How To Break Quartz Combustion Tubes Scott Hughes, EA Consumables Ian Smith, Paul Stevens, Jon Davies, Elemental Microanalysis

Transcription

1 How To Break Quartz Combustion Tubes Scott Hughes, EA Consumables Ian Smith, Paul Stevens, Jon Davies, Elemental Microanalysis Quick Answer: Use them in an Elemental Analyzer

2 What is quartz? Second to feldspar as the most abundant mineral in the Earth s continental crust. We use Fused quartz or silica for our high temperature furnace tubes A glass consisting of silica in amorphous, non-crystalline form Pure SiO 2 without any other additives High working temperature and very low coefficient of thermal expansion, make it ideal for use in elemental l analyzers

3 Manufacture of fused quartz and silica Fused Quartz vs Fused Silica Fused quartz uses mined natural quartz deposits (Brazil, Madagascar) or high quality silica sand (Pennsylvania, West Virginia) as starting material Fused silica uses synthetically produced d silica sand as starting material. This is a purer material. High purity silica sand is produced from gaseous SiCl 4 4, burned in the presence of oxygen to form silicon dioxide. Fusion Process The sand is melted at high temperature, usually under vacuum and then drawn over a refractory mandrel to form tubes. In a batch process, the quality of the product is reduced as the process nears the end.

4 Fusion Heating source is either electrical or by flame fusion Translucent material is from the flame fusion, where small bubbles are included in the material, and more OH groups are formed into the glass. Transparent material is produced in the cleaner electric fusion process.

5 Polymorphic Forms Quartz Tridymite Cristobalite 870 o C 1470 o C 1710 o C( (m.p.) All 3 forms are SiO 4 tetrahedra linked such that every oxygen atom is common to two tetrahedra (ie SiO 2 ) The arrangement of the tetrahedra is different in the different forms. The change from quartz to tridymite involves the breaking of Si-O-Si bonds, and relinking. (Sluggish process) Quartz has a α/β form (just twisting) with 573 o C transition temperature

6 Impurities Impurities effect the viscosity of the glass H 2 O can create a terminal OH non bridging oxygen H 2 O HO HO Alkaline oxides (e.g Na 2 O) also creates a non bridging oxygen This is the process of devitrification

7 How to break a quartz combustion tube? Put it in an elemental analyzer Bring it up to 1,000 o C 1700 o C Add l hi h t i Add many samples which contain Alkaline salts and oxides 1700 o C

8 Material Quality Does Matter Not all Quartz is the same Specifications other than size includes: Devitrification, size and frequency Inclusions Surface impurities Chemical impurities (Na, Mg, K, H, Fe etc) User cannot tell good quartz from poor quality quartz The more non bridged OH groups, the more susceptible to devitrification ifi ti in the presence of cations Must trust supplier to ensure quality

9 Inclusions and surface defects show up during heating.

10 Transparent versus Translucent Fact or Folklore? Historically many felt that translucent combustion tubes were more resistant to devitrification compared to transparent tubes. Translucent tubes are from a flame fusion process, and therefore have more bubbles (ie the translucent part) as well as more non bridged OH groups. Last century, the translucent tubes sized for Carlo Erbas/Costech, happened to have a larger wall thickness. Today, this is not the case. Translucent tubes are still available but at a higher cost due to larger waste and time of manufacturing caused by more inclusions.

11 What to do? Get your combustion tubes from a supplier with known long term quality standards and systems (ISO 9001) Let me know if you need a recommendation! Be aware that some combustion reagents can lead to issues (next slide). Be aware of the type of samples with often lead to shorter tube life (Na, Ca etc). Use of thicker walled tubes Use of ash crucibles Use of liners

12 Chromium Oxide Key impurities in the Chromium Oxide can cause devitrification of quartz Devitrification is accelerated in the presence of Na & P ions. Most Cr 2 O 3 is formed from sodium dichromate, so Na is always present Elemental Microanalysis uses a different process to form Cr 2 O 3 to results in lower sodium impurities. After granulation, it is further heat treated to drive off other impurities The results is much longer tube life The tube on the left showed no signs of devitrification after 1,000 dt determinations. ti

13 Thicker walled tubes C1022 C1078 (71 premium)

14 Nickel Sheet Liner 130 x 130mm sheet $25 Cut in quarters roll into cylinder Insert into problem area of tube One time use (can t be removed).

15 Ash Crucibles Many to choose from Lengths Number of slits Orientation of slits Larger Tube size Remove ash soon after analysis

16 Conclusions Use trusted supplier to ensure high quality quartz Avoid crossing the 870 o C Quartz/Tridymite transition Consider using: Thicker combustion tubes, Larger combustion tubes Nickel Liner Ash crucibles and remove after use Keep a ready stock of more combustion tube. Devit Happens!