Make Crystals via Chemical Vapor Transport(CVT)

Table of content

  1. Introduction
  2. CVT Theory
  3. Experiment
  4. The End
  5. Reference

Introduction

Chemical vapor transport, the most luxury way of making crystals for element collection, has fascinated me with its gorgeous products at the first glance. Pictures of those expensive and precious CVT crystals are used by various element collectors and Wikipedia—the featured picture for elements such as platinum[3]—as a representative of the beauty of the elements. The price of CVT gold crystals on onxymet.com , for example, is 162.5€ per gram. However, they are sold out long ago and the price are soaring (up to three times higher)recent years because of the low yields—as far as I know, the only producer before my success is Ivan Timokhin[3]. In this project, I aim to produce CVT crystals on my own to provide more element collectors with those marvelous samples at a lower price.

Palladium crystals made by CVT[1]
Gold crystals made by CVT[1]
The platinum crystal on Wikipedia [2]

CVT Theory

Chemical vapor deposition(CVD) is a technology known for making artificial crystals and various coating layers. Chemical vapor transport(CVT) is a special type CVD that summarizes a variety of reactions that show the following features: a solid reactant is vaporized in the presence of a gaseous reactant, the transport agent, and deposits elsewhere through a temperature gradient in a close system[4]. Most CVD reactions, on the other hand, involves gaseous reactants and an open system.

Scheme of CVT experiments for crystallization of solids in a temperature gradient[5]

CVT reactions are valuable due to their uses in various fields. In labs, popular 2D materials such as black phosphorus and hundreds of single crystals are produced via CVT, which helps us to expand the boundaries of science. Also, CVT paves the way for the natural formation of multiple important minerals such as Fe2O3[5].

CVT is characterized by the existence of a transport agent, which is required for the dissolution of the source(reactant). For example, HCl is used as a transport agent in the transport of Fe2O3[4].

CVT of Fe2O3[4]

The second important component of the CVT process is the temperature gradient—T1 is the lower temperature and T2 is the higher—which only allows a single equation in the reverse reaction to fit in the Chatelier’s principle, thus “moving the substances” from side to side. In an exothermic reaction, the source travels to sink(product side) from T1–>T2; in an endothermic reaction, the source travels to sink from T2–>T1.

To maximize the transporte rate, the following steps are required:

  1. Find out the suitable transport agent
  2. Calculate the optimum temperature

Select a Transport Agent


Normally, the transport agents are halogens(Cl2, I2…), halogen compounds(HCl, HBr, HI). Other less common agents are mercury halides(HgCl2…), sulfur, and so on[5].

Two thermodynamic data(from diverse softwares or using Hess’s law) of the transport equation should be considered for choosing a transport agent:

  1. The equilibrium constant Kp——range from 10^-4 to 10^4
  2. Gibbs free energy ΔrG0——range from -100 to 100kJ/mol

ΔrG0<–100kJ/mol or Kp>10^4——a high dissolution of the solid into the gas phase but the deposition is impossible

ΔrG0>100kJ/mol or Kp<10^-4——the solid is unable to transfer into the gas phase

Select an Optimum Temperature

The optimum temperature calculated is the average transport temperature. The experimental temperature gradient is generated by adding or subtracting 100k to the optimum temperature, which is practically applicable.

To obtain the optimum temperature, Van’t Hoff equation is applied to establish a link between the equilibrium constant and the reaction’s enthalpy and entropy.

Van’t Hoff equation[5]

When K = 1 or ΔrG0T = 0, the optimum temperature is obtained:

Final equation for calculating the optimum temperature[5]

However, many experiments have proven that a temperature slightly higher than the optimum temperature calculated will lead to a higher transport rate since at the calculated temperature, condensation of unwanted reaction products would occur or crystal growth would be kinetically inhibited[4].

Example

Choosing the right transport temperature and agent for WO2 by analyzing the thermodynamic data.

Transport WO2 using different transport agents[5]

I2 should the best transport agent since Cl2’s and HCl’s equilibrium constants are too high, while the free energy of Br2 is farther from zero than I2. The optimum transport temperature is about 1300k(112400/84.6). Normally 298k is used to calculate the optimum temperature, but 1000k can also work since the result will not deviate too much[5].

This section only introduces you to some of the most basic calculations that allow you to model a very simple CVT reaction. You can learn new concepts like gas-phase solubility in articles in my reference.

Experiment

Warning: CVT involves flames, toxic waste gases, high temperatures, high pressures and more!!!

DO MATH TO PREVENT OVERPRESSURE EXPLOSIONS!!!

Equipment

1. A two zone furnace

It is the only laboratory equipment required and can be DIY by putting two furnaces and two temperature control systems together.

Remember to use quartz blocks at the two ends to stop the heat, put quartz wools at the opening ends of the tube to further stop the heat and indicate any leakage (wools change color when chemicals leak) and never open a furnace >800ºC.

2. Quartz tubes

Quartz tube is the only commercial tube that can withstand the high temperature (up to 1100ºC) and high pressure (few to tens of MPa). However, quartz has high melting point, so only flames >2000ºC can seal the tubes.

Normally I use a quartz tube with size: 10mm diameter, 150mm long, and 1mm thick. It is easier to seal the tube with the help of a suitable quartz block.

This is further explained in the “tube sealer” section.

3. Tube sealers

I have found oxyhydrogen flam generator and portable plasma flame generator the best options for sealing the quartz tubes due to their relatively low price and high flame temperature.

Oxyhydrogen flame generator:

Oxyhydrogen generator produces flame of 2500º to 3000ºC. It works by electrolyzing H2O and fires the H2 and O2 generated. The price of such devices increases when the rate of gas production increases. My device is a second-hand 90L/hour generator, $70. However, it is not very efficient in sealing quartz tubes. I think the ones higher than 200L/hour are ideal but they are normally very expensive.

Portable plasma generator:

Portable plasma generators are normally used in welding, producing plasma flame >8000ºC using H2O. The new ones are really expensive, but my second-hand one only costs about $300. Provided the extremely high temperature flame, it is ideal for sealing tubes. However, you need a sunglasses as the flame is incredibly bright, and you need to be brave enough to work under its massive heat and loud working noise.

3. A vacuum system—vacuum pumps, connectors, spark leak detectors…

For any CVT system, it is recommended to have a vacuum environment to prevent overpressure and produce purer crystals. A small cheap oil pump is enough for normal purposes, but you can upgrade it to a turbo pump (more in the fusion reactor post), costing hundreds or thousands of dollar more, to produce crystals of research-grade purity.

Vacuum connectors like the one in the picture above allow you to connect the tube to the vacuum pump with a rubber hose and disconnect to it during the sealing. It is composed of several elements; you can either by a complete module or DIY one using individual parts.

Applying glues like the one in the picture on the threads between individual parts can prevent air leakage.

In addition, VCR connectors or flanges (KF/CF) should be used in the case of a turbo pump due to their low leaking rate (more in the fusion reactor blog).

4. Other things: e.g. oven glove, electrical balance…

Well, a balance with scale 0.01g is necessary because you will need to start from about 0.05g of transport agents (using my tube size). Never put in too much transport agents (<0.1g is usually safe). It will explode!

Please do math to make sure the equilibrium pressure safe before putting in any transport agent!!!

You will need a balance with 0.1mg(0.0001g) scale to achieve optimum reaction rate. Any small difference in the mass of a transport agent can cause great varies in the equilibrium pressure, thus the reaction rate. Sometimes small differences even stop your reactions when you are using small reaction systems like my 10*150mm tube. If you can not afford the balances (in fact a second-hand 0.1mg can be quite cheap, around 120$).

I love Sartorius’ design

Result

I‘m currently making money by CVT crystals to support other my projects, so I will only post the trial of making iron crystals as an example. More will be updated in the future.

CVT Iron[6]

Material and method

Material

  1. 10*150*1mm quartz tube, sealing to have a 100mm close system.
  2. Source: 1.5g of Fe
  3. Transport agent: 0.10g or 0.20g of I2

Based on the trial, 0.10g I2 produces feather-shaped crystals and 0.20g I2 produces crystals of normal shapes but big sizes. Do your own trials to uncover more crystal types.

Method

  1. Temperature gradient: 800ºC to 1000ºC
  2. Time: one week

The End

I2 is only the simplest transport agent for iron—easy calculation, easy measurement (solid at the room temperature). Nevertheless, the reaction rate of this system is relatively slow. Use your own calculation and experimentation to find better temperature gradients, better transport agents, and better original masses.

CVT fascinates me because every time I use a new reaction system, I can find new treasures such as different crystal shapes or faster reaction rates. Although mathematics and thermodynamics help me a lot in designing an ”optimal” system, I can find better ones by trying new systems. Luck is a key factor in the experiments because you can never build a perfect model. I am always passionated for CVT since often when I open the furnace after weeks or even months or reacting, I can shout out ”OMG! I haven’t seen this before” or ”What! Why this time they grow so fast”.

Reference

[1] Photo from Sodium_Azide(also the featured image)

[2] Wikipedia contributors. (2022, August 7). /Platinum/. Wikipedia. https://en.wikipedia.org/wiki/Platinum#/media/File:Platinum_crystals.jpg

[3] /The collection Ivan Timokhin in the Periodic Table/. (2010). Theodore W. Gray. https://periodictable.com/Elements/Source-timokhin/index.html

[4]Binnewies, M., Glaum, R., Schmidt, M., & Schmidt, P. (2017). Crystal Growth Via the Gas Phase by Chemical Vapor Transport Reactions. /Handbook of Solid State Chemistry/, 351–374. https://doi.org/10.1002/9783527691036.hsscvol2020

[5]Schmidt, P., Binnewies, M., Glaum, R., & Schmidt, M. (2013). Chemical Vapor Transport Reactions–Methods, Materials, Modeling. /Advanced Topics on Crystal Growth/. https://doi.org/10.5772/55547

[6]Photo from Chao