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行星式球磨仪 高能量输入粉碎

行星式球磨仪满足并超过快速、可重复性研磨到精确分析的所有需求,从常规的样品加工到胶体研磨和先进的材料开发,它们被用于实验室中最严苛的任务。

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行星式球磨仪-功能原理

在行星式球磨仪中,每个罐子代表一个"行星"。这颗行星位于一个圆形平台上,即所谓的太阳轮。太阳轮转动时,研磨罐绕自己的轴以相反的方向旋转。因此,离心力和科里奥利力被激活,研磨球得以快速加速,产生研磨出极细颗粒所需的高粉碎能。研磨球摩擦研磨罐一侧的内壁撞击另一侧内壁产生的巨大加速度对样品材料产生强烈的冲击效应,并通过摩擦发生附加的粉碎效应。
对于胶体研磨和大多数其他应用,太阳轮的速度与研磨罐的速度之比为1:-2。这意味着在太阳轮的一次旋转过程中,研磨罐以相反方向旋转两次。这个速度比通常在行星球磨机中是很常见的。
RETSCH还提供速度比为1:-1的行星式球磨仪,又被称为离心模式,能够以较少的能量输入产生更加温和的研磨效果,适用于加工温度敏感样品;能量输入较高、速度比为1:-2.5甚至1:-3的行星式球磨仪则主要用于机械化学应用。

行星式球磨仪-应用领域

行星式球磨仪用于干湿模式下软、硬、脆、纤维材料的粉碎。极高的离心力产生很高的粉碎能量,因此研磨时间较短。
行星式球磨仪非常适合机械化学(机械合成、机械合金化和机械催化)或纳米级超细胶体研磨等研究应用,也非常适合混合和均质等常规应用。
行星式球磨仪的一个关键优势是它的灵活性。行星式球磨仪可以提供不同数量的研磨平台;研磨罐和研磨球有各种尺寸和材料。

行星式球磨仪——研磨罐和研磨球材质

如何选择最合适的材料?

If, for example, a sample is analyzed for its heavy-metal content, the abrasion of steel grinding jar and balls might introduce chromium into the sample which would lead to falsified analysis results. Therefore, a metal-free material like zirconium oxide should be selected. Another point to consider is the influence of the tool on the grinding efficiency. Here two aspects are important:

  • Energy input (related to the density of the material)
  • Hardness of the material

能量输入

The energy input grows with increasing density of a material. If the material of the grinding jars and balls has a high density, like tungsten carbide, acceleration of the grinding balls is higher at a given speed compared to materials of lower density. This means the energy input is higher when the ball hits the sample and, consequently, the crushing effect is higher with dense materials. This effect is beneficial for pulverizing hard-brittle samples.

For soft sample materials, on the other hand, too much energy input can prevent effective crushing. In such cases, the sample is not really pulverized into a fine powder but rather forms a layer that sticks to the jar walls and covers the grinding balls. Homogenization is not possible that way and sample recovery is difficult. For soft sample materials, other mill types, for example rotor mills, are better suited.

硬度

To find a jar and ball material with suitable hardness, the consideration is simple: The material must be harder than the sample. If the material is less hard, the grinding balls could be ground by the particles of the sample material.

不同材料的研磨工具

不建议使用不同材料的配件。例如钢制研磨罐搭配氧化铵材质的研磨球,首先,两种材料的磨损都会影响分析结果,其次,会加速配件的磨损。

Planetary ball mills - recommended jar fillings

用于干磨

For dry grinding, the best results are usually obtained with the so-called one-third-rule. This means, that approximately one third of the jar volume should be filled with balls. Following this rule, the smaller the balls are, the more must be taken to fill a third of the jar. Another third of the jar volume should be filled with sample material. The remaining third is free space to allow the ball movement inside to achieve the required comminution energy for fast pulverization of the sample.

Following this rule, the required crushing energy is provided while at the same time sufficient sample material is in the jars to prevent wear. 

1. One third free space
2. One third sample
3. One third grinding balls

用于含纤维的样品

For fibrous materials, or materials which lose their volume drastically when pulverized, a higher sample filling level is advisable. Sufficient material needs to be in the jar to minimize wear. If necessary, it is possible to add more material after some minutes to maintain the minimum required volume. 

1. Two third sample
2. One third grinding balls

用于湿磨

To produce particle sizes down to 100 nm or less, wet grinding and friction is required rather than impact. This is achieved by using many small balls with a large surface and many friction points. Consequently, the one third filling level, which is recommended for dry milling processes, is exchanged by the 60 % rule, meaning that 60 % of the jar are filled with small balls. The sample amount should be approx. 30 %. First, the small balls are added to the jars (by weight!) and then the material is added and mixed. Finally, the dispersant liquid is mixed carefully.

1. One sixth to one third sample + liquid
2. Two thirds grinding balls

如何选择合适的研磨球尺寸

另一个经验法则是,研磨球应该至少比最大的样品大三倍。这样就保证了研磨球能快速粉碎样品。
要为所需的最终出料粒度找到合适的研磨球尺寸,通常可以应用大约1000倍的系数。如果想要研磨出30 µm(D90),最适合的研磨球尺寸会在20毫米至30毫米之间。如果需要较小的颗粒,则必须更换较小的研磨球代替,以进行第二个工艺步骤。
由于较大的球会压碎较小的球,因此不宜在一个研磨过程中组合使用不同尺寸的研磨球。

Wet and nanoscale grinding in planetary ball mills

Nanotechnology deals with particles in a range from 1 to 100 nm. These particles possess special properties due to their size, as their surface is greatly enlarged in relation to their volume (so-called “size-induced functionalities”). Ultrafine particles are, for example, harder and more break-resistant than larger particles.

With dry grinding the particle size of a sample can only be reduced to a certain extent as small particles tend to get charged on their surfaces and agglomerate. Therefore, liquid or dispersant is used to keep the particles separated. Salt solutions are used to neutralize the surfaces charges. Long chain molecules in the liquid can keep the particles separated thanks to steric hindrance.  

Due to their significantly enlarged surface in relation to the volume, small particles are drawn to each other by their electrostatic charges. Neutralization of surface charges is only possible by adding a buffer (electrostatic stabilization, left) or by adding long-chained molecules (steric stabilization, right).

Use of planetary ball mills for co-crystal screening

Co-crystals are solid materials composed of two or more molecular components. Co-crystal screening is the process of identifying suitable co-formers that form stable and desirable co-crystals with a target molecule. Co-crystal screening can be used to improve the physicochemical properties of, e.g., pharmaceuticals or agrochemicals such as solubility or stability. With a special adapter, co-crystal screening can be carried out in a planetary ball mill, using disposable vials such as 1.5 ml GC glass vials. Typically, a few 3 mm or 4 mm steel balls are used to mix the substances at low to moderate speed. If required, a few µl solvent are added. The process is usually finished in 30-120 min.

The adapter features 24 positions arranged in an outer ring with 16 positions and an inner ring with 8 positions. The outer ring accepts up to 16 vials, allowing for screening up to 64 samples simultaneously when using the Planetary Ball Mill PM 400. The 8 positions of the inner ring are suitable to perform trials with different energy input, e.g. for mechanosynthesis research.

As the vials are made of glass, the speed of the mill should be selected carefully, we recommend a maximum of 500 rpm in the PM 300 and 550 rpm in the PM 100. The maximum speed of 400 rpm in the PM 400 is not critical.

For co-crystal screening high energy input generated by high speed is disadvantageous as this might lead to alterations of the chemical compounds of the substances. Consequently, optimum results are obtained at low and moderate speed.

行星式球磨仪-FAQ

What is a planetary ball mill?

Planetary ball mills are used for pulverizing solid sample materials by impact and friction. The extremely high centrifugal forces result in very high pulverization energy and therefore short grinding times. Planetary ball mills are available with one, two or four grinding stations.

Which applications require a planetary ball mill?

Planetary ball mills are used wherever highest demands are placed on speed, fineness, purity, and reproducibility. They pulverize and mix soft, medium-hard to extremely hard, brittle and fibrous materials and easily achieve grind sizes in the low micron or even in the nanometer range. They are perfectly suited for mechanochemical applications.

How does a planetary ball mill work?

In the planetary ball mill, every grinding jar represents a “planet”. This planet is located on a circular platform, the so-called sun wheel. When the sun wheel turns, every grinding jar rotates around its own axis, but in the opposite direction. Thus, centrifugal and Coriolis forces are activated, leading to a rapid acceleration of the grinding balls.