Carbon Felt Insulation for High-Temperature Vacuum Furnaces

SGL describes cellulose-derived soft felt carbonization at 800–1000°C and further treatment above 2000°C for its graphite-like soft felts. Those are processing descriptions, not universal service limits. SGL material explanation

Mersen describes CBCF as short rayon-derived fibers joined by a carbonized resin matrix, and offers both soft graphite felt and purification options. These product families illustrate why precursor, heat treatment, form and purity must be separate RFQ fields. Mersen product description

The tool uses a constant effective conductivity in a steady planar model: q = k × (T hot − T cold) / t, with t in metres for W/m². With t entered in mm, the same numerical expression gives kW/m². Thermal power Q = q × area; annual thermal energy = Q × hours at condition. Target thickness in mm = k × (T hot - T cold) / target q in kW/m².

For k varying only with temperature, the equivalent interval value is k effective = integral[k(T) dT] / (T hot - T cold). It must describe the same gas, pressure, density and heat-flow direction. Using a value measured only at the hot face is an approximation, not an integrated model. The example k = 0.4 W/m·K is an editable assumption, not a quoted product property.

For a cylindrical wall with constant k, use radial resistance ln(r outer / r inner) / (2 × pi × k × L), plus ends and penetrations separately. A planar area estimate cannot silently substitute for this geometry. Multilayer walls likewise need layer-specific resistances and interface temperatures.

C177 addresses steady heat flow through flat specimens; E1461 measures diffusivity, from which conductivity can be derived with density and heat capacity. Their public scopes do not establish a universal percentage difference between methods. C177 / E1461

SGL publishes soft-felt thermal data measured by laser flash. This directly rules out a blanket claim that laser flash is always unsuitable for felt. Request the laboratory treatment of porosity, anisotropy, optical transmission and heat-loss corrections. SGL brochure, pp. 10–11

The historical Mersen CBCF 15-2000 sheet gives the following typical values for one rigid grade. It is dated February 2009, not 2026; the review date does not make it current. The table does not specify test pressure or uncertainty, so it illustrates condition dependence and is not used to populate the calculator. Original grade sheet

Engineering recommendation: request the actual pressure and gas species with each k(T) curve. Neither the historical points nor an assumed cubic temperature curve resolves an unmeasured operating condition. Do not extrapolate these values to a different precursor or grade.

Soft felt adapts to shapes; a rigid board or cylinder supports a defined geometry. Mersen lists surface treatments intended to limit gas permeability and erosion-related dust. This supports evaluating a facing, but does not establish a universal jet-velocity rating. Rigid/soft construction and surfaces

For gas quench, specify nozzle direction, local flow, peak pressure, pressure ramp, venting and facing retention. Evaluate quenching separately from static hot hold. Do not release unqualified foil, fastener spacing or washer dimensions based on the calculator.

Ash residue, individual elements, shed particles and desorbed gases answer different questions. C561 concerns ash and is not intended for determining ash in purified graphite. It cannot be used here as evidence of a boron limit or model validation. ASTM public scope

Mersen describes optional purification for its insulation. The appropriate acceptance limit remains process-specific; no universal boron, aluminum, wafer-yield or outgassing guarantee is established by this page. Supplier purification options

• SiC PVT: agree critical elements, detection limits, batch sampling and vapor-exposure compatibility with the growth-process owner.
• Brazing: qualify the carbon environment against workpiece chemistry, particle tolerance and vacuum requirements.
• Outgassing: request the test temperature, time, sample area, conditioning history, pumping arrangement and measured species; it is not calculable from an ash tier.

Engineering review is required for air ingress, moisture, reactive gases and process vapor exposure. This guide supplies no oxygen partial-pressure threshold that guarantees service life, no universal oxidation rate, and no material-life multiplier.

Commissioning: follow the furnace OEM and selected material supplier procedure for storage, drying, ramp rates, holds, vacuum checks and acceptance. Do not add a generic “100°C above operating temperature” bake-out: material and equipment ratings still apply. Record a baseline before production release.

All examples below are hypothetical. Common assumptions: planar geometry, hot/cold faces 1800/60°C, area 2.45 m², 6000 h/year at steady conditions and target 1.85 kW/m². The tool scenario buttons reproduce them. No cost, yield or service-life improvement has been measured.

The large calculated thicknesses are useful flags: an arbitrary heat-flux target and a guessed k can be mutually impractical. Confirm measurements and boundaries, consider a different architecture, or revise the target with the designer. A silent 160 mm cap would hide that conflict.

Use total installed cost plus replacements, labor, downtime, qualification and measured cycle energy over a common study period. Compare equal part quality, workload, gas cycle and availability. Model thermal-energy differences only as sensitivity inputs; electricity conversion and tariffs need site data.

Unknown on this page: purchase prices, cycle life, replacement intervals, electricity tariff and yield effect. Request them before making a payback claim. A cheaper felt may increase maintenance or qualification effort; a more expensive facing may also add handling complexity. Neither outcome is assumed.

Copy the study summary above and attach it with these inputs. Missing data should remain “to be confirmed” in the quotation; estimated values should not become guaranteed purchase-order limits.

Reviewed November 1, 2026. Dates refer to this review unless a document date is named. Links below are primary manufacturer publications or public standards scopes. No full paid standard, proprietary test report or customer result is claimed to have been audited.

1. SGL Carbon — SIGRATHERM soft felt

   Carbonized versus graphitized processing; vacuum/inert-atmosphere applications. Does not qualify every felt for 2400°C.

2. SGL Carbon — Specialty graphites for high-temperature furnaces

   Soft felt discussion on pp. 10–11 includes laser-flash measurements. Manufacturer examples, not proof of a universal precursor ranking.

3. Mersen — Rigid and soft carbon insulation

   CBCF construction, purification options, surface treatments and graphite soft felt. Published product descriptions are not batch certificates.

4. Mersen — CALCARB CBCF 15-2000 datasheet (February 2009)

   Historical typical conductivity values in nitrogen and vacuum for this one rigid grade. Pressure and uncertainty are not specified in the table; request current data.

5. ASTM C177-19(2026) — Guarded hot plate

   Public scope for steady-state flat-specimen heat-flow testing. Apparatus conditions and uncertainty matter; this calculator is not an ASTM test.

6. ASTM E1461-13(2022) — Flash method

   Public scope for diffusivity measurement; conversion to conductivity needs density and heat capacity. Porous/anisotropic specimens require appropriate experimental design.

7. ASTM C561-23 — Ash in graphite

   Public scope concerns nonburnable residue and states limits for purified graphite. It does not certify elemental purity or heat-loss calculations.

Evidence gaps: no matched-batch rayon/PAN comparison, furnace-specific oxidation threshold, universal siliconization expansion/stress prediction, outgassing rate, commissioned bake-out schedule or measured ROI is established here. The report therefore makes no numerical guarantee for them. Request process-specific evidence before procurement release.

Send the study assumptions, drawings and operating conditions. Ask for grade suitability, thermal data, an assembly proposal and explicit acceptance criteria before ordering.