysis reduced significantly.

2. Li YQ, Qiu T, Xu J. Effect of thermal oxidation treatment in air on the hydrolysis of AlN powder. Materials Research Bulletin. 1997;32(9):1173–1179. doi:10.1016/s0025-5408(97)00093-7

https://www.sciencedirect.com/science/article/pii/S0025540897000937?via%3Dihub#

An older and possibly slightly incorrect paper, however it still has value for our purposes because it uses infrared analysis to measure the speed of hydrolysis with temperature. They suggest the layer deposited by oxidation is Al2C3, an Aluminum carbide. No other study found Al2C3. The internet suggests Al2C3 does not exist or the Aluminum carbide is the wrong name for that compound, it is possible the article is wrong.

3. Bartel CJ, Muhich CL, Weimer AW, Musgrave CB. Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping. ACS Applied Materials & Interfaces. 2016;8(28):18550–18559. doi:10.1021/acsami.6b04375

https://pubs.acs.org/doi/pdf/10.1021/acsami.6b04375

This is an incredibly complicated study going into the kinetics of the hydrolysis reaction with AlN. They study the activation energy and types of bonding within the Aluminum oxide and Aluminum nitride - going so far as to find the specific limiting agent, which is the diffusion of protons across the surface of AlN.

4. Kocjan A, Krnel K, Kosmač T. The influence of temperature and time on the AlN powder hydrolysis reaction products. Journal of the European Ceramic Society. 2007;28(5):1003–1008. doi:10.1016/j.jeurceramsoc.2007.09.012

https://www.sciencedirect.com/science/article/abs/pii/S0955221907004839

To study morphology - the specific products created - this group of scientists measure pH and look at the effect of time and temperature on what specific crystals are formed. This is useful for ensuring we know what material is protecting the insert. With this information, we can study the chemical reactions the material could be involved in.

5. Yeh C-T, Tuan W-H. Oxidation mechanism of Aluminum nitride revisited. Journal of Advanced Ceramics. 2016;6(1):27–32. doi:10.1007/s40145-016-0213-1

https://link.springer.com/content/pdf/10.1007/s40145-016-0213-1.pdf

This paper is significantly newer than many of the others, and builds on older work. They make the discovery that the oxidation is driven by diffusion, and not a chain reaction. The diffusion drives from high concentration to low concentration, depositing an even layer of oxidation. It additionally gives us an understanding of specifically how the oxidation stops hydrolysis. If the oxidation reaches maximum diffusion, the water cannot diffuse through the oxidation, onto the AlN.

6. Korbutowicz R, Zakrzewski A, Rac-Rumijowska O, Stafiniak A, Vincze A. Oxidation rates of aluminium nitride thin films: effect of composition of the atmosphere. Journal of Materials Science: Materials in Electronics. 2017;28(18):13937–13949. doi:10.1007/s10854-017-7243-5

https://link.springer.com/article/10.1007/s10854-017-7243-5

These scientists tested how the moisture content of the Aluminum nitride affects oxidation. They compared wet, dry, and mixed AlN, with interesting results. The dry Aluminum nitride oxidized in a logarithmic way; the wet oxidized in a linear way; and the mixed Aluminum nitride oxidized in a parabolic way and. The wet Aluminum nitride had the thickest oxide layer, capping at around 200nm.

7. Yeh C-T, Tuan W-H. Accelerating the oxidation rate of AlN substrate through the addition of water vapor. Journal of Asian Ceramic Societies. 2017 [accessed 2017 Aug 22];5(4):381–384. doi:10.1016/j.jascer.2017.08.001

https://www.sciencedirect.com/science/article/pii/S2187076417300076

A study about the effect of water vapor on AlN oxidation. They found in the presence of water, vapor oxidation was sped up by an order of magnitude. Important to note for us, “The presence of surface oxide reduces the thermal conductivity by ∼15% when the thickness of the oxide layer is only 3 μm.”

8. Gu Z, Edgar JH, Wang C, Coffey DW. Thermal Oxidation of Aluminum Nitride Powder. Journal of the American Ceramic Society. 2006;0(0). doi:10.1111/j.1551-2916.2006.01065.x

https://ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/j.1551-2916.2006.01065.x

This group of scientists studied the oxidation of AlN intensively. They compared the kinetics, morphology and crystallinity of AlN at different temperatures. This is the speed of the reaction, the depth of the oxidation layer, and the types of crystals deposited using X-ray diffraction.

Important to note from this study, “the density of oxide grains increased with temperature.”

9. Fukumoto S, Hookabe T, Tsubakino H. Hydrolysis behavior of Aluminum nitride in various solutions. Journal of Materials Science. 35(11):2743–2748. doi:10.1023/a:1004718329003

https://link.springer.com/article/10.1023/A:1004718329003

Here they compared the effects, at different temperatures, of two acids and a base on hydrolysis. They found the higher temperature, the lower hydrolysis. HCl and NaOH sped up the reaction while H3PO4 slowed hydrolysis.

10. Nosaka A, Hiraki T, Akiyama T. Hydrolysis Rate of Aluminum Nitride in a Sodium Hydroxide Solution. High Temperature Materials and Processes. 2011;30(4-5). doi:10.1515/htmp.2011.054

https://www.degruyter.com/downloadpdf/j/htmp.2011.30.issue-4-5/htmp.2011.054/htmp.2011.054.pdf

This paper studies the effect of Sodium hydroxide on hydrolysis of Aluminum nitride. It includes a written-out reaction for the base catalyzed chemical reaction. They found that the presence of 1M NaOH, a strong base, greatly speeds up hydrolysis.

11. Pyun S-I, Moon S-M. Corrosion mechanism of pure aluminium in aqueous alkaline solution. Journal of Solid State Electrochemistry. 2000;4(5):267–272. doi:10.1007/s100080050203

https://www.semanticscholar.org/paper/Corrosion-mechanism-of-pure-aluminium-in-aqueous-Pyun-Moon/c922e7b87a1da82cd33022ad13707cbc97d746c3

A study on the effects of alkalinity on pure aluminum. They found that in the presence of a strong base, the surface oxide is dissolved. The most important part of this paper is a mechanism for degradation of aluminum. They found that the base creates a layer of AlO3 from the aluminum, then breaks it apart, then creates another layer of AlO3 - then continues this pattern until all of the aluminum is dissolved.

12. Aluminum Nitride, AlN Ceramic Properties. Aluminum Nitride | AlN Material Properties. 2013 [accessed 2020 Feb 2]. https://accuratus.com/alumni.html

https://accuratus.com/alumni.html

This is a manufacturer's website that shows the physical properties of Aluminum nitride. It has some pretty basic information about the stability of AlN, as well as a table of numbers that describe the physical properties of the ceramic.

13. Kocjan A. The Hydrolysis of AlN Powder – A Powerful Tool in Advanced Materials Engineering. Wiley Online Library. 2018 Apr 27 [accessed 2020 Mar 10]. https://onlinelibrary.wiley.com/doi/full/10.1002/tcr.201800001

https://onlinelibrary.wiley.com/doi/abs/10.1002/tcr.201800001 - Here we see an unusual use of hydrolysis. Instead of trying to stop hydrolysis entirely, they set out to use it. They found a superior synthesis path for an aluminum powder. This powder is used in dental ceramics.