One of possible scenario is rearrangement of the glass-surface structure, which is possible if in the tube is high temperature regime. If the tube made of sodium metasilicate then the problem can be K ions from sample.

Which metal oxide(s) are you using?  I think you're simply forming a glass with different thermal properties to the quartz. Reduce the temperature and time of heating and the lifetime of your quartz will improve.  Otherwise see it as a sacrificial piece, replacing after breakage.  You may want to try borosilicate (Pyrex or 7740 from Corning) as this has good heat resistance.

Thank you all for your suggestions. Dr. Rawle i am using NaNiCoMnO2. 

Seems my material form a layer in the tube. Kindly see the attached pic. 

Layer that is formed inside of the tube can have a different thermal expansion coefficient and the consequence is tension across all the tube.

To my mind, you are dealing with two separate issues:

1.      Chemical reaction of quartz glass with Na+ ionic modifier resulting in a lower melting point Na+ modified silicate glass or the mix of crystalline sodium silicate encapsulated with the low -softening T Na+ modified glass. That is why HCl is not cleaning it. The other components of your reagent would create black or brown-colored films. If the reagent directly placed into the tube I would recommend using quartz/ceramic boat/liner. The best high-T lining material is Pt foil. Crucibles/boats are usually made of Al2O3 ceramic or forsterite.

2.      Cracking of the quartz tube. Diffusion furnaces made of quartz glass/fused silica are working safely at ~1500C. However, quartz is sensitive to thermal shock. Temperature ramps up and down should be watched out. Also, material fatigue might be a factor.

Is there a common pattern for the fracture? e.g. the direction of crack growth and the fracture.

It seems that the quartz tube has a directional strength but a relatively homogeneous thermal expansion which might be responsible for a consistent fracture pattern

Dear Vivek,

Quartz is resistant to heat, but if the temperature gets to too high, you can damage your quartz countertop. A quartz countertop is made up of nearly 93% quartz with the rest being the resin and pigments. The resin can only withstand temperatures of around 150 degrees. The principle of quartz heating works with the heating element becoming very hot and the quartz tube experiencing a sudden build up of heat. When the quartz tube reaches the appropriate temperature, the heat is emitted out of the quartz tube.

Quartz crystals lose the piezoelectric effect when heated above the Curie temperature of quartz—573 degrees C. At that temperature alpha-quartz becomes beta-quartz. The Constants of Alpha Quartz. This is reversible (the beta-quartz crystal reverts to alpha-quartz). However, alpha-quartz exists in both right-handed and left-handed versions. When the temperature decreases below 573 C, regions of the crystal become right-handed and other regions become left-handed, thereby ruining the crystal for piezoelectric applications. “Overheat” a quartz crystal beyond 1710 degrees C it will melt, thereby permanently destroying its crystalline properties.

***In my opinion, possible reason may be your metal oxide (if oxide is alkaline) vapor reacts at surface vigorously with resin mixture which bring frequent breakdown. Overheating can cause damage to the resins. Resin used to bind the quartz matrix will discolor (turn white) if heated too high.

Hope it help you out.

Ashish

It is due to a phase transformation in the quartz from alpha-silica to tridymite.