How to judge the degree of hydrolysis of aluminum nitride and improve its hydrolysis resistance?

When it comes to ammonia, many people may cover their noses for the first time.

Ammonia and aluminum nitride powder

AlN is a covalently bonded nitride with [AlN4] tetrahedron as its structural unit.

Aluminum nitride ceramic substrate (Source: MARUWA)

However, a big problem of AlN is that it is prone to deliquescence - the surface of AlN powder is extremely active, and it will hydrolyze with water vapor in the air to generate aluminum hydroxide and ammonia gas

In the past, this reaction has been used to produce ammonia

1.

For the AlN hydrolysis reaction, domestic and foreign scholars have carried out a lot of research

KRNEL and KOSMAC studies found that there is an induction period before the hydrolysis reaction of AlN, which shortens with the increase of temperature and prolongs with the decrease of the pH value of the solution

By analyzing the hydrolysis products at different temperatures, it can be seen that the hydrolysis behavior of AlN will change at 351K: below this temperature, Bayerite is the main crystalline phase; when the temperature is higher than this temperature, crystalline boehm is formed.

2.

In the past, to judge whether aluminum nitride was hydrolyzed, many people relied on the smell of ammonia gas, and some people said that it could be filled into the composite material to see the change of thermal conductivity

① Since the AlN powder will generate ammonia gas after hydrolysis, the ammonia gas will ionize NH4+ and OH- in water, and the pH value in the solution will change, so the pH value is an important indicator to characterize the degree of AlN hydrolysis;

②The phase analysis before and after AlN hydrolysis can be carried out by XRD, which can be used to qualitatively judge whether there is a new phase after hydrolysis, judge the products of hydrolysis, and judge the degree of hydrolysis;

③ The morphology of AlN powder particles before and after hydrolysis can be observed by SEM, and the degree of hydrolysis can be qualitatively judged according to the change of morphology.
Generally, if aluminum hydroxide is formed, its specific surface area will become larger;

④ TEM can be used to analyze the crystal structure of products, etc.
in addition to morphology analysis
.

TEM micrographs of AlN powder before and after hydrolysis

3.
Methods to avoid hydrolysis of aluminum nitride

As mentioned above, the hydrolysis method generally occurs in the stage of powder, and the degree of hydrolysis will be greatly reduced after being made into ceramics
.
But don't think that all you need to do is to guard against contact with moisture during powder preparation and storage
.
For example, in the preparation process of aluminum nitride ceramics, since the wet grinding step also needs to add sintering aids, binders, dispersants and other aids, so the grinding solution medium ethanol is essential, and ethanol It will also contain some moisture, and the subsequent process will inevitably be exposed to the air, resulting in a hydrolysis reaction of AlN
.
Therefore, in order to ensure the quality of AlN ceramics, the most reliable thing is to improve the hydrolysis resistance of the powder itself
.

There are many surface modification technologies for AlN powder.
The basic principle is to perform corresponding physical adsorption or chemical treatment on the surface of the powder, and coat or form a thinner reaction layer on the AlN particles to prevent the hydrolysis reaction of AlN powder and water
.
The main methods are coating modification method, surface chemical modification method, heat treatment method and so on
.

Figure 3.
Surface modification method of aluminum nitride

01 Coating modification method

Coating modification is a traditional modification method that has been used for a long time.
It is to coat the surface of AlN powder with inorganic compounds or organic compounds, which can weaken or shield the agglomeration of particles.
Due to the steric repulsion, it is very difficult to re-aggregate the particles, so as to achieve the purpose of surface modification
.
The modifiers used for coating modification include surfactants, inorganic substances, hyperdispersants,
etc.

① Surfactant: According to the nature of the surface charge of AlN particles, the addition of cationic or anionic surfactants can change the interfacial tension of gas-liquid and solid-liquid in the powder dispersion system.
thickness of cladding
.

②Inorganic coating: The inorganic surface modification of AlN powder is to deposit inorganic compounds or metals on its surface by certain means to form a coating film, or to form core-shell composite particles, so that the surface of the modified powder presents a coating material properties and improve surface properties
.

③ Hyperdispersants: Hyperdispersants are similar to traditional surfactants in amphiphilic structure, but the hydrophilic and lipophilic groups of surfactants are replaced by anchor groups and solvation chains
.

.

02 Surface chemical modification

Surface chemical modification is accomplished by chemical reaction or chemical adsorption between surface modifiers and particle surfaces
.
The polymer long chain is grafted on the surface of the powder, and the long chain containing the hydrophilic group in the polymer acts as a three-dimensional barrier in the aqueous medium through hydration, so that the dispersion of AlN powder in the medium is stable in addition to relying on electrostatic repulsion.
And relying on steric hindrance, the effect is very obvious
.

①Coupling agent: A coupling agent is a compound that has both a polar group that can react with the surface of inorganic particles and an organic functional group that is reactive or compatible with organic matter
.
Its function is that one end can be combined with the surface of the powder and the other end can have a strong interaction with the dispersion medium, so it can improve the affinity of the AlN powder and the polymer material, and realize the dispersion of the powder in the polymer material
.

② Hydrophobization treatment: The hydrophobization treatment is to select organic substances with hydrophobized groups (such as long-chain alkyl groups, chain hydrocarbon groups, and cycloalkyl groups) to surround the surface of the AlN powder, so that the alkyl groups are firmly bound to the surface of the powder.
The surface exhibits strong hydrophobicity
.

③Surface grafting method: Surface grafting polymerization is to graft polymer chains to the surface of AlN powder through chemical reaction, which can significantly improve the dispersibility of particles in organic solvents or polymers
.

④Inorganic acid modification: The surface of AlN powder is treated with phosphoric acid, dihydrogen phosphate, etc.
, and it is found that it can not only make AlN resistant to hydrolysis, but also strengthen the dispersibility of the powder
.

03 Heat treatment method

The heat treatment method is to heat the powder to oxidize the surface to form a dense alumina protective film, thereby producing hydrolysis resistance
.
Li Yawei et al studied the effect of heat treatment process on the hydration resistance of aluminum nitride in the range of 700-1050 ℃ in air
.
AlN was heat-treated in air atmosphere, and it was found that AlN started to oxidize at 700 °C, and was further oxidized as the temperature increased.
When it reached 1050 °C, it was completely oxidized
.
The water resistance of the AlN powder modified by heat treatment is related to the ambient temperature.
As the water temperature increases, the water resistance decreases.
The AlN powder treated at 700-850 ℃ has certain hydration resistance
.

04 Other modification methods

There are also many other modification methods, such as high-energy treatment, ultrasonic wave, encapsulation modification, etc.
, the surface modification of AlN powder can also be carried out
.
Usually these methods are combined with other methods to improve the surface modification effect of powder
.

In addition, it should be noted that there are many factors that affect the surface modification effect of AlN powder, such as temperature, time, and the amount of modifier
.
The modifiers all interact with the surface groups of the nano-powder to achieve the effect of modifying the powder, but the chemical structure of different modifiers and the length of the chain segments may affect the nano-powder in the polymer matrix.
dispersion
.

In addition, the molecular weight of the modifier has a great influence on the surface modification of the powder.
If the molecular weight is too small, the thickness of the coating layer will be relatively thin, and sufficient steric hindrance will not be generated, and the dispersion of the modified powder will not be good enough; Modified by a large modifier, a thicker covering layer can be formed on the surface layer, and the molecular chain can be well compatible with the organic matrix
.

Fig.
5 Nano-powders coated with different molecular weight modifiers

(Left, small molecule coated powder; right, macromolecule coated powder)

When a silane coupling agent is used to modify the surface of the AlN powder, in order to improve the modification effect on the powder surface, a small amount of anhydrous ethanol and other solvents will be added to accelerate the reaction between the coupling agent and the powder.
When the solvent is different, this may change the interaction between the AlN powder and the coupling agent
.